LOCOMOTOR BEHAVIOR AND HABITAT SELECTION IN

LOCOMOTOR BEHAVIOR AND HABITAT SELECTION IN INTERTIDAL GASTROPODS FROM VARYING SHORE HEIGHTS

Thomas W. Bates

with Dr. David W. Hicks, Dr. John W. Tunnell, Jr., Dr. Kim Withers, and Dr. David A. McKee

Center for Coastal Studies Texas A&M University-Corpus Christi 6300 Ocean Drive, NRC 3200 Corpus Christi, Texas 78412

Sian Ka'an Series, No. 8 Center for Coastal Studies Texas A&M University-Corpus Christi

May 2003 ..

11

PREFACE Texas A&M University-Corpus Christi has a long histnry nf scientific, researc.h in the marine and coastal environments of Mexico. Starting with research by Dr. Henry H. Hildebrand in the late 1950s on Alacrhn Reef and Laguna Madre de Tamaulipas to our Inure recenl work during the 1970s, 1980s, and 1990s on the coral reefs and coast of' Veracruz, we have been dedicated to studying the biodiversity and marine ecology of Mexico and providing graduate research opportunities in Mexico. Through distribution of theses, dissertations, leolulical reports, a ~ i dscic~itilicjournal articles, we have pl-ovidcd our research to Mcxicaii scientists aid natural resource maiagcrs. Most recently, starting in 1996, we have established a long-term study site at Rancho Pedro Paila, near Boca Paila, in the northern part of the Sian Ka'an Biosphere Reserve in the state of Quintana Roo on the Caribbean side of the Yucatan Peninsula. In order to efficiently and effectively get our research to interested Mexican scientists, natural resource managers, and other interested persons, we have created the Sian Ka 'an Series. Since peer reviewed journal articles take one to three years to be published, this series will allow quick dissemination of the information. Additional copies may be obtained with instructions on the next page of the document.

John W. Tunnell, Jr., Ph.D. Director, Center for Coastal Studies and Professor of Biology at Texas A&M University-Corpus Christi

Sian Ka'nn Scrics Center for Constnl Studies Texas A&M University-Corpus Christi 6300 Ocean Drive, NRC 3200 Corpus Christi, Texas 78412 Phone: 361-825-2736 Fax: 361-825-2770 Email: j [email protected]

Title No. 1

No. 2

No. 3

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Keeney, Talitha S. 1999. Coral reef macroalgae in northern Sian Ka'an Biosphere Reserve, Quintana Roo, MCxico. M.S. Thesis. Biology Program, Texas A&M University-Corpus Christi, Corpus Christi, Texas. Milroy, Scott P. 1999. Effects of light availability on reef community structure of hermatypic corals within Sian Ka'an Biosphere Reserve, Quintana Roo, MCxico. M.S. Thesis. Biology Program, Texas A&M University-Corpus Christi, Corpus Christi, Texas.

Price (USD)

$7.00

$7.00

Hilbun, Nancy L. 2000. Distribution and abundance of echinoderms fkom Sian Ka'an Biosphere Reserve, Quintana Roo, Mexico. M.S. Thesis. Biology Program, Texas A&M UniversityCorpus Christi, Corpus Christi, Texas.

$7.00

Koltermann, Amy E. 2000. Ecological characterization of northwestern Caribbean ironshores, Sian Ka'an Biosphere Reserve, Quintana Roo, Mexico. M.S. Thesis. Biology Program, Texas A&M university-corpus Christi, Corpus Christi, Texas.

$7.00

I

No. 4

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Tunnell, Kathryn D. 2001. Epibiont flora and fauna associated with two Rhizophora mangle forests, Veracruz and Quintana Roo, MCxico. . M.S. Thesis. Biology Program, Texas A&M UniversityCorpus Christi, Corpus Christi, Texas.

$7.00

Campbell, Matthew D. 2001. A dry season analysis of larval and juvenile fish assemblages of the Sian Ka'an Biosphere Reserve, Quintana Roo, MCxico. M.S. Thesis. Biology Program, Texas A&M University-Corpus Christi, Corpus Christi, Texas.

$7.00

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No. 7

No. 8

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Childs, Catl~erine. 2002. Development of a natural resource conservation plan far Punta Allen peninsula, Sian Ka'an Biosphere Rcscrvc, Quintma Roo, MCxico. M.S. Tl~tsis. Biology Prugraiun, Texas A&M University-Corpus Christi, Corpus Christi, Texas. Bates, Thomas W. 2003. Locomotor behavior and habitat selection in intertidal gastropods from varying shore heights. M.S. Thesis. Biology Program, Texas A&M University-Corpus Christi, Corpus Christi, Texas. Ledford, Chns. 2003. Comparison of coral species diversity and abundance between patch reefs and shallow reefs of the Sian Ka'an Biosphere Reserve, Quintana Roo, Mexico. M.S. Thesis. Biology Program, Texas A&M University-Corpus Christi, Corpus Christi, Texas.

$7.00

$7.00

$7.00

Van Sant, Scott. 2003. Community structure, abundance, and biomass of fishes on a Caribbean coral reef, Sian Ka'an Biosphere Reserve, Quintana Roo, Mexico: An analysis by depth zone and habitat. M.S. Thesis. Biology Program, Texas A&M UniversityCorpus Christi, Corpus Christi, Texas.

$7.00

Reed, Addie L. 2003. Implementation of a long-term coral reef monitoring plan, Sian Ka'an Biosphere Reserve, Quintana Roo, Mexico. M.S. Thesis. Biology Program, Texas A&M UniversityCorpus Christi, Corpus Christi, Texas.

$7.00

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ABSTRACT Microhabitat preference and daily and diurrlal r~lovementpatterns of two tropical (Nerita versicolor and Tectarius antonii) and two temperate (Siphonaria pectinata and Nodilittorina riisei) littoral gastropod species were compared in relation to shore position allowing for an assessment of the influence of physical stressors (heat and desiccation) in controlling their behaviors and vertical distributions. Two, 12-day monitoring efforts (one in Mexico and one in Texas) of individually marked specimens indicated that movement andlor feeding activities of intertidal gastropods is initiated during periods of dccrcascd desiccation and themla1 stress. Near corltinuous mobility was observed ill snails inhabiting the moist low-shore zone in Mexico. In contrast, tropical snails inhabiting the high-shore zone exhibited movement only after shores were wetted during periods of increased wave action and sea state. Further, in diurnal surveys both tropical species indicated a preference for nocturnal movement while desiccation and thermal stress were reduced. The narrow zone widths and heights, and hence smaller stress gradient, of Texas hard shores allowed continuous movement independent of shore position and time of day. The propensity for increased activity under conditions of reduced desiccation stress was also supported by an experiment in which wetting of the substrate (simulating inundation) encouraged activity in all tested species. All species exhibited a preference for sheltered microhabitat (crevice and pit) and avoided exposed surfaces, but the mechanism motivating these preferences differed between geographical locals. On the broad, platform-like rocky shores of Mexico, low and high-shore species use of sheltered microhabitat appeared to be a behavioral adaptation to reduce desiccation stress. In contrast, use of sheltered microhabitat on steeply sloped Texas hard shores was more likely related to the mechanical stress resulting from wave action reachng all shore positions.

RESUMEN Las preferencias por el microhibitat y 10s patrones diumos de desplazamiento de dos especies de gasteropodos tropicales (Nerita versicolor y Tectarius antonii) y dos de aguas templadas (Siphonaria pectinata y Nodilittorina riisei) heron comparadas en relacion con su posicion respecto a la costa, lo que permiti6 hacer una evaluation de la influencia de 10s factores fisicos limitantes (calor y desecacion) responsables de su comportamiento y sus patrones de distribution vertical. Se realizaron dos muestreos de doce dias cada uno (una en Mexico y 11no en Texas), donde 10s animales heron marcados para identificarlos individualmenle, e~icunlrhicloseque 10s movimientos y las actividades para alimentarse se iniciaron en periodos de baja desecacion y baja tension termica. Una mobilidad casi continua h e observada en 10s caracoles de Mexico que habitan en las porciones mas bajas y humedas de la costa. En contraste, 10s caracoles tropicales que habitan en 10s niveles mas altos se movian solo cuando la costa quedaba humedecida con la mar en calma, despu6s ciertos periodos de oleaje intenso. Mas a h , las observaciones diumas mostraron que las especies tropicales mostraban preferencia por por el movimiento durante las noches, cuando la desecacion y la tension termica eran reducidas. La estrechez de la zona de mareas y por lo tanto el menor gradiente de tension ambiental de las costas rocosas texanas permite a 10s animales tener movimiento continuo, independientemente de su posicion en la costa y de la hora del dia. La propension por una mayor actividad en condiciones de baja tension de desecacion tambi6n se apoya en un experiment0 en el que el sustrato h e humedecido (simulando inundation), lo que estimul6 la actividad en todas las especies con las que se probo la respuesta. Todas las especies mostraron preferencia por microhabitats protegidos (huecos y hendiduras), evitando las superficies expuestas, per0 el mecanismo que motiva este comportamiento es diferente entre las especies de cada localidad. Sobre las amplias costas rocosas de las localidades mexicanas, el uso de 10s microhabitats protegdos por las especies de las partes altas y bajas parece ser un patron de comportamiento adaptativo orientado a reducir la tension por la desecacion. En contraste, el uso del microhabitat protegido en las abruptas costas rocosas texanas parece estar relacionado con la tension mechnica provocada por la accion del oleaje que alcanza todas las posiciones de la costa.

LIST OF TABLES PAGE Table 1.

Site and shore level characterization showing averages between low and high-shore zones at Mexico sites, 12-25 May 2002.. .....

Table 2.

Relative tide heights (m) measured during two diurnal surveys at Mexico Sitc 1.. ............................................................

Table 3.

Percentage of distances from the initial mark (home mark) of low and high-shore species at Mexico sites, 12-25 May 2002.. ....

Table 4.

Differential use of microhabitats by low and high-shore species at Mexico sites, 12-25 May 2002.. .....................................

Table 5.

Differential use of microhabitats by low and high-shore species at Mexico sites, 12-20 May 2002 with respect to degree of substratum moisture.. ....................................................

Table 6.

Shell length and width measurements of low and high-shore snails pooled from Mexico sites, 12-25 May 2002.. .................

Table 7.

Statistical results of aggregation calculations for Nerita versicolor at Mexico sites, 12-25 May 2002.. ........................

Tab19 8.

Statistical results of aggregation calculations for Tectarius antonii at Mexico sites, 12-25 May 2002.. ............................

Table 9.

Site and shore level characterization showing averages between low and high-shore zones at Texas sites, 12-25 June 2002.. ........

Table 10.

Relative tide heights (m) measured during two diurnal surveys at Texas Site 3... .............................................................

Table 11.

Percentage of distances from the initial mark (home mark) of low and high-shore species at Texas sites, 12-25 June 2002.......

Table 12.

Differential use of microhabitats by low and high-shore species at Texas sites, 12-25 June 2002.. .......................................

Table 13.

Differential use of microhabitats by low and high-shore species at Texas sites, 12-25 June 2002 with respect to degree of substratum moisture.......................................................

Table 14.

Table 15.

Table 16.

Table 17.

Rayleigh's test of unifornlity or1 rnovemenl direction of low and high shore species at Texas Site 3 with respect to flooding and ebbing tides.. .............................................................

42

Shell length and width measurements of low and high-shore snails pooled from Texas sites, 12-25 June 2002.. ...................

42

Statistical results of aggregation calculations of Siphonaria pqctinntn at Texas sites, 12-25 June 2002 ..............................

43

Statistical results of aggregation calculations of Nodilittorina riisei at Texas sites, 12-25 June 2002 ..................................

44

PAGE Figure 1. Figure 2.

Comparison of zonation of rocky shore habitats in Texas and the Yucatan, Mexico.. ...................................................

6

Map showing the Mexico based study sites, Punta Yu Yum (Sitc 1) and Punta Xamach (Site 2), Quiiltana Roo, Mcxico.. ............

8

Figure 3.

Map showing the Texas based study sites, McGee Beach (Site 3) and University of Texas Marine Science Institute (UTMSI) breakwaters (Site 4), Texas, USA .................................................

Figure 4.

Example of how rugosity was used to measure relief of substrate on the rocky shore.. ......................................................

Figure 5.

Photograph on the left (a) represents the spray collection gauge and on the right (b) the data logger 1 stand assembly ...............

Figure 6.

Temperature and relative humidity data recorded electronically 12-25 May 2002 Quintana Roo, Mexico. .............................

Figure 7.

Mean movement rate of low and high-shore species recorded daily at Mexico sites, 12-25 May 2002. ................................

Figure 8.

Angular vectors representing the mean directions measured daily, 12-25 May 2002 Quintana Roo, Mexico.. ........................

Figure 9.

Proportions of daily microhabitat use by Nerita versicolor in the low-shore zone'at Mexico sites, 12-25 May 2002.. .................

Figure 10.

Proportions of daily microhabitat use by Tectarius antonii in the high-shore zone at Mexico sites, 12-25 May 2002.. ................

Figure 11.

Mean movement rate of low and high-shore species during two diurnal experiments at Mexico Site 1, 16-17 and 21-22 May

Figure 12.

Temperature and relative humidity data recorded electronically 12-25 June 2002 Texas, USA.. .........................................

36

Mean movement rate of low and high-shore species on a daily basis at Texas sites, 12-25 June 2002.. .................................

38

Figure 13.

ACKNOWLEDGEMENTS

I am deeply indebted to the members of my thesis committee, Dr. David W. Hicks, Dr. John W. Tunnell, Dr. Kim Withers, and Dr. David A. McKee, for their leadership and guidance in all my endeavors as a graduate student. I am grateful to Dr. Tunnell for introducing me to the tropical ecology of Mexico and to Dr. Hicks for his assistance throughout the duration of this project. I would also like to thank Suzanne Dilworth for her much needed assistance in the field and with preparation of this thesis report. Thmks also to Mr. Bart German and Ms. Melissa Castaficda for thcir assistance in the field and Dr. Harley Moody and Dr. Roy Lehman for providing photography. My deepest gratitude goes to my family for their continuing love and support, financially and emotionally. Thank you for encouraging me to accomplish higher goals and strive for success.

LOCOMOTOR BEHAVIOR AND HABITAT SELECrl'1ONIN INTERTIDAI, GASTROPODS FROM VARYING SHORE HEIGHTS INTRODUCTION

Rocky shore intertidal habitats are subject to varying intensities of physical stress, particularly with respect to temperature extremes and desiccation, which increase with increasing shore height (reviewed by Lcwis, 1964; Newell, 1979). This gradient or physical stress is partially responsible for the zonation exhibited by many rocky shore organisms because each vertical zone is set apart by a unique suite of selective pressures. Intertidal gastropods have accumulated a number of specialized adaptations in response to periodic emergence, many of which correlate with vertical shore position (reviewed by Underwood, 1979; McMahon, 1988; 1990). Initial studies of rocky shore ecology, dating back to the early 2othCentury, described patterns of vertical zonation and species distributions that form in horizontal bands on the shore (reviewed by Lewis, 1964; Newell, 1979; Underwood, 1979; McMphon, 1990). The widths and heights of these zones are related to shore slope and tidal range, respectively. Thus, patterns of vertical zonation vary geographically as well as locally over relatively short distances on the same shore. The classic zonation scheme characterized zones with respect to tidal amplitude, distinguishing a eulittoral zone as the shore area between mean high and low water (i.e., the intertidal zone). Shoreward and seaward of the eulittoral are two additional zones referred to as the eulittoral-fringe and sublittoral-fringe, respectively (Stephenson and Stephenson, 1949; Newell, 1979). Since, researchers have used and modified these classic approaches primarily by adopting sitespecific terminology for describing zonation patterns on geographically distant shores. For example, at some locations zones are defined by their dominate species (e.g., 1

balanoid zone), whereas zonation at other locations is defined by substrate color (e.g., yellow zone). Further, some authors (McMahon, 1988) characterize zones by their physical attributes (e.g., proportion of time spent emersed). Gastropods occupying different shore positions (above and below the high tide mark) are subjected to varying degrees of desiccation and thermal stress brought upon by the contrasting effects of solar radiation and tidal inundation (McMahon, 1990; Britton, 1995). As a result, species inhabiting the eulittoral-fiinge (individuals above the high tide mark) can be exposed to dry air for periods of several days or even months and have developed adaptations to compensate for desiccation and heat stress (McMahon, 1988; 1990). Such adaptations in response to these stressors may be morphological, physiological, or behavioral or some combination thereof. Morphological adaptations which have been suggested to mitigate effects of thermal and desiccation stress include shell pigmentation and/or ornamentation, individual size, and presence of opercula (Vermeij, 1973.). Such morphologies are I

generally assumed to ameliorate the effects of heat stress while eliminating the need for evaporative cooling and allowing for extended periods of emergence while conserving water (McMahon, 1990; Britton, 1995; Lang et al., 1998). Physiological adaptations to periodic emersion among gastropods that have received recent study include metabolic thermal regulation (McMahon, 2001), capacity for aerial gas exchange (McMahon and Russell-Hunter, 1977), and increased thermal (McMahon 1990) and desiccation (Britton, 1992) tolerance. Studies related to behavioral adaptations to reduce the effects of thermal and desiccation stress in tidally emersed gastropods include selection of favorable microhabitats (Kensler, 1967; Raffaelli and Hughes, 1978), spatial distribution (Chapman

and Underwood, 1996), individual posture (Britton, 1995), evaporative cooling (McMahon, 1 WX), prodliction of mucus holdfasts (Bingham, 1972), and restricting movement to periods of reduced stress (Peck01 and Guarnagia, 1989; Lang et al., 1998). It is generally assumed that the lower limits of species distributions on rocky shores are controlled by biological interactions. Indeed, many studies have attempted to establish a functional relationship between biological factors (spccies interactions) and distribution of rocky shore species (Underwood, 1972; 1975; 1977; 1981). For example, investigations concerned with effects of predation upon intertidal gastropods have shown that individuals occupying zones lower than their normal shore height are at an increased risk of mortality (McCormack, 1982; Behrens Yamada and Boulding, 1996). Likewise competition for food and space are also suggested as important factors controlling the lower limits of species' distributions (Underwood, 1980; Underwood and Jernakoff, 1984). Conversely, the upper limits of species distributions on rocky shores are generally assumed to be controlled by physical factors. Many studies have attempted to correlate I

species distributions with resistance to environmental stress, particularly with respect to temperature (McMahon, 1990; Britton, 1992) and desiccation stress (Britton, 1992), often with inconclusive results (Dobson-Moore and Britton, 2001). While many studies have utilized various physical and biological factors in an attempt to explain the distributions of rocky shore gastropods, relatively few have examined the influence of environmental factors on behavioral adaptations. In most cases, behavioral adaptations a& fashioned by the complex interrelationship of physical and biological factors and as a result, it is difficult to discern which and to what extent specific environmental factors effect mollusc behavior (Minton and Gochfeld, 2001).

One method of analysis is through the use of experimental transplantation out of the subject's home range zorlc. For csanlplc, snails were obseived to return Lu llicir original shore height after being transplanted into areas higher than normally occupied, suggesting that such behaviors are a response to the physical stresses encountered at high shore positions (Peck01 and Guarnagia 1989; Chapman, 1999). Behavioral responses often appear to be adaptations to reduce thc cffccts of thermal and desiccation stress. Activity of the high shore gastropod, Cenchritis

muricatus, commenced at night during periods of elevated humidity and decreased temperature (Emson et al., 2002). Similarly, Voltolina and Sacchi (1990) observed an increase in mobility during nocturnal periods among two Littorina species. Gastropods often seek out shelter (e.g. crevices, cracks) within their intertidal areas to reduce the effects of thermal and desiccation stress (Kensler, 1967; Addy and Johnson, 2001). Unfortunately, single-species behavioral response studies limited to one shore location makes interpretation of the influence of environmental stress on behavioral f

adaptation difficult to discern. Further, comparisons of behavioral response data by different authors for species from geographically distant shores are also problematic due to varying environmental conditions and sampling methodologies. Only by comparing multiple species on geographically distant shores can the adaptive significance of different behavioral responses in relation to shore position be assessed in a biologically meaningful manner. Very few studies have been conducted that define actual movement rates for species and no investigations have compared these rates with respect to position on the shore. In this study, locomotor behavior and habitat selection are utilized to examine the role of physical stressors in structuring behavioral adaptations in gastropods

from eulittoral and eulittoral-fringe areas. Specifically, to tcst thc hypothesis that individuals are most likely to move when desiccation and heat stress me reduccd. STUDY SITES Comparisons of locomotor behavior and habitat selection in eulittoral and eulittoral-fringe species (referred to hereafter as low and high-shore species, respectively) were examined on natural limestone hard shores of the eastern Yucatan Peninsula, Caribbean Sea and the man-made hard shores of the Texas, Gulf of Mexico, coast.

MEXTCO Most Caribbean rocky shores are remnants of ancient coral reef communities consolidated over time by intense geologic pressure. The resulting limestone rock is continuously weathered by wave action creating pits, cracks, and tide pools that provides a range of habitats for many littoral organisms (Kaplan, 1988). The zonation scheme used to describe tropical hard-shore communities is based upon the distinct gradations in rock color from low to high shore positions (Stephenson and Stephenson, 1950; Kaplan, t

1988; Tunnel1 et al., 1993; Koltermann, 2000). The white zone is the uppermost region and in many localities is occupied by upland andlor maritime vegetation (Fig. 1). The grey and black zones of tropical hard shores correspond to the eulittoral-fringe of more traditional classification schemes. The grey zone is the highest and often widest area and because of its height on shore, remains dry for most of the year, except when inundated

by tropical storm surge. The black zone occupies the lower portion of the eulittoralfringe and remains mostly dry due to a lack of wave spray and daily tidal inundation. However, during spring high tides and storm surge, the black zone can remain wet for extended periods. Below the eulittoral-fringe (grey and black zones) is the yellow zone.

Two rocky points, Punta Yu Yum and Punta Xamach, hereafter referred to as study Sites 1 and 2, respectively, were sampled in the Sian Ka'an Biosphere Reserve on the eastern side of the Yucatan Peninsula (Fig. 2). Punta Yu Yum (19'55' N and 87'28' W) is characterized as a large, gently-sloped rocky shore that widens to form two large rocky outcroppings. All five color zones are represented and visibly distinct. Punta Xamach (19'55' N and 87'26' W), situated to the south of Site 1, is a small, high-energy rocky shore with numerous tide pools. A sandy beach that transcends directly into upland vegetation replaces the grey zone at Site 2. Both rocky shores are bordered on the north and south side by large lagoon-type bays with sandy bottoms and seagrasses. Rocky shores in this region of Mexico are platform-like and as a result exhibit vertical relief that is very gradual, slowly increasing as shore height increases. Climate in this area is characterized as subtropical to tropical with air temperatures ranging 25-28 'C and an annual rainfall of 143 cm y-' (Slemko, 20 April 2002). The Caribbean Sea east of the peninsula is dominated by easterly winds averaging 11 k& and north to northwesterly flowing currents at an average of 2 kph. Sea surface temperatures range 26-30 "C and tide cycles follow a semidiurnal pattern with a tidal range of 0.5-1 m. (Samuels, 23 April 2002). At Sites 1 and 2, Nerita versicolor Gmelin, 1791 and Tectarius antonii Philippi, 1846 were used as representatives of low and high-shore habitats, respectively. These species were the dominant gastropods in each of their respective zones. Nerita versicolor (Prosobranchia: Neritidae) are up to 2.5 cm in length, coarsely sculptured, and white with red and black spiral cords. The parietal area is white to yellowish with 4 strong teeth and a brownish gray operculum. It has been reported that snails in the genus Nerita occupy

high-shore positions (Garrity, 1984), but at the sites reported herein and shores in Jamaica (McMahon, 2001) and the Bahamas (Pec,koland Guarnagia, 1989), Nerita spp. occupy lower shore positions within areas effected by daily tidal inundation. Tectarius antonii (Prosobranchia: Littorinidae) are up to 2.5 cm in length, squarish at the base, with

a multispiral opcrculum. They are com~loillyfound high on the rocks in Southeast Florida and the West Indies ( W a d e and Abbott, 1961). TEXAS

Man-made rocky shores, jetties, groins, breakwaters, and bulkheads are the primary hard shore habitats on the Texas Gulf of Mexico coast. Before jetties were constructed around inlets joining the Gulf of Mexico to bays, hard shore communities were essentially absent along the Texas coast (Britton and Morton, 1989). Rocky shores in Texas follow the classic zonation pattern described by Stephenson and Stephenson (1949) with two distinct zones. The lower balanoid zone, corresponding to the eulittoral zone, is dominated by barnacles and limpets where as the higher littorinid zone (= eulittbral-fringe) is dominated by gastropods (Fig. 1; Britton and Morton, 1989; Tait and Dipper, 1998). Two study sites were selected on the lower Texas coast, one in Corpus Christi Bay and the other on the Aransas Pass Ship Channel (Fig. 3). Site 3 was located on a breakwater protecting the Corpus Christi Marina near McGee Beach (27'47.2' N and 97'23.6' W). The breakwater extends 1.3 km and is composed of stacked limestone rock capped by a cement walkway. The eastern side of the breakwater receives daily wave action fkom the predominating southeasterly wind. Site 4 (27'49.8' N and 97'2.0' W) was located along a small jetty extending 30 m into the Aransas Pass Ship Channel at

Science Institute (UTMSI). The low-shore zones at both sites were composed of large, flat rocks with numerous small pits whereas rocks in 11igl1zones were smoother, containing ridges carved by constant wave and wind action. Texas man-made shores are very steep compared to the natural platform-like hard shores of the eastern Yucatan Peninsula. The steepness and unconsolidated construction of Texas hard shores, combined wit11 a relatively small tidal rangc, results in zones that are poorly defined compared to those sampled in Mexico. Climate of the area ranges seasonally from semi-arid, temperate to sub-humid and tropical, with air temperatures ranging 13-28 "C and precipitation averaging 90 cm y-' (NDBC, 23 April 2002; TWC, 20 April 2002). Southeasterly winds averaging 22 kph prevail throughout most of the year. The tidal cycle is mixed depending on time of year with a 0-1 m tidal range W B C , 23 April 2002). Sea surface temperatures range 14-30 "C and prevailing southerly currents average 1 kph (NDBC, 23 April 2002; GERG, 23 April 2002). At Sites 3 and 4, Siphonaria pectinata LinnC, 1758 and Nodilittorina riisei Marrch, 1876 were used as representatives of the low and high-shore habitats, respectively. Both of these species were the dominant gastropods encountered at study sites in each of their respective zones. Siphonaria pectinata (Pulmonata: Siphonariidae) are up to 2.5 cm in length, conic shaped, and whtish with numerous brown bifurcating lines. This species is a common inhabitant of the intertidal rocks and jetties throughout shores of eastern North ~merica;Gulf of Mexico, Mexico, and the West Indies.

Siphonaria pectinata resembles a true limpet, but utilizes aerial respiration (Andrews, 1981). Nodilittorina riisei (Prosobranchia: Littorinidae) range 1-2.5 cm in length and are

grey in color with oblique dark brown zigzag lines. They are common inhabitants of intertidal rocks and jetties of Bermuda, South Florida, Texas, Costa Rica, ard Bruil, often with large colonies occupying crevices (Andrews, 1981).

METHODS AND MATERIALS Site and Shore Level Characterization. Rugosity and zone width measurements were used to characterize low and high-shore zones at Mexico study sites. However, the uniform shape of the rocks and steepness of man-made hard shores precluded the utility of rugosity and zone width measurements at either Texas site. Rugosity is defined as the length of surface contour divided by the total linear length measured (Rooker et al., 1997). Nine rugosity measurements in the low and high-shore zones (n = 18) were taken at Sites 1 and 2. A meter stick with an attached piece of twine was used to take the measurements. The meter stick was haphazardly placed on the substrate and a length of twine was used to measure the surface contour of the rock immediately below and between the ends of the stick (Fig. 4). Nine measurements of zone width (m) were collebted at Sites 1 and 2 in the low and high-shore zones (n = 18). Low zone width was measured as the linear distance from the uppermost boundary of the pink zone to the uppermost boundary of the yellow zone. High zone width was measured as the distance from the uppermost boundary of the yellow zone to the uppermost boundary of the black zone. The temperature of rock surfaces was measured in each zone to test the assumption that substratum temperature increased with shore height. Each day, the bulb of a mercury thermometer was placed against the rock surface at three locations in each zone for 5 minutes and the temperatures were recorded.

were kept sealed in plastic bags except for the duration of the hour-long survey to eliilliilate the &l-tlier.lvss or gain of water between weight measurements. The amount of water lost or gained was calculated by subtracting the combined weight of the sponge and 75 ml of saltwater (76.74 g at 35%0)from the final weight. Seawater spray resulting from crashing waves was collected using rain gauge-type devices constructed from PVC pipe and secured into cement bases that were haphazardly placed above and below the mean high water line (Fig. 5a). At each site, the devices were deployed for a period of one hour on three different days to capture the day-to-day variation in wave strength over the course of the 12-day experiment (3 devices x 3 different days x 2 zones = 18 total measurements at each site). The volume of spray collected (ml) above and below the mean high water line was used as an additional indicator to define varying degrees of moisture conditions occurring among low and high-shore zones. A one-way analysis of variance (ANOVA) was used to compare low and high-shore zones with respect to rugosity, zone width, rock surface temperature, and desicbation potential (sponge-survey and seawater spray collection). Meteorological and Oceanographic Data Collection. A data logger (model

HOBO H8 from Onset Computer Corporation) deployed at Rancho Pedro Paila, the base camp for Mexico field operations, 2 km north of Site 1 (20'02.6' N and 87'28.8' W, Fig. 2), was used to characterize ambient temperature and relative humidity. For the Texas portion of the study, a data logger was deployed on northern Padre Island, 15 km southeast of Site 3 (27'35.7' N and 97'14.0' W, Fig. 3). Because the loggers are not waterproof, a case made of clear PVC and Plexiglass was used to protect them. Holes were drilled in the side to allow fresh air exposure to the humidity sensor. A stand was

staff. l'irle data Ibr the Texas portion of the study was taken from the Conrad Bluchcr

Institute tidal station 008 at the Texas State Aquarium (27'48.9' N and 97'23.9' W). Each hourly sample time used in statistical analyses represented a mean of all tide readings for the hour during which data was collected for the diurnal sampling intervals. Pearson's correlation analysis was used to examine relationships between ambient temperature, relative humidity, and relative tide height with movement behaviors during the diurnal surveys. This analysis was attempted solely for diurnal experiments as the shorter time intervals (4 h) allowed more precise correlations between activity and environmental parameters. Movement and Microhabitat Selection. Field observations were made fiom 12-

25 May 2002 in Mexico and 12-25 June 2002 in Texas. Three sub-sites representing lower shore positions and three representing higher shore positions were sampled at each of the four study sites (2 in Mexico and 2 in Texas). Within each sub-site, five adult individuals of each species were haphazardly selected and marked with a uniquely f

colored enamel paint to ensure individual distinction (methodology after Gochfeld and Minton, 2001). Initial location of each snail was marked by drilling a hole approximately 5 cm south of each individual and installing a colored nail corresponding to the color of the painted snail (home mark). Three initial records were documented for each marked specimen: (1) distance to the water and (2) compass heading to the water (degrees) and (3) a description of the microhabitat in which it was found. All distances were estimated using a fiberglass tape measure. Microhabitat was classified based upon surface topography and substratum moisture. Surface topography was classified as exposed surface (flat rock), crevice (depression in the rock surface large enough for two or more

individuals to occupy), or pit (depression in rock large enough for a single individual to occupy). Suhstrah~mmoisture was classified as wet (wet but no standing water), at the waterline, underwater, or dry. Each subsequent day, as many individual snails as possible were relocated. A second nail of the same color was used to mark the new location of each individual (day mark). Distance and directioil traveled fiom the previous day's mark (colored nail) ns well as the microhabitat occupied were recorded daily for each individual located. Distance and direction fiom the home mark was also recorded to determine a 'home area' measurement. For those individuals that traveled, the day mark was moved to the snail's new location. Daily movement rates were calculated by dividing the distance traveled by the time elapsed since the previous day's record. Average daily movement rates (cm - hr -') were estimated fiom a linear mixed model that accounted for serial correlation of errors due to the repeated sampling of individuals in time. The model employed a reference cell /

parameterization and indicator variables to analyze the effects of shore position (low vs. high) on rate of movement. The model was estimated using maximum likelihood, via the MIXED procedure in SAS@(Cary, North Carolina). Directional preference was examined using a circular analysis (Oriana for WindowsTM, Wales, United Kingdom). All headings were standardized by adjusting the heading so that the direction to the waterline was zero degrees. For example, if circular analysis indicated the 95% confidence interval of the mean heading was 102 to 2004 it was concluded that there was a movement away (180") from shore. Directional measurements fiom each site within

the same area (Mexico and Texas) were pooled and analyzed with respect to shore position.

The observed vs. expected proportions of microhabitat use wcrc co~nparcdusing a method based on Bonferroni's inequality after Haney and Solow (1992). Expected proportions of availablc microhabitat were estiinated using a 25 x 25 cm PVC quadrat divided into 25 equal 5 x 5 cm squares by a plastic cord. Sixteen points were created where each of the plastic cords intersected one another. The quadrat was haphazardly placed on the substrate and the microhabitat and degree of substratum moisture (see above) present beneath each of the sixteen points was recorded. A total of 192 points (12 quadrats) were sampled in each zone at each site. The distances between daily locations and home marks were analyzed for the presence of homing behavior by calculating the percentages of home area measurements falling into each of four categories (< 1, 1-3,3-5, and > 5 m). Simple comparisons of these percentages were made between low and high-shore zones at Mexico and Texas sites.'

Diurnal Survey. Two 24-hr diurnal surveys of locomotor behavior were conducted to test the hypothesis that movement increases when desiccation and heat stress are reduced. One site within each area was chosen (Site 1, Mexico and Site 3, Texas) to conduct .diurnal,surveys. Surveys were conducted from 16-17 and 21-22 May in Mexico and 20-21 and 25-26 June in Texas. The movements of 15 low-shore and 15 high-shore individuals were monitored at 4 h intervals for a 24 h period using the same methodology described above for daily locomotor rates. The SAS@MIXED procedure outlined in the previous section was utilized to compare movement rates among low and

high-shore positions and time of day (day vs. night). Thc cffcct of water level and shore circular g analysis position (low or high-shorc) on movcment direction was analyzed u s i ~ ~ (Oriana for WindowsTM, Wales, United Kingdom). Pearson's correlation analyses were performed to examine potential associations between locomotor rates and several environmental parameters (rock temperature, tide level, ambient temperature, and relative humidity). Size, Density, and Aggregation. The propensity for aggregation in low and highshore species was estimated to test the hypothesis that aggregation is a behavioral adaptation to reduce desiccation and thermal stress. Spatial patterns of distribution were analyzed using Goodall's (1974) variation of Pielou's (1964) two-phase patchworks method. The modification employs presencelabsence data in random pairs of quadrats (without replacement) over increasing quadrat distances to determine patch size. In this study, a 25 x 25 cm PVC quadrat, separated into twenty-five equal 5 x 5 cm cells, was haphazardly placed on the substrate and the presence or absence of each snail species was I

noted in each of the twenty-five squares. Random pairing of cells was analyzed over distances of 5-10, 10-15, 15-20, and 20-25 cm. The observed proportions of specimens occupying cell pairs significantly exceeding expected proportions were taken as evidence of a continuous patch. Density was estimated by counting the number of individuals of each species found within 25 x 25 cm quadrats. Twelve replicate quadrats were surveyed in the low and high-shore zones at each study site. The size of individuals occupying low and high-shore zones was compared by measuring the 30 specimens marked for use in the daily movement experiments. Prior to the onset of the daily movement survey, the shell length and width of each snail was

measured using a dial caliper (rnm). Shell sizes of species occupying low and high-shore zones were compxud using a11 ANOVA, Wetling Experiment. 'l'wo wetting experiments were conducted, one in Mexico and one in Texas, to estimate the probability of snails moving under different environmental conditions (dry-substratum, freshwater moistened substratum, and seawater moistened substratum). Forty-five individuals of each species were collccted from low and high-shore zones at Punta Yu Yum (Site 1). Thirty minutes prior to testing, three groups of five snails (15 total) were placed on an outdoor testing surface (cement slab). Ten minutes before testing, each group of snails was placed in a 50 x 50 cm treatment box that was drawn on the testing surface. Five minutes prior to testing, the snails were moved to the middle of their respective boxes and left untouched for the duration of the test. One of three treatments was randomly applied to each group. Treatments tested were dry-substratum (control), 250 ml of fiesh water, and 250 ml of saltwater. Freshwater and saltwater treatments were designed to represent 1 mm of rainfill and saltwater inundation andor spray, respectively, and were applied to the entire treatment box using a water bottle. A perforated plastic cap was used to create a sprinkling effect and evenly cover the entire area. Specimens were observed for periods of 10-minutes during which the total number of individuals exhibiting movement was recorded. The experiment was replicated three times for each species (15 snails x 3 replicates = 45 individuals of each species). The same experiment was performed in Texas, however, the low-shore species was not tested because of the difficulty in removing Siphonaria pectinata from the substrate without harm. The CATMOD procedure was used to analyze data via categorical modeling (SAS@,Cary, North

Carolina). To be more specific, a weighted-least-squares analysis was used to model the probability of moving as a linear response function.

RESULTS MEXICO Site and Shore Level Characterization. Rocky shores in Mexico consisted of a

con-lplex combinatiorlo1tide pools and miniature crevasses as a result of time-scour from constant wave action. The habitat available to low and high-shore species was similar with respect to morphology (measured by rugosity) and area (zone width; Table 1). The substratum surface in both zones was rough, weathered rock with many small pits and sharp edges. In contrast, low and high-shore zones differed significantly with respect to desiccation and heat stress. Rock temperature increased significantly with increasing shore height (Table 1). Weather conditions permitted collection of only 2 days of wave splash and spray samples at Site 1 and one day at Site 2. However, wave splash and spray was not collected in high-shore areas at either site demonstrating dry conditions that :revail in high-shore habitats (Table 1). Desiccation potential, measured as the amount of seawater lost by evaporation (or gained fiom sea spray) using the spongesurvey method, was markedly different with respect to shore height. On the average, sponges in low-shore zone absorbed an additional 0.27 g of water, whereas sponges in the high-shore zone lost 6.97 g of water (Table 1). Rainfall was observed on 19,20,23, and 25 May 2002 at both Mexico sites. Temperature averaged > 30 "C throughout the study period with the exception of three days (20,22, and 25 May 2002). When rainfall was recorded, temperature declined and

Table 1. Site and shore level characterization showing averages between low and highshore zones at Mexico sites, 12-25 May 2002.

R~lgosity(m) Zone Width (m) Rock Temperature (OC) Splash and Spray (ml) Sponge Surveys (g)

Low-Shore

Kgh-shore

F-Value

df

P

1.25 3.97 26.77 37.44 0.27

1.19 4.97 27.82 0.00 -6.97

1.03 2.85 7.93 1.50 5.146

1,34 1,34 1, 327 1,16 1, 16

0.318 0.105 0.005 0.238 0.038

relative humidity increased to 2 80% (Fig. 6). Tide data measured during diurnal surveys was a relative value and followed a semi-diurnal tidal pattern (Table 2). DaiCy Movement and Direction. All 60 snails marked at Sites 1 and 2 (15 low

and high-shore specimens at each of the two sites) were relocated at least once over the course of the 12-day observation period. A total of 593 (86%) of the 690 possible observations (30 individuals x 11 days at Site 1 + 30 individuals x 12 days at Site 2) were recorded over both sites. Similar numbers of low and high-shore individuals were relocated (e.g. 84% of N. versicolor and 89% of T.antonii, respectively). Average daily movement rates were similar within the low and high-shore habitats at both sites (t = 0.61; df = 559, P = 0.544), thus these data were pooled to analyze the effects of shore position. N. versicolor and T.antonii moved during 94 and 53% of the daily intervals examined, respectively. The mean rate of the low-shore species (3.12 cm . h-') was significantly higher than the rate of high-shore species (0.27 cm . h-'; t = 10.79, df = 58, P < 0.0001). Interestingly, a storm developed 6 days into the observation period (19 May 2002) beginning with 2 days of rainfall followed by 4 days of increased sea state causing higher than normal tide conditions. This provided a unique opportunity to examine the effects

12

13

14

15

16

17

18

19

20

21 22

23

24 25

MAY 2002 DATE

Figure 6. Temperature (OC;black circles) and relative humidity (%; grey triangles) data recorded electronically (HOBO H8 Data Logger) 12-25 May 2002 Quintana Roo, Mexico (20'02.6' N and 87'28.8' W). Arrows indicate days when rainfall was recorded. Table 2. Relative tide heights (m) measured during two diurnal surveys at Mexico Site 1.

Time

16-17 Mav 2002

21-22 Mav 2002

of increased wave action on the behavior of low-shore species and an increase in substratum moisture on behavior of the high-shore species. Using the same analysis, but introducing a new variable that divided the daily observations into normal and high water conditions, the effect of shore position on the rate of movement was re-evaluated with respect to increased substratum moisture. The rise in water level effected the rates of low and high-shore species differently (Fig. 7). Under heavy sea conditions, rate of

2

3

4

5

6

7

8

9

1

0

1

1

1

2

DAY Figure 7. Mean movement rate of low (black bars) and high-shore species (grey bars) recorded daily at Mexico sites, 12-25 May 2002. Cross-hatched area represents days when high water conditions were observed. Error bars represent standard error of the mean.

movement in the low-shore species was similar to that recorded during normal sea state averaging 2.79 cm . h-' (t = -0.77, df = 502, P = 0.444). In contrast, snails in the highshore zone exhibited an increase in mobility to 1.63 cm . h-'. Although the rates of lowshore snails remained higher than that of the high-shore species (t = 2.83, df = 502, P = 0.005), the mobility of T. antonii increased 6 times over that observed during normal sea conditions (t = 6.83, df = 502, P < 0.0001). Daily directional movement was analyzed as a two-factor experiment with respect to shore position and substratum moisture. Daily calculations having fewer than 10 observations were removed fiom the analysis following Underwood and Chapman (1985). Some evidence of directional movement to a position higher on the shore was observed by the low-shore species as water level on the shore increased, but broad patterns of movement were not observed (Fig. 8). Little movement, and to no particular

To Water

Sites 1 and 2 - Nerita versicolor

To Water

Sites 1 and 2 - Tectarius antonii

Figure 8. Angular vectors representing the mean directions measured daily (1- 12), 12-25 May 2002 Quintana Roo, Mexico. All compass headings were standardized and the direction to the water was refigured as 0" north. Days 1-8 are marked with solid black points for normal tidal conditions and days 9-12 are marked with white points denoting days the shore was inundated by high tidal conditions. The length of the line represents the sqength of the correlation and the directions marked with an asterisk (*) were significantly different using Rayleigh's test for uniformity. direction, was observed by high-shore species prior to elevated substratum moisture. As sea state increased, some directional movements were evident, but followed no pattern in relation to increasing water height. Percentages of home area measurements falling into each of four categories (< 1, 1-3, 3-5, or > 5 m) indicated that only 29% of the low-shore specimens remained within 1 m of their initial mark, 55% ranged from 1-3 m from their home mark, and 16% ventured further than 3 m (Table 3). A few low-shore snails (8%) were observed to travel more than 5 m from their home mark with a maximum distance of 17 m recorded. In contrast,

Table 3. Percentage of distances from the initial mark (home mark) of low and highshore species at Mexico sites, 12-25 May 2002. Zone

0-1 m

1-3 m

3-5 m

51 m

Low-Shore High-Shore

28.5 83.3

55.2 16.7

8.0

8.3 -

-

83% of high-shore species were found within 1 m of their home mark and the remaining 17% never moved farther than 3 m.

Microhabitat Selection. Availability versus the use of three major microhabitat categories (exposed rock, crevice, and pit) was compared among low and high-shore habitats using quadrat data pooled over both sites. Exposed rock surface comprised the majority of available microhabitat in the low and high-shore zones (54 and 66%, respectively), followed by crevice (43 and 30%) and pit (4% at both sites; Table 4). Prior to the increase in sea state, low-shore individuals showed a preference for pit (26%) and avoided exposed surface habitat (20%; Fig. 9). After water height on the shore increased, I

low-shore snails did not exhibit preference or avoidance for any particular habitat, the majority of individuals used exposed surface (44%) and crevice (43%), with the remaining 12 % using pit. The increase in substratum moisture had little effect on the use of microhabitat by high-shore species, individuals avoided exposed surface (27 and 29%) and preferred crevice habitat (63 and 56%) during times of normal and increased tidal height, respectively (Fig. 10). The preference individuals had for a particular level of substratum moisture (wet, waterline, underwater, and dry) was considered only for those observations taken before the onset of increased water level. Wet substrata comprised 95% of available

Table 4. Differential use of microhabitats by low and high-shore species at Mexico sites, 12-25 May 2002. --

.-

Zone Low-Shore (normal)

Low-Shore (high water)

High-Shore (normal)

High-Shore (high water)

Microhabitat

Ex~ected

Observed

'

95% Confidence Interval 011 Observed Pro~ortions

Surface Crevice Pit

0.537 (206) 0.425 (163) 0.039 (15)

0.199 0.539 0.262 A

(38) (103) (50)

0.025 0.322 0.070

5p 5 5P 5 5p 5

0.373 0.757 0.454

Surface Crevice

0.537 (206) 0.425 (163) 0.039 (15)

0.443 0.433 0.124

(43) (42) (12)

0.227 0.217 -0.020

5p 5 5p 5 5p 5

0.660 0.649 0.267

0.662 (254) 0.297 (114) 0.042 (16)

0.269 0.626 A 0.105

(59) (137) (23)

0.076 0.414 -0.029

5P 5 5p 5 5 p -