Seasonal variations of the Spionida - Wiley Online Library

Marine Ecology. ISSN 0173-9565

ORIGINAL ARTICLE

Seasonal variations of the Spionida (Palpata: Canalipalpata) in the sublittoral zone of the Gulf of California Pablo Hernańdez-Alcańtara & Vivianne Solı´s-Weiss Laboratorio de Ecologıá y Biodiversidad de Invertebrados Marinos, Instituto de Ciencias del Mar y Limnologıá, Universidad Nacional Autońoma de Me´xico, Me´xico D.F., Me´xico

Keywords Gulf of California; distribution; Polychaeta; seasonal variations; Spionida. Correspondence Dr Vivianne Solı´s-Weiss, Laboratorio de Ecologıá y Biodiversidad de Invertebrados Marinos, ICMyL, UNAM, Me´xico D.F. 04510, Me´xico. E-mail: [email protected] Accepted: 22 September 2005 doi:10.1111/j.1439-0485.2005.00061.x

Abstract The aim of this study was to analyze the seasonal and spatial variations of the Spionida, one of the most abundant and diverse polychaete groups in the Gulf of California. A total of 6527 organisms from 39 stations taken during the winter-spring and summer periods were identified. The number of species was similar in winter-spring and summer (29–30), but abundance was significantly higher during the winter-spring season (4791 versus 1736 in summer). The overall highest density values were recorded in the central-eastern region (33.45–164.66 org./0.1 m2) where some of the areas with the highest species richness were found (12–16 species). A similar pattern was observed during summer, but with lower values of density. Paraprionospio pinnata, the key species, contributed with 16.24% to the total average dissimilarity between seasons and defined most assemblages of species. It was mainly distributed at stations deeper than 64 m in the eastern shores. The density and composition of species change spatially as a response to changes in depth and sand percent; seasonally, the temperature influenced the variation in the faunal distribution.

Problem The Gulf of California is ecologically one of the most complex marine systems in the tropical eastern Pacific; a large variety of habitats occur there as a result of the confluence of water masses of different origins (sub-arctic, subtropical and tropical) combined with its physiographic and topographic heterogeneity. As a consequence of the proven direct relationship between the ecologic, oceanic and coastal processes, with local high primary productivity (2–3 times higher than that found in the Atlantic and Pacific Oceans at similar latitudes), one of the most abundant and diversified faunas in the tropical eastern Pacific are found in this Gulf (Hendrickx 1992). The fauna of the Gulf of California is mainly composed of tropical eurythermic species (Steinbeck & Rickets 1941;

Rioja 1962; Keen 1971; Briggs 1974; Brusca 1980; Hendrickx 1992), with high affinities with the fauna of the Tropical Eastern Pacific region. The characterization of the faunal distribution models operating at different scales (both spatially and seasonally) is important to understand their interactions in the composition and structure of the faunal assemblages (Broitman et al. 2001). In the Gulf of California, the study of the polychaetous annelids’ geographical variations at small spatial scales has increased our knowledge of some local ecological dynamics (Enrı´quez-Ocanã 1990; Padilla-Galicia & Solı´s-Weiss 1992; Bastida-Zavala 1993; Meńdez-Ubach 1997). However, only a few studies have investigated the distribution of these invertebrates at a larger scale (Brusca 1980; Hernańdez-Alcańtara 2002). That is why the aim of this study is to analyze the variations in the composition and abundance of the Spionida at a regional scale, in the

Marine Ecology 26 (2005) 273–285 ª 2005 The Authors. Journal compilation ª 2005 Blackwell Publishing Ltd

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Hernańdez-Alcańtara & Solı´s-Weiss

Seasonal variations of the Spionida

two climatic cycles that characterize the Gulf of California: winter-spring and summer-autumn. The polychaetes are one of the dominant macrobenthic groups in most marine habitats (Mackie & Oliver 1996; Hutchings 1998), so that their distribution patterns often reflect those of the benthic fauna as a whole (Mackie et al. 1997; Glasby & Read 1998). Among them, the Spionidae is one of the best represented families of polychaetes, both in abundance and number of species in the Gulf of California and in general in the Mexican Pacific (Hernańdez-Alcańtara et al. 1994; HernańdezAlcańtara 2002). The Spionidae and several related families are grouped in a reasonably well defined group named Spionida (Pettibone 1982; Blake 1996; Blake & Arnofsky 1999). This group includes the families Spionidae, Apistobranchidae, Longosomatidae, Poecilo-

chaetidae, Trochochaetidae and Uncispionidae. As a result of recent cladistics studies, the families Chaetopteridae and Magelonidae have been included too (Rouse & Fauchald 1997).

Material and Methods Study site

The Gulf of California, a basin of 225,000 km2 (Lindsay 1983), located at the northwestern end of Mexico between 2030¢–3150¢ N and 10515¢–11435¢ W is considered an interior sea, only connected to the Pacific Ocean at its southern end (Fig. 1). Its climatic and hydrographic characteristics derive from its geographic position (Roden & Emilsson 1979).

Fig. 1. Study area showing the collection sites.

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Sampling methods

The samples were collected on board the R/V ‘El Puma’ in 39 stations on the sublittoral zone of the Gulf. Two or three samples were taken at each station during winterspring (10–23 March 1985) and summer (28 August to 9 September 1985) with a Smith-McIntyre grab (0.1 m2) and sieved through 1.0 and 0.5 mm screens. The stations’ nomenclature is comprised of: (a) their location in the continental (C) or peninsular (P) side of the Gulf, (b) their position on the continental shelf [inner (1), middle (2) or outer (3)], and (c) the original number of the station. Following fixation in formaldehyde (4%) the organisms were preserved in ethanol 70% (Fauchald 1977) and incorporated in the Collection of Polychaetes of the Instituto de Ciencias del Mar y Limnologıá (CPICML-UNAM, DFE.IN.061.0598). At each station, depth, temperature and salinity were measured with a Niels Brown CTD; dissolved oxygen was determined by the Winkler method (Strickland & Parsons 1977), and the organic matter by the Walkley & Black (1934) acid digestion method. The sand percent was used as the granulometric parameter, and processed following the method for wet sieving (Folk 1980). The data are provided in Table 1. Data analyses

Distribution of the density and number of species per station was plotted on histograms. A one-way anova was performed using the density and species richness to explore the differences between winter-spring and summer periods. All data were tested for homocedasticity using Cochran’s test. Significance level was set at a ¼ 0.05. These analyses were performed using the statistica v6 software. The ranked matrix of similarities among samples for each sampling period was constructed using the Bray– Curtis index. They were used to determine the species assemblages with the upgma sorting strategy using the software Plymouth Routines in Multi-Variate Ecological Research (primer v5) (Clarke & Gorley 2001). Species contributing to dissimilarity between faunal groups were determined by the similarity percentage procedure (SIMPER routine) of the primer software (Clarke & Warwick 2001). The influence of the environmental factors on the distribution of the Spionida was assessed with the canonical correspondence analysis (CCA). The forward selection procedure was employed to find the variables explaining the most variance in the species data using the software Canonical Community Ordination (canoco) v4.5. Plots were made by the drawing program canodraw v4.0 (ter


Braak & Smilauer 1998). The square root of the species density was used to reduce the influence of the highly abundant species. The environmental variables were standardized to obtain comparable scales (Clarke & Warwick 2001). Results A total of 6527 specimens were identified: 36 species were found to belong to Spionida: Spionidae (27 species), Longosomatidae (1 species), Magelonidae (5 species), Poecilochaetidae (1 species) and Chaetopteridae (2 species) (Table 2). Species richness was similar in winterspring and summer (29 and 30), but the number of individuals was distinctly higher in winter-spring (4791 specimens), while in summer sampling, only 1736 organisms were collected. The heterogeneous spatial distribution of the polychaetes is evident in the Gulf, since nearby stations showed large differences in density and species richness (Figs 2 and 3). The variations in geographical distribution were more pronounced in winter-spring (1–224.60 ind./0.1 m2, x ¼ 26.35 ind./0.1 m2; 1–16 species, x ¼ 5.18 species) (Fig. 2) than in summer (1–61.50 ind./0.1 m2, x ¼ 8.34 ind./0.1 m2; 1–10 species, x ¼ 4.36 species) (Fig. 3). Spatially, the organisms were more abundant in front of the eastern shores of the Gulf (between Guaymas and Topolobampo) and the stations with higher densities [C261 (224.6 ind./0.1 m2), C215 (164.67 ind./0.1 m2), C314 (97.33 ind./0.1 m2), C204 (65.00 ind./0.1 m2) and C238 (30.14 ind./0.1 m2)] were located in zones deeper than 64 m. The highest densities could not be directly related to high values of species richness, except at station C261 (11 species). The highest species richness values were recorded at stations C152 (16 species), C116 (12 species), C251 (12 species), C261 (11 species) and C137 (8 species). These stations are located on the continental side of the Gulf, but in contrast with those of the highest densities, they are shallow (< 64 m) (Fig. 2). During the summer, the highest densities were always found in the southeastern shores of the Gulf in stations deeper than 64 m (Fig. 3): C305 (61.50 ind/0.1 m2), C350 (53.20 ind./0.1 m2), C314 (28.55 ind./0.1 m2) and C260 (26.63 ind./0.1 m2). The highest densities were not associated with high values of species richness, since the latter are located in the inner and middle sublittoral zones [C204 (10 species), C152 (9 species), C137 (9 species)]. Variations in density and number of species between the winter-spring (WS) and summer (S) samplings were analyzed with one-way anova. Differences in mean densities between interannual periods were significant (P ¼


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Table 1. Environmental variables of the stations sampled during winter-spring (WS) and summer (S) periods in the Gulf of California.

depth (m)

salinity (&)

temperature (C)

dissolved oxygen (mlÆl)1)

organic matter (%)

sand (%)

station

WS

S

WS

S

WS

S

WS

S

WS

S

WS

S

C137 C238 C339 C142 C243 C344 C127 C226 C325 P132 P134 P333 C147 C248 C346 C116 C215 C314 C152 C251 C350 C103 C204 C305 P119 P220 P321 P149C P249B P349A P110 P208 P209 C261 C260 P155 P257 P356 P162C

30.3 71.9 106.4 29.9 68.8 104.1 34.9 71.9 102.1 37.2 32.9 81.8 36.9 60.2 105.0 22.2 49.8 92.0 28.6 49.5 97.0 32.0 79.0 120.0 30.4 54.1 104.1 28.9 68.8 100.0 39.0 52.0

21.5 64.0 93.0 23.5 66.3 106.0 34.4 65.8 83.0 21.6 26.3 72.8 21.4 54.6 96.0 18.0 39.0 80.7 22.1 42.0 80.0 23.5 54.9 97.0 31.6 43.9 112.0 17.0 63.3 94.0 28.4 48.0 77.5 12.2 67.7 32.5 55.2 104.6 29.7

35.51 35.45 35.16 35.54 35.45 35.26 35.46 35.35 35.22 35.48 35.38 35.33 35.06 35.09 35.00 35.46 35.22 35.09 35.19 35.15 34.99 35.04 35.00 34.98 35.30 35.28 35.24 35.40 35.11 35.10 35.51 35.50

36.06 35.71 35.54 35.63 35.60 35.63 35.59 35.58 35.52 35.90 35.93 35.58 35.56 35.44 35.31 35.95 34.80

16.0 14.5 13.2 16.4 15.2 14.2 15.1 14.4 12.7 15.1 15.1 13.8 13.8 13.2 12.9

29.6 26.5 20.8 28.1 22.5 19.4 26.6 26.8 17.2 28.2 28.9 22.7 29.8 22.5 16.0

5.40 3.17 1.73 5.11 3.03 2.40 3.09 2.55 1.90 4.21 4.30 1.93 1.54 0.63 0.91

2.4

28.1 15.5 30.0 32.0 17.6 30.2 25.3 16.3 23.6 24.4 16.3 24.5 21.4 18.9 24.7 19.8 16.7 28.3 21.7 21.3 17.2 16.3 22.1

1.04 0.92 5.40 1.80 1.47 1.02 0.80 0.54 4.00 3.25 2.97 4.70 1.33 1.34 4.93 3.62

5.0 3.0 5.0 4.8 8.4 8.4 5.3 4.8 6.5 5.1 2.9 5.1 1.5 1.9 3.0 3.8 6.1 7.2 5.3 4.8 3.8 2.9 5.0 5.1 5.0 4.0 6.9 3.8 4.4 5.7 5.0 5.0 5.3 8.4 7.4 3.8 6.1 4.0 4.2

85

14.1 13.6 16.8 14.8 13.2 14.0 13.2 12.9

4.26 3.51 2.25 4.03 2.75 2.56 4.28 4.64 2.54 4.10 3.88 3.18 4.53 3.01 1.62 4.82 3.83 1.23 4.34 4.58 2.22 4.76 3.58 2.12 3.59 3.80 2.22 3.87 3.09 2.77 5.06 3.21 3.60 3.67 1.47 5.20 2.65 1.60 5.29

91 74 64 80 71 52 95 89 99 40 16 64 96 89 96 87 81 88 48 46 49 87 84 69 95 84 97 93 74 93 86 97 96

50.4 76.0 55.2 101.0

34.92 34.99

34.80

34.20 35.12 35.22 35.00 35.23 35.07 35.64 35.76 35.48 35.44 35.46 35.41 34.45 34.30 35.44 33.34 33.50 34.70 33.48 33.53 35.10

13.6 17.2 13.7 13.2 17.5 18.7 16.8 15.3

13.9

0.037). Changes in density can be explained by the climatic variations associated with seasons, but the variations in the number of species was not significantly different between winter-spring and summer (P ¼ 0.217). During the winter-spring season (WS), five main faunal assemblages were identified with cluster analysis (Fig. 4): group WS was formed by the stations located towards the western shores of the Gulf at depths around 30 m. The species A. cf. oxycephala and P. (P.) heterobranchia determined the similarity within the assemblage. Stations of group WS2 were basically located in the inner sublittoral zone of the northern region; these stations were grouped 276

1.03 0.76

1.10

3.0 3.6 8.9 7.2 6.9 1.5 3.0 7.2 6.9

5.7 2.9 3.9 5.3 3.6 7.2 5.7 5.7 3.0 6.4 1.8 4.5 3.6 3.6 4.1 4.2 5.5 4.8

5.7

82 91 74 74 94 94 96 46 18

89 95 92 85 58 58 62 96 77 77 96 97 90 96 87 99 94 92

94

89 97 98 97 89

by the low densities of the species P. pinnata found in that area. Group WS3 included stations deeper than 64 m located in the vicinity of the Ańgel de la Guarda and Tiburoń islands; their similarity was defined by L. cirrata and P. (P.) ehlersi (Fig. 4). Group WS4 was formed by stations C103, C127 and C116 located in the inner sublittoral zone and was characterized by the presence of the species S. bombyx and A. dayi. Group WS5 was basically formed by stations deeper than 64 m located along the eastern coast of the Gulf; P. pinnata was the dominant species, with average density values of 52.48 orgs./ 0.1 m2.



Table 2. List of Spionida species from the Gulf of California (classified after Rouse & Fauchald 1997).


climatic period winter-spring

species Palpata Canalipalpata Spionida Family Spionidae (1) Aonidella sp. 1 (2) Aonides cf. oxycephala (Sars 1862) (3) Apoprionospio dayi Foster 1969 (4) Apoprionospio pygmaea (Hartman 1961) (5) Dipolydora socialis (Schmarda 1861) (6) Dispio uncinata (Hartman 1951) (7) Laonice cirrata Sars 1851 (8) Laonice pugettensis Banse & Hobson 1968 (9) Malacoceros indicus (Fauvel 1928) (10) Microspio pigmentata (Reish 1959) (11) Paraprionospio pinnata (Ehlers 1901) (12) Polydora cornuta Bosc 1802 (13) Prionospio (Minuspio) lighti Maciolek 1985 (14) Prionospio (Minuspio) multibranchiata Berkeley 1927 (15) Prionospio (Prionospio) dubia Day 1961 (16) Prionospio (Prionospio) ehlersi Fauvel 1928 (17) Prionospio (Prionospio) heterobranchia Moore 1907 (18) Prionospio (Prionospio) jubata Blake 1996 (19) Prionospio (Prionospio) steenstrupi Malmgren 1865 (20) Scolelepis (Parascolelepis) texana Foster 1971 (21) Scolelepis (Parascolelepis) tridentata (Southern 1914) (22) Scolelepis (Scolelepis) squamata (Mu¨ller 1806) (23) Spiophanes berkeleyorum Pettibone 1962 (24) Spiophanes bombyx (Clapare`de 1870) (25) Spiophanes duplex (Chamberlin 1919) (26) Spiophanes kroeyeri Grube 1860 (27) Spiophanes wigleyi Pettibone 1962 Family Longosomatidae (28) Heterospio sp. 1 Family Magelonidae (29) Magelona californica Hartman 1944 (30) Magelona marianae Hernańdez-Alcańtara & Solı´s-Weiss 2000 (31) Magelona pacifica Monro 1933 (32) Magelona tehuanensis Hernańdez-Alcańtara & Solı´s-Weiss 2000 (33) Meredithia spinifera Hernańdez-Alcańtara & Solı´s-Weiss 2000 Family Poecilochaetidae (34) Poecilochaetus johnsoni Hartman 1939 Family Chaetopteridae (35) Chaetopterus variopedatus (Renier 1804) (36) Mesochaetopterus sp. 1

The faunal groups showed differences between summer (S) and winter-spring samplings. P. pinnata remained the key species in the whole Gulf, but with lower densities in summer. Five main groups were identified (Fig. 5). S1 included the stations located in the middle and outer sublittoral zone of the peninsular margin; their similarity was determined by S. berkeleyorum and S. duplex. The stations of group S2 were deeper than 64 m, in the eastern region and with the highest densities of P. pinnata (x ¼

X X X X X X X X X X X X X X

summer

X X X X X X X X X X X X

X X X X

X X X X X X X X X X

X

X

X X X X X

X X X X X

X

X

X X X

X X

27.76 ind./0.1 m2). In group S3 the stations were in front of the northwestern and center-east regions and associated with the presence of L. cirrata. Group S4, the largest, has most stations of the inner and middle sublittoral zone in the eastern region; their similarity was determined by P. pinnata, P (P.) dubia, and P. steenstrupi. Finally, P. (P.) dubia was the most important species within group S5, basically occupying stations with low densities located along the western coast.


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Fig. 2. Density and species richness per sampling station in the winter-spring period [density data were transformed into log(x + 1) to standardize the scale].

Fig. 3. Density and species richness per sampling station in the summer period [density data were transformed into log(x + 1) to standardize the scale].

The SIMPER analysis showed that 79.96% of the total average dissimilarity (AvD) between seasons was accounted for by a few species (Table 3), with nearly 60% accounted for by nine species of which P. pinnata had the highest contribution. It is important to emphasize that the average density of each of these species was always lower in summer. The relationship between the faunal data and the environmental parameters during both sampling seasons (the 278

winter-spring and the summer) was analyzed through CCA. The first two CCA axes explained 53.6% of the species/environment variation and 7.5% of the variability in the species data. The latter seems very low, even by ecological standards, but species data are often very ‘noisy’, and a low percent variation can often be quite informative about the heterogeneous faunal composition. The first two axes were well correlated with the environmental data (r ¼ 0.737 and 0.715, respectively). The environmental




Fig. 4. Group-average clustering of the sampling sites with the Bray–Curtis similarity index in the winter-spring period.

Fig. 5. Group-average clustering of the sampling sites with the Bray–Curtis similarity index in the summer period.

variables explained nearly 90% of the variability in the polychaete fauna. Depth (r ¼ 0.469) and sand percent in the sediments (r ¼ )0.461) were the variables with the highest correlation with axis I, while depth (r ¼ )0.499), temperature (r ¼ 0.418) and sand percent (r ¼ )0.409) were the most important parameters for axis II (Table 4). In the ordination diagram (Fig. 6) the stations’ distribution was determined initially by depth. This was the most important variable for axes I and II; a positive correlation can be observed with axis 1 but there is a negative

correlation with axis II (Table 4). The distribution pattern due to depth can be observed with the differences among the stations of the sublittoral inner zone having low densities (on the upper side of the diagram) and the stations located at depths higher than 64 m with high densities (on the lower central side). Sand percent was the other important factor and had a negative correlation with axes I and II. This vector had a perpendicular orientation with respect to depth and also reflects the differences among the stations with highest


279



Table 3. Average dissimilarity between seasons indicating which species contributed most (up to 60%) to the overall dissimilarity, presented as average density (AvDen), individual contribution towards the average dissimilarity (AvD), contribution towards the dissimilarity between seasons (AvD/standard deviation of AvD) and cumulative AvD expressed in percentage. AvDen species

winter-spring

Average dissimilarity ¼ 76.96 P. pinnata 19.80 P. dubia 0.55 L. cirrata 0.59 P. steenstrupi 0.94 P. ehlersi 0.69 S. duplex 0.14 S. kroeyeri 0.51 S. bombyx 1.13 Heterospio sp. 1 0.19

summer

AvD

AvD/SD (AvD)

AvD Cum.%

5.72 0.45 0.33 0.42 0.31 0.03 0.01 0.12 0.11

12.50 6.61 6.41 5.60 5.31 2.92 2.75 2.63 2.62

1.23 0.86 0.95 0.85 0.79 0.63 0.44 0.42 0.57

16.24 24.83 33.16 40.44 47.33 51.13 54.70 58.11 61.51

Table 4. Intraset correlations of environmental variables with the first two axes of canonical correspondence analysis in the winter-spring and summer periods. Axis Axis variable

I

II

Depth Salinity Temperature Dissolved oxygen Organic matter Sand (%)

0.469 )0.092 )0.014 )0.245 0.177 )0.461

)0.499 0.151 0.418 0.213 0.183 )0.409

sand (on the left lower side) and mud (on the right upper side) percentages. Due to the effect of these two factors on the species variability, the stations with higher densities were associated with zones deeper than 64 m and sediments with sand percent slightly lower (77–94%) are plotted in the lower central part of the diagram. The second important factor in the correlations with axis II was the temperature; a remarkable separation between stations due to the effect of this variable was observed. The temperature vector is almost parallel to the vertical axis, which is why most summer samples (upper side of the diagram) with lower densities were positively correlated with it. In contrast, most of the stations sampled in winter-spring (on the lower side) with higher density values were associated with a reduction in temperature. Discussion These results constitute the first quantitative description of the Spionida assemblages at a regional scale in the Gulf 280

of California. The fact that the Spionida are so abundant, diversified and widely distributed in the Gulf of California implies that the faunal assemblages, their associated distribution areas and their seasonal variations are important to understand the function of those invertebrates in the benthic system. The distribution of the Spionida in the Gulf of California is not homogeneous, with changes in composition due to spatial and seasonal effects. The environmental complexity found there has a differential effect on abundance or species richness. This has been shown to interact with biotic factors (e.g. reproductive rates, recruitment patterns, competition, etc.) and can change the spatial distribution patterns (Pascual & Caswell 1997). The centers of higher density in front of the northern and central eastern shores include a large number of species but with no consistent correlation between density and species richness. The composition of the faunal groups and the spatial distribution models of density depend on the differential effects of the environmental factors and on the particular distribution patterns of each species (Sagarin & Gaines 2002). Here, the species occur in discrete groups rather than exhibiting gradual changes in composition along latitudinal or depth gradients. During the winter-spring, the most abundant groups (WS2 and WS5) were the result of P. pinnata’s abundance. This species was found along most of the eastern coasts and in the northern region and peaked in the middle and outer zones of the continental shelf. During the summer, P. pinnata was reduced almost to half and thus, the faunal assemblages defined by that species (S2 and S4) were modified. Even if they are distributed in similar zones, their abundance decreased significantly at shallow depths (around 30 m). The oceanic circulation causes a net flow towards the south. Waters at higher temperatures and salinity come from the northern region on the peninsular side of the Gulf. This, added to the narrow continental shelf with rocky profiles (Lavıń et al. 1997), can have a negative effect on the fauna from the western region of the Gulf, since the number of organisms and species is lower than in the extensive fine sands of the littoral eastern margin. The similarity of groups WS1 and S1 can be explained by the presence of species with reduced densities which were only present in one of the periods sampled: A. cf. oxycephala, P. (P.) heterobranchia in winter-spring and S. berkeleyorym in summer. The fauna exhibited a dramatic decrease in abundance during the summer, but with no significant changes in the number of species. Despite this fact, the faunal differences between winter-spring and summer were also detected in the composition of the fauna between both periods. Thirteen species (36%) were only recorded in




Fig. 6. Canonical correspondence analysis (CCA) ordination diagram showing stations and environmental variables relative to axes I and II in the winter-spring and summer periods.

one period: Aonidella sp. 1, A. cf. oxycephala, D. socialis, P. cornuta, P. (P.) heterobranchia and Mesochaetopterus sp. 1 were only present in winter-spring, while A. pygmaea, L. pugettensis, M. pigmentata, P. (P.) jubata, S. (P.) texana, S. berkeleyorum and C. variopedatus were only collected in summer (Table 2). The high percentage of dissimilarity obtained with the SIMPER analysis confirms the existence of significant differences between the composition of the winter-spring and summer fauna. P. pinnata, P. dubia, L. cirrata and P. steenstrupi were the species with the largest AvD/Standard deviation (AvD); these species not only contributed significantly to the dissimilarity between periods but they also consistently contributed to the similarity within the assemblages of species in each season. The CCA for the climatic cycles reveals that nearly 90% of the variation in the species was explained by the environmental variables. Distinct relationships were found between species variability and the environment. However, significant relationships do not depend on the amount of the variability explained and might be detected using CCA even when the percentage is low (ter Braak & Verdonschot 1995).

Although synergetic influences cannot be overlooked, here depth was the most important variable, accounting for 24% of the variation in the species data. The arrangement of the samples in relation to depth showed a gradual change in their distribution along the continental shelf. This distribution pattern was related to faunal changes and thus the samples near the central lower end of the ordination, deeper than 64 m, had the highest densities. Depth could be a non-specific descriptor since it is related to other factors such as salinity, nutrients, temperature, currents and light, among others. In the Gulf of California, dissolved oxygen and salinity decrease with depth. The position of the arrows of the environmental factors suggests a correlation among variables, but it should be emphasized that the CCA diagram is based on the effect that those factors have on the variation of the fauna. Dissolved oxygen and salinity do not help in the explanation of the species variability. The sediment composition is one of the most important factors in the distribution of the benthic communities (Etter & Grassle 1992). Here, the sand percentage can explain 22% of the species variation. Density increased


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when sand content decreased slightly (77–94%). Sand content is reduced in the north and central eastern Gulf, where high densities occur. In contrast, on the peninsular margin of the Gulf where the highest sand percentage was found, density values were reduced. The CCA shows that organic matter is inversely related to sediment size. Usually, organic matter concentration is inversely related to sediment particle size and offers the same information as the fine fractions of the sediment. However, even though spionids are predominantly detritivores, the organic matter content in the sediments adds little information about the distribution patterns of the Spionida in the Gulf of California. Depth and sand percentage have the highest correlation with axes I and II; the length and direction of these vectors seem to represent changes from shallow muddy habitats located at the upper end of the diagram down to deep sandy areas located at its lower end. The relation between the depth-sediment factors and the fauna shows that the density increases in the stations plotted in the central lower part of the diagram. Those localities are in the medium and outer sublittoral zones where a slight reduction in the percentage of sand can be observed. High environmental correlations do not necessarily have a causal effect (Clarke & Ainsworth 1993) but here, changes in depth and sediment directly influenced the spatial density patterns. Most of the stations with a similar geographic position are located very close to each other in the diagram at each sampling period, because of the effect of these two factors. However, a separation of the stations correlated to the season was also observed as a result of the influence of the temperature, the second most important factor for axis II. Temperature explains an additional 16% of the species variability. The effect of this factor on the distribution of the species, however, acts at the interannual climatic level, since 66.67% of the stations sampled in the summer are correlated with an increase in temperature, while during winter-spring, 65.8% of the stations were associated with a decrease in temperature. Temperature can also reflect the influence of related factors such as surface heat flux, latitude, and season (Mackie et al. 1997). Throughout the year, the shallow waters of the Gulf are subjected to remarkable changes in their physical and chemical properties. During the summer, the heat flow across the surface of the Gulf of California represents a net gain which causes an increase in temperature in the superficial layers of more than 10 C (Lavıń et al. 1997; Salas-de Leoń et al. 2003). In the Gulf of California, the links between temperature and fauna can be the result of the changes in the spatial boundary of the water masses as well as the circulation patterns, rather than direct effects on species metabolism. 282

During summer, the warm equatorial shallow water (ESW), found in the first 150 m, reaches its maximum intrusion inside the Gulf, up to Guaymas Basin, while during the winter it goes back to the Gulf’s opening. The intrusion of the superficial equatorial water is a clear seasonal signal that could be responsible for the changes observed in the faunal distribution since it can influence transportation and dispersion of the species with tropical affinities. The effects of depth and type of sediment on seasonal variations of benthic polychaetes have been documented elsewhere: Hylleberg & Nateewathana (1991) in Phuket Island waters, where the Spionidae were the most abundant family, could not explain the fact that their density was higher at certain depths and attributed it to the type of sediment. The spionids were more abundant when the silt-clay content in the sediment decreased. In another study carried out in southern Brazil (Barros et al. 2001), in sandy beaches where spionids were also very abundant, zonation patterns were not significantly different between summer and winter and the changes in composition and abundance were attributed to the influence of waves, but also to sediment type and beach slope (associated to depth). In the western Gulf of Mexico’s continental shelf, where spionids were dominant, in particular P. pinnata, the same trend observed in this study was found; a larger number of species and organisms were present in spring and there was a significant decrease in summer-autumn (Delgado-Blas 2001). In this case, the spatial distribution was correlated with the type of sediment, the number of species and organisms being higher in sandy bottoms. According to the existing information, the influence of other environmental factors such as salinity and temperature is not significant for the variations of faunal density. However, since most spionids are suspension feeders and interface feeders (which switch between suspension feeding and deposit feeding at the sediment–water interface), they could well react to the spatial and seasonal variations of their food sources by way of physiological or behavioral adaptations (Shimeta et al. 2004). In a study of how growth rate and temperature (5 and 15 C) affect Polydora cornuta feeding efficiency, both factors were significant – the organisms with lower growth rate and at higher temperature (15 C) increased their food-capture efficiency (Shimeta et al. 2004). Even if the results of this study do not enable us to confirm or deny the effects of these factors on the local fauna, they suggest that further studies are necessary to assess the real effect of temperature. In the Gulf of California temperature clearly has an influence on seasonal density variation in Spionida, supporting the importance of temperature for organisms with those feeding habits.



Summary A series of quantitative samples revealed 36 species of Spionida belonging to the families Spionidae, Longosomatidae, Magelonidae, Poecilochaetidae and Chaetopteridae in the sublittoral zone of the Gulf of California. Populations were more abundant in front of the eastern shores of the Gulf; the highest species richness was recorded at stations shallower than 64 m, but the areas with higher densities were located in the middle and outer sublittoral zone. Polychaete density varied significantly between seasonal cycles, decreasing drastically in summer, but the number of species was similar in winter-spring and summer. The polychaete assemblages showed seasonal and spatial differences in species composition. During the winter-spring period, the most abundant groups were related to P. pinnata’s abundance but in summer the average abundance of this spionid was reduced almost to half and thus the faunal assemblages defined by that species were modified. Depth and sand percent were the most important spatial factors determining the Spionida distribution. Temperature explained a substantial part of the variation in the species data between periods. Acknowledgements Thanks are due to Laura Gonza´lez Ortiz for the identification of some of the biological material and to Michel Hendrickx, head of project CORTES for inviting us to participate and collect the material used in this study. Lisa Levin made many useful comments which greatly improved the manuscript for which she is especially thanked. Kristian Fauchald inspired so many of us, and VSW in particular to persevere in this field, that we are glad to acknowledge him and pay tribute to his decisive role in our formation. References Banse K., Hobson K.D. (1968) Benthic polychaetes from Puget Sound, Washington, with remarks on four other species. Proceedings of the United States National Museum, 125(3667), 1–53. Barros F., Borzone C.A., Rosso S. (2001) Macroinfauna of six beaches near Guaratuba Bay, Southern Brazil. Brazilian Achieves in Biology and Technology, 44(4), 351–364. Bastida-Zavala J.R. (1993) Taxonomıá y composicioń biogeogra´fica de los poliquetos (Annelida: Polychaeta) de la bahıá de La Paz, Baja California Sur, Me´xico. Revista de Investigaciones Cientı´ficas, 4, 11–39. Berkeley E. (1927) Polychaetous annelids from the Nanaimo district. Pt. 3, Leodicidae to Spionidae. Contributions to Canadian Biology, 3, 405–422.


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