Population Dynamics and Life History Characteristics ...

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Callinectes similis and C. sapidus co-occur in the Mobile Bay-Mississippi ... Hines et al” 1990),little information is available for the Gulf of Mexico (see.
P.S.Z.N. I: Marine Ecology, 14 (3): 239-257 (1993) © 1993 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0173-9565

Accepted: January 11,1993

Population Dynamics and Life History Characteristics of the Blue Crabs Callinectes similis and C. sapidus in Bay Environments of the Northern Gulf of Mexico Pan-Wen Hsueh1* ,James B. M cC lintock1

& Thomas S. Hopkins2

1 Department of Biology, University of Alabama at Birmingham, Birmingham, AL 35294-1170, U.S.A. 2 Department of Biological Sciences, University of Alabama, Tuscaloosa, AL 35487-0344, U.S.A. With 13 figures and 1 table

Callinectes similis,C. sapidus, population dynamics, life history, northern

Key words: gulf of Mexico.

Callinectes similis sapidus similis Callinectes C. similis

Abstract. The population dynamics and life history characteristics of and C. in the Mobile Bay-Mississippi Sound estuarine system of the northern Gulf of Mexico were studied. We report that C. is numerically dominant in open-bays,while C. is more abundant in shallow salt marshes. Juvenile C. recruit into both open-bays and salt marshes, but juvenile C. primarily recruit into shallow salt marshes. Sex ratios of C. and were generally skewed towards higher proportions of males than females. appears to have a life cycle similar to that of Egg-carrying migrate to higher salinity waters and presumably release larvae to nearshore shelf waters; following develop­ ment, the larvae re-invade estuaries as megalopae. Juvenile C. grow more slowly than This may be attributable to reduced feeding rates. Marked differences in fecundity and reproductive effort were found between these species. and C. respec­ tively, produced 2.4-5.5 x 105 and 2.1-3.2 x 106 eggs and invested 24-49 and 171-372 kJ per brood. Rhizocephalan barnacle infestation by was encountered in while sympatric C. were free of this rhizocephalan parasite.

sapidus

similis

C. sapidus similis C. sapidus.

similis

sapidus

similis

C. sapidus.

similis Callinectes similis Loxothylacus texanus

sapidus, C. sapidus,

Problem

Callinectes similis W illiam s is widely distributed along the east coast of North

America and within the Gulf of Mexico. The distribution of C. similis ranges along the Atlantic from Delaware to southern Florida and in the Gulf of Mexico from northwestern Florida to Campeche, Yucatan. W illiam s (1984) also

reported C. similis from Isla de Providencia, Columbia and northern Jamaica. * Present address: National Museum of Natural Science, 1 Kuan Chien Rd., Taichung, Taiwan ROC 40419.

U.S. Copyright Clearance Center Code Statement: 0173-9565/93/1403-0239$02.50/0

H su e h , M cC lin to ck & H opkins

240

In many areas, C. similis co-occurs with its congener, C. sapidus (T agatz ,1967; W illiam s, 1974; Perry, 1975; Hsueh

et al” 1992a). Considering distributional

records,there is a remarkable lack of information on population dynamics and life history characteristics of C. similis. This has been due, in part, to the tendency to lump individuals of both species together. Callinectes similis and C. sapidus co-occur in the Mobile Bay-Mississippi Sound estuary system (Hsueh, 1992). This provides an opportunity to compare

the population dynamics and life history of the two closely related species. In addition,although the life history and population dynamics of C.

sapidus have et al”

been well studied in coastal estuarine and bay systems on the northeastern coast of the United States M illio n & W illiam s , 1984; Hines 1987; Hines et al” 1990),little information is available for the Gulf of Mexico (see

(e. g”

references in Steele & Perry, 1990). Significant differences in both physical and

biotic factors characterize these two geographic regions and may influence aspects of life history and population dynamics. The present study focuses on Mobile Bay-Mississippi Sound estuaries as model systems to compare the biology of Callinectes similis and C. sapidus. Aspects of the investigation include seasonal occurrence and abundance, habitat partitioning by sex and size, feeding and growth rates, size at sexual maturity, fecundity,energy invested in egg masses, and reproductive castration through rhizocephalan parasitism.

Material and Methods 1 ■ Distribution, abundance, sex ratio, and size frequency

Callinectes similis

sapidus

and C. were collected monthly from May 1989 to April 1991 by otter trawl (6.1-m opening and 3.0-cm mesh with 0.5-cm mesh cod end; triplicate 10-min trawls at approxi­ mately 3.5 km • h_1) at study sites in Mobile Bay (Fort Morgan, 10m depth) and east Mississippi Sound (Tall Range D, 3m depth) (sites I and II , Fig. 1). The two species were also collected monthly from Channel Marker # 28 in south-central Mobile Bay (site III, Fig. 1) over a one-year period (July 1990 to June 1991). Crabs at this site were collected monthly by otter trawl (7.9-m opening and 3.5-cm mesh with 2.5-cm mesh cod end; single 20min trawl at 3.6km . h_1). Blue crabs were also collected monthly at Dauphin Island Airport marsh using a 6-m seine (5-mm mesh) (site IV , Fig. 1). Beach seining was conducted monthly by pulling the net towards the shore over a distance of 25 m. The selected sampling sites represent a variety of habitats including open mud bottom (sites I ,II and III) and salt marsh wetland (site IV). Water temperature and salinity measurements were taken during each sampling period at each site. The numbers of individuals collected, as well as sex and size [carapace width (CW)] were recorded for both species. A Mann-Whitney U-test was used to compare abundances for each sampling date at Fort Morgan and Mississippi Sound sites (Zar, 1984). Male and female sex ratios of each species were compared using a Sign test (Z ar, 1984).

2. Feeding and growth rates, size at sexual maturity, fecundity, and energy invested in egg masses

Callinectes similis

Feeding rates of three size classes (11一 20, 21-40, 41-70 mm CW) of juvenile and C. were examined in the laboratory from May to July 1989. The crabs were collected from Mobile Bay and Dauphin Island Airport Marsh (sites III and IV). They were held individually in 1.1 liter plastic containers on racks in aquariums maintained at a constant temperature of 23 °C and a salinity of 30 ppt. Crabs were fed an diet of 5 % gum agar containing a 5 % concentration of fish meal. Agar food blocks were pre-weighed, placed in each container for 24 h, and then blotted

sapidus

ad libitum

Ecology of blue crabs

241 Fig. 1. Mobile Bay and east Mississippi Sound, Alabama. Four permanent sampling sites in this study are noted in Roman numerals.

similis

sapidus

sapidus Callinectes similis

sapidus

dry and re-weighed. Mean rates of food intake were compared between C. and C. within each size class using a Student’s t-test (Zar, 1984). Using a similar experimental protocol as in the feeding experiments, the growth rates of 10 juvenile (20-25 mm CW) male and C. were examined. Wet weight and carapace width of crabs were measured at each molt interval over 100 days. An ANCOVA was employed to compare growth rates (Z ar, 1984). The size at which female crabs become reproductively mature (oval-shaped abdomen) was ascertained from field collections. A total of 1800 and 577 C. were examined. A total of 390 C. and 72 male testes were examined microscopically to determine the size at which males become reproductively mature. Ovigerous and C. were collected from Mobile Bay in summer 1989 for measurements of fecundity and reproductive energy output. The numbers of eggs produced per female was estimated by weighing sub-samples of egg masses of twelve crabs of each species and counting the numbers of eggs under a dissecting microscope. These counts provided an assessment of the number of eggs per unit wet weight egg mass. Multiplying by the total wet weight of the egg mass, we obtained an estimate of total fecundity. In order to calculate the total energy invested per egg mass, sub-samples of twelve egg masses of each species were analyzed for organic composition (soluble and insoluble protein, soluble carbo­ hydrate, lipid, and ash). Protein and carbohydrate concentrations were determined using colori­ metric techniques ( L o w r y 1951 and D u b o is 1956),while lipid concentration was determined gravimetrically (Freeman 1957). Ash was determined by placing tissues in a muffle furnace for 4h at 500°C. Energetic composition (kJ • g_1 dry wt) was computed indirectly by multiplying the dry weight of the egg mass tissue by the level of each organic class and their energetic equivalents (Brody, 1945; Lawrence ., 1984). Total reproductive energy output was calculated by multiplying the amount of kJ • g_1 dry wt egg mass tissue by the entire dry weight of the egg mass. The reproductive output (total kJ) of C. and C. was compared using a Student’s t-test.

Callinectes similis

similis

C. similis

C. sapidus

et al”

sapidus

et al”

et al similis

et al”

sapidus

3. Parasitism

Callinectes similis

Numbers and sizes of were recorded.

Loxothylacus texanus

and C.

sapidus infected by the

rhizocephalan barnacle

H su e h , M cC lintock & H opkins

242

Results 1. Environmental conditions Bottom sea water temperature at all four sampling sites varied greatly with season during the study (Figs. 2 and 3). The highest sea water temperature (35 °C) occurred at the Mississippi Sound site in August 1989. The lowest temperature (11 °C) occurred at both Dauphin Island Airport Marsh and the Mississippi Sound sites in December 1989. Sea water temperatures were gener­ ally similar at each of the four sampling sites on a given date. During the

40

s sadEs m t o o )

3 0 2 0 1 0

saw Auulls

Fig. 2. Temperature and salinity at Fort Morgan, Mississippi Sound, and Dauphin Island Air­ port Marsh sites from May 1989 to April 1991. No data were col­ lected in September 1989 for these three sites or in January 1991 for the Fort Morgan and Mississippi Sound sites.

3 0 2 0 1 0

A S O N D J F MA M J J A S O NDJ F M A 1989

35 {Ids^c=g *00 e )JssadEs co

1990

temperature salinity

1991

Channel Marker # 28

30

25

20

1990

1991

Fig. 3. Temperature and salinity at the Channel Marker site from July 1990 to June 1991. No data were col­ lected in May 1991.

Ecology of blue crabs

243

summer of 1990, however,sea water temperatures were consistently higher at the Dauphin Island Airport Marsh than at the other three sites, ranging from 32 to 34 °C. Salinity at the four sampling sites also varied with season. The lowest salinities generally occurred during the early spring (February—March) of 1990 (Figs. 2 and 3). The maximum salinity (34 %o) was recorded at Fort Morgan in January 1990, minimum salinity values (1-2 %o) at Mississippi Sound in February and March of 1990. Differences in seasonal salinity fluctuations were lower at the channel site (26-32 %o) than at shallower sites (e. g.y Mississippi Sound: 1-26 %o). Overall, salinities were consistently lower at the Dauphin Island Airport Marsh than at the other three sites,with values ranging from 4 to 26 %o (文 = 14±7%o). Salinities were consistently highest at the deeper-water, openbay Fort Morgan and channel sites,with values ranging from 17 to 35 %o (文 =26 ± 5 and 30 ± 2 %o, respectively) •

2. Distribution, abundance, sex ratio, and size frequency Callinectes similis and C. sapidus co-occurred at all four sampling sites. The former was generally collected in significantly (P < 0.05) greater numbers than the latter at open-bay sites (Fort Morgan,Mississippi Sound, and Channel Marker). This trend was particularly evident at the Channel Marker site from July 1990 to February 1991,where C. similis accounted for at least 95 % of the total crabs collected. Minor exceptions occurred in June 1989 at Mississippi

一 qsolo JaqEnu USE IMS}

c o

Fig.4. M ean(± lS.E ., n = 3) abundances of and C. at Fort Morgan and Mississippi Sound from May 1989 to April 1991. No data were col­ lected in September 1989 at these sites.

similis

1989

1990

1991

Callinectes sapidus

H su e h , M cC lintock & H opkins

244

Callinectes similis sapidus

Fig. 5. and C. abundance at the Channel Marker (July 1990 to June 1991) and Dauphin Island Airport Marsh sites (May 1989 to April 1991).

1990

1991

Sound, March 1990 at Fort Morgan and Mississippi Sound, and April 1991 at Fort Morgan (Figs. 4 and 5). Blue crab abundance varied over both season and site. At Fort Morgan, both species were most abundant during the summer of 1989 and spring of 1990 and 1991. Peak abundances reached a mean of 71 Callinectes similis per trawl during the summer of 1989,and 64 and 32 C.

similis per trawl in the spring of 1990 and

1991,respectively (Fig. 4). Callinectes sapidus were most abundant at Fort Morgan during the spring of 1990 and 1991 (x = 30 and 43 crabs per trawl). At Mississippi Sound, C. similis peaked in abundance (x = 89 crabs per trawl) in fall of 1989 and remained low throughout the remainder of the sampling period (Fig. 4). Callinectes sapidus had low densities throughout the sampling period at Mississippi Sound. At the Channel Marker site, C. similis was abundant in all seasons except in spring 1991,when C. sapidus became abundant here (Fig. 5). In contrast to open-bay sites,Callinectes sapidus was always more abundant than C. similis at the Dauphin Island Airport Marsh (Fig. 5); peak abundance occurred during the summer and winter of both 1989 and 1990, as well as in spring 1991. Callinectes sapidus was found on all sampling dates, while C. similis was absent at the marsh site in the spring and winter of 1989 and 1990. Peak abundance of C. similis occurred in early summer 1989 (Fig. 5). Sex ratios of Callinectes similis varied among sites (Fig. 6). Males were significantly (P < 0.05) more abundant at all study sites (except Channel Marker); m ale: female ratios in open-bays,however,were much lower than in

Ecology of blue crabs

245

Callinectes similis

17

697

□0

1

00 80 60 40 20 00 80

Males Immature Females 11 Mature Females

1610

Fig. 6. Percent frequencies of males, and immature and mature females of and C. at four sampling sites. Data are com­ bined over all sampling dates. Sample sizes are given at the top of each bar.

similis

Callinectes sapidus

1

{%) Aouanbsl

Callinectes sapidus

Fort Morgan

Mississippi Sound

Dl Airport Marsh

Channel Marker #28

shallow marshes. These ratios were further reduced from 2.9:1 and 2.5 :1 at the Fort Morgan and Mississippi Sound sites’ where the greatest number of sexually mature females were collected. Few sexually mature females were collected at Mississippi Sound and Fort Morgan (Fig. 6). No sexually mature female C. similis were collected at the Dauphin Island Airport Marsh. In the openbays, sexually mature females were collected almost year-round, with a peak abundance in October 1990 (Fig. 7). Among the numerous sexually mature females collected from open-bay sites,none carried an egg mass. Sex ratios of Callinectes sapidus also varied among sites (Fig. 6). Males numerically dominated over females (P < 0.05) only at the Dauphin Island Airport Marsh. In contrast, females were more abundant (P < 0.05) at Fort Morgan and the Channel Marker, with no difference (P > 0.05) at the Missis­ sippi Sound site. Sexually mature females were more abundant at Fort Morgan and the Channel Marker than at Mississippi Sound. None were collected at the Dauphin Island Airport Marsh. In the open-bays, sexually mature females were collected almost year-round, except November and December in 1989 and 1990 (Fig. 7). In contrast to C. similis, ovigerous females were collected from May to August 1989,July to October 1990,and January through March 1991. The percentage of females with egg masses varied over both season and year (Fig. 7). The values ranged from 31 to 100 % (summer 1989 and summer and fall of 1990) and 44 to 100 % (January, February, and March 1991). Sexually mature females

H su e h , M cC lintock & H opkins

246 50

Fig. 7. Abundances of ma­ ture female and C. with and with­ out egg masses from May 1989 to June 1991. Data are combined for all sampling sites.

Callinectes similis sapidus

Callinectes similis

0 Females w/ egg masses 40 □ Females w/o egg masses

30

20

sqsolojequlnu

Da

ix

1991

1990

1989

Callinectes similis Fort Morgan N = 937

g n

Fig. 8. Size frequency dis­ tributions of at Fort Morgan and Mississippi Sound. Data are combined over all sampling dates.

similis 10

20

30

40

50

60

70

80

carapace width (mm)

90 100 110

Callinectes

Ecology of blue crabs

247 Fig. 9. Monthly size fre­ quency distributions of at the Channel Marker site from July 1990 to June 1991. Sample sizes are given.

JAN 91 N=230

I

Callinectes similis

FEB 91 N=654

n

AUG 90 N=224



1

SI

8EPM N»191

N=128

I

JilliiL

..

mm

lii^

APR 91

OCT 90 N=271

N^62

| I MAY 91 NO DATA

NOV 90 N=404

li

lliL



JUN91 N=47

i

DEC 90 N=537

n n

_國 __1 carapace width (mm)

Callinectes sapidus



□ ■ □ □

I

-

Parasitized Individuals (n=398) Immature Females (ns202) Males (n=142) Mature Females (ns92)

1 ^ -

CZ3 n

{%) Aouan s l

cp



80 60 40 20

10

20

30 40

50 60

70

80 90 100 110 120 130 140 150 160+

carapace width (mm)

Callinectes sapidus.

Fig. 10. Size frequency distribution of Data are combined from Fort Morgan, Mississippi Sound and Channel Marker sites and over all sampling dates. Sample sizes are given.

H su e h , M cC lintock & H opkins

248 1 1

00 MAY 89 80 N:107 60 40 20 00 80 60 a n 40 NOV 89 20

{% )>oc 3

o )

J 一

cr

a )

N=63

JUN 89 N=66

JUL 89 N=90

Ml DEC 89 N=105

JAN 90 N=113

I i H in 10 30 50 70 90 110 130

10 30 50 70 90 110 130

10 30 50 70 90 110 130

carapace width (mm)

Callinectes sapidus

Fig. 11. Size frequency distributions of at the Dauphin Island Airport Marsh. Data are presented only for months in which more than 28 crabs were collected.

collected in January, February, and March 1990,and April 1990 and 1991 were not carrying egg masses. Callinectes similis reached larger sizes in open-bays. Sizes at Fort Morgan ranged from 4-107 mm CW. The largest mode occurred in the 31-40 mm CW size class (Fig. 8). At Mississippi Sound, C. similis ranged from 8 to 91 mm CW; the largest mode occurred in the 21—30 mm CW size class (Fig. 8). Recently settled C. similis (< 10 mm CW) were observed only in April 1991 at Fort Morgan and Mississippi Sound. At the Channel Marker site, C. similis size ranged from 21 to 125 mm CW. With few exceptions, the population structure was nearly unimodal,with peak sizes in the 41-60 mm CW size classes (Fig. 9). Individuals larger than 90 mm CW were collected most frequently at the Channel Marker site. The population size structure of C. sapidus at the openbay sites formed trimodal distributions, with peak mean size class values of 46 ± 10 (n = 3),92 ± 32 (n = 7),and 152 ± 10 (n = 4) mm CW (Fig. 10). At the Dauphin Island Airport Marsh, Callinectes similis were small (size range: 4-36mm CW). Most C. similis (85%,n = 124) here were in the 11-22 mm CW size class. Larger individuals (> 30 mm CW) were rare,and none larger than 40 mm CW were observed. Callinectes sapidus at the Dauphin Island Airport Marsh were larger,ranging from 5—187 mm CW, although most were ©

carapace width (mm)

Callinectes similis

l l£ i CSE E(E )UJaEaloulfPIM ueaE 6} E&ou6 {s

~

Callinectes sapidus

o)

20

40

60

time (days)

80

100

Fig. 13. Mean ( 士 1 S.E.,n = 10) laboratory growth (width weight) increments of juvenile (20-25 mm CW) and C. over a 100-day period.

knd

sapidus

Callinectes similis

similis than C. sapidus (Table 1). Fecundity was ten times lower in C. similis (2.4-5.5 x 105 eggs per brood) than C. sapidus (2.1-3.2 x 106 eggs per brood). Energy invested in each brood was also much lower,with only 24-49 kJ per brood invested by C. similis compared with 171-372 kJ per brood for C. sapidus. On a per gram basis, there was no difference (P > 0.05) in the amount of energy invested in egg production (Table 1).

Ecology of blue crabs

251

Table 1. Life history characteristics of Mexico.

Callinectes similis and C. sapidus from the northern Gulf of similis

characteristic

C.

maximum carapace width (CW) female size at sexual maturity (CW) male size at sexual maturity (CW) number of eggs/brood energy invested/g dry wt egg mass energy invested/egg mass parasitism by rhizocephalan barnacles

125 mm > 60 mm > 70 mm 2.4-5.5 x 105 25.3 ± 0.19kJ 24-49 kJ absent

C.

sapidus

190 mm > 100 mm > 120 mm 2.1-3.2x106 24.8 ± 0.41 kJ 171-372 kJ present

4. Parasitism Callinectes sapidus was observed to be parasitized by the rhizocephalan barnacle Loxothylacus texanus. Infected crab sizes ranged from 35 to 80 mm CW, with the mode of infected crabs ranging from 40-50 mm CW (Fig. 10). Incidence of infection of C. sapidus collected from open-bay sites was 62%,but at the Dauphin Island Airport Marsh it was less than 1 % . The incidence of infection varied seasonally and interannually. At Fort Morgan, infected blue crabs were collected in June, July, and December 1989, January-June 1990,and March-April 1991,with high percentages (95% ,n = 20) in June 1989, 57 (n = 58) in February and 40 % (n = 108) in March 1990,and 70 (n = 27) and 82% (n = 130) in March and April 1991. At Mississippi Sound, rhizocephalaninfected crabs were collected in June, July, October,and November 1989, March and June-August 1990,and April 1991; they were never common here. At the Channel Marker site,infected C. sapidus were collected in December 1990,and January-April and June 1991, with high percentages (76 % ,n = 37) in February, 72% (n = 89) and 56% (n = 61) in March and April 1991. No rhizocephalan-infected C. similis were found.

%

%

Discussion Both Callinectes similis and C. sapidus were widely distributed in the Mobile Bay-Mississippi Sound estuarine system. Callinectes similis was numerically dominant in open-bays, C. sapidus in marshes. Distinct habitat preferences of the two blue crab species may be related to differences in osmoregulatory ability

et al”

(E ngel, 1977; Hsueh 1992 b; Hsueh, 1992). Based on an analysis of the ion regulating ability of both species,E ng el (1977) suggested that C. is

similis

less tolerant of low salinity environments. In the present study, most C. similis occurred in the Mobile Bay Channel and at bay-mouth sites where salinity is consistently high (2 25%) year-round. These observations suggest that C. similis may prefer high salinity environments. We report, however, that 22 % of C. similis (12 to 67 CW) were collected from relatively brackish water (8-15% ). In addition , Hsueh

et al.

(1992b) found that both juveniles

(20-30 mm CW) and adults (> 70 mm CW) are able to tolerate salinities as low

H su eh , M cC lintock & H opkins

252

as 10 %o for one week. Callinectes similis occurred in shallow marshes during seasons of low freshwater influx. These habitats support abundant C. sapidus (D a rn e ll, 1959; Perry ,1975; O rth & van M ontfrans ,1987; W illiam s

et al”

1990). The opportunity to utilize shallow, vegetated habitats may increase survival of juvenile C. similis. It is well known that juvenile C. sapidus utilize shallow,vegetated habitats as refuges from predation (Heck & W ilso n , 1987; W ilson 1987; O rth & van M ontfrans, 1990) and possibly as a source of detrital nutrition (M cC lintock 1991).

et al”

et al”

In contrast to Callinectes similis, C. sapidus were seldom observed in openbays except in the spring of 1990 and 1991. This season was correlated with periods of heavy freshwater influx into Mobile Bay, which may have flushed C. sapidus from shallow marshes or grassbeds into open-bays. It is possible that C. similis abundance declines when C. sapidus move into their habitats. Preda­ tion pressure from the more aggressive C. sapidus may play a role in excluding C. similis from both marshes and open-bays. Field caging experiments indicate a significant predatory impact of the former on the latter (Hsueh

et al” 1992 b).

Interannual and season variation in numbers of both species were observed in this study. At the Fort Morgan site,the summer peak abundance of C. similis in 1989 was not observed in 1990,but its spring peak abundance in 1990 was repeated in 1991. Similarly, the summer peak abundance of C. sapidus at Dauphin Island Airport Marsh was observed in both 1989 and 1990,the winter peak abundance only in 1989. Interannual and seasonal variations have also been reported in C. sapidus from the northern Atlantic (Hines et al” 1987; O rth & van M ontfrans, 1987) and the northeastern Gulf of Mexico (Living­ 1976). Factors such as food availability and recruitment success have ston

et al”

also been considered responsible for interannual and seasonal variations in blue crab abundance (O rth & van M o n tfran s ,1987; Hines

et al” 1990). Although

numbers of C. similis were reduced in Mississippi Sound in 1990 versus 1989,the abundance at the more stenohaline Fort Morgan site was consistent over the two year period. A stable population was even more evident at the Channel Marker site, where salinity remained high (>25 %). Sex ratios were generally skewed towards higher proportions of males. Female C. sapidus were more abundant at open-bay than shallow marsh sites. Similar skewed sex-ratio patterns have been reported for C. sapidus in

et al, 1987) and in Weeks Bay, Alabama (M cC lintock et al (1987) noted that female C. sapidus were more

Chesapeake Bay (Hines 1993). Hines

et al”

common than male crabs in open river basin habitats, but male crabs preferen­ tially utilized tidal creek habitats as molting grounds. The present study indi­ cates that the predominance of males in shallow marshes and tidal creeks is geographically widespread. Spring and fall spawning seasons for C. similis in Florida, North and South Carolina, and Texas estuaries have been suggested by W illiam s (1966) and T agatz (1967). Low numbers of sexually mature female C. collected in

similis

Mississippi Sound indicate that females are migrating to deeper waters in Mobile Bay or into nearshore coastal waters, where salinities are relatively high.

similis spawn in nearshore waters of the C. similis is similar to that of C. sapidus (M illik in & W illiam s 1984; W illiam s

T agatz (1967) found that female C.

Atlantic Ocean. Migration to higher salinity waters by sexually mature female ,



Ecology of blue crabs

253

et al” 1987). Numerous egg-bearing C. similis were collected from Sand Island (approximately 1km offshore of Mobile Bay) in September 1989 (M. D ardeau pers. comm.). Larvae of C sapidus cannot survive in low salinity environments (C ostlow & Bookhout 1959). We suggest that C. similis larvae 1984; Hines





may also be intolerant of low salinities. More egg-bearing Callinectes sapidus were collected than male and immature female crabs at the Channel Marker and Fort Morgan bay-mouth sites. This suggests that sexually mature female C. sapidus behave similarly to conspecifics in the North Atlantic and other regions of the Gulf of Mexico, e. g” they migrate to the mouths of bays to extrude and carry their eggs (D a r n e ll, 1959; Perry, 1975; M illik in & W illiam s, 1984; W illia m s ,1965,1984; Hines

et al”

1987). This facilitates the release of zoeae into high salinity offshore waters where development to megalopae occurs (Epifanio W illiam s ,1984).

et al”

1984; M illik in &

Population size structures of Callinectes similis differ between marshes and open bays: only small (8% ). In contrast, C. sapidus occurs abundantly only in open-bays in spring during freshwater influxes. Distinct habitat preferences of the two species may be related to differences in osmoregulatory ability. 2. Sexually mature C. similis and C. sapidus females migrate to high salinity habitats to extrude and carry their eggs. Egg-bearing females of both species extrude and release larvae into offshore water,and their megalopae re-invade estuaries. 3. Recruitment of C. similis at Dauphin Island Airport Marsh occurs from late spring to early fall,whereas recruitment of C. sapidus occurs almost yearround. Juvenile C. similis apparently recruit into open-bays and to a lesser • extent salt marshes,juvenile C. sapidus, however,into shallow salt marsh and grassbed habitats. 4. Significant differences in maximum body size, size at sexual maturity, fecundity, and total reproductive energy output occur between C. similis and C. sapidus. 5. Callinectes sapidus are selectively parasitized by the rhizocephalan barnacle Loxothylacus texanus; parasitic barnacles do not infect the sympatric C. similis.

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255

Acknowledgements We wish to thank the crew of the RV Deborah B and the staff at Dauphin Island Sea Laboratory, Alabama. Patrick Harper, Sharon Delchamps, A nita Owen, Jenny Cook, Meredith Cornett, Emily Knott, Scott Rikard, Jeff Vernon, Mike Lares,Marc Slattery, Gottfried Schinner,and Chi-Mei Hsueh kindly assisted with field collections of blue crabs. Drs. Richard M odlin, George Cline, Robert Angus, and Ken Marion provided valuable comments on earlier drafts of this manuscript. We also acknowledge the Department of Biology, UAB, for providing logistical and financial assistance. Dr. Ken Marion kindly assisted with the preparation of a Sea Grant Proposal awarded to James McClintock and Pan-Wen Hsueh to conduct this study. This work is a result of research sponsored in part by NOAA National Sea Grant College Program, U.S. Department of Commerce under grant # NA89AA-D-CZ052 by the Mississippi-Alabama Sea Grant Consortium to the University of Alabama at Birmingham. The U.S. Government and the Mississippi-Alabama Sea Grant Consortium are authorized to reproduce and distribute reprints notwithstanding any copyright notation that may appear hereon. This is MESC Contribution No. 209 and Contribution No. 181 of the Aquatic Sciences program at the University of Alabama.

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