0045258 JOURNALOF CRUSTACEANBIOLOGY, 11(1): 103-112, 1991 SEX-SPECIFIC GROWTH AND SURVIVAL IN THE MOLE CRAB EMERITA PORTORICENSIS (SCHMITT) Miguel P. Sastre ABSTRACT The anomurancrab Emerita portoricensis often occursin the intertidalzone of sandybeaches in the Caribbean.Four sites on PuertoRico, within Mayagiiezand Aiiasco bays, were sampled for 2 yearsin orderto constructsize-frequency distributions and determinerelative growth and survival of each sex. Emeritaportoricensis was regardedas a dioecious species, since no inter- mediateforms were found in any of the 30,755 specimens.Size frequencydistributions of male and female mole crabs were not significantlydifferent between sites. However, the maximum length of the carapaceof male mole crabs (11 mm) was shorterthan that of females (19 mm). Growthcurves based on size-frequencydistributions indicated that females grew significantly fasterthan males. Densities of males were greatestfollowing recruitment. The percentagesur- vival of youngindividuals (< 5 months of age)was higherfor males than femalesand percentage survival of older mole crabs was greaterfor females. Results of this study indicate that sexual heterogeneitiesin size structureof populations can be explained by differentialgrowth and survival of the sexes. Mole crabs of the genus Emerita (Ano- spermatophoresto pleopods of larger fe- mura: Hippidae) are commonly encoun- males (MacGintie, 1938; Wharton, 1942; tered in the intertidalzone of temperateand Knox and Boolotian, 1963; Efford, 1967; tropical sandy beaches. These crabs usually Diaz, 1980; Subramoniam, 1984). aggregateand migrate alongshore (Dillery Caribbean mole crabs, E. portoricensis and Knapp, 1970). Smaller crabs typically (Schmitt, 1935), spawn continuously occur in the upper portion of the littoral throughout the year. The proportion of zone, while largerorganisms are usually lo- ovigerous females increases with growth to cated at lower levels within this zone (Ef- larger size classes. The largest carapace ford, 1965). Like other beach organisms, length observed for megalopae is approxi- species of Emerita migrate synchronously mately 3 mm, which coincides with the with the tide and maintain zonation smallest carapacelength of the firstjuvenile (MacGintie, 1938; Wharton, 1942; Efford, instar (Sastre, 1988). 1965; Cubit, 1969). The hypothesis of whether sexual heter- Within the genus, male crabs generally ogeneities in size structuresof E. portori- are smaller than female crabs (Goodbody, censis are caused by differentialgrowth and 1965; Barnes and Wenner, 1968; Wenner, survival rates between sexes is examined. 1972; Subramoniam, 1977; Diaz, 1980). The possible occurrenceof protandricher- This sexual differencein size has been ex- maphroditismis also examined in this spe- plained in terms of differentialsurvival and cies. growth rates for Emerita analoga (see Ef- ford, 1967) and Emerita talpoida(see Diaz, 1980). Subramoniam (1981) proposed re- MATERIALSAND METHODS version of sex from male to female for the A descriptionof study sites and methods employed case of Emerita asiatica. Although protan- to monitor populationsof E. portoricensiswere pre- dry also was suggested for E. analoga (see viously described(Morelock et al., 1983;Sastre, 1988). Barnes and Wenner, 1968; Wenner, Briefly,four study sites wererandomly selected at Ma- 1972), yagiiezBay and two at Aiiasco Bay, Puerto Rico. Sed- a further study did not support this hy- iment enclosed in 30 quadrats(1,254 cm2each) 10 cm pothesis (Wennerand Haley, 1981). deep was sampled at monthly intervals for 2 years at Precocious sexual maturity (neoteny) in each site. All sediment was sieved through2-mm and males has been reportedin at least five spe- 1-mm mesh size sieves. Emeritaportoricensis retained in each quadratwere sexed and the carapacelength cies of Emerita (not includingEmerita por- was measured. These data were used to construct toricensis) (see Efford, 1967). Neotenous monthly size-frequencydistributions (Figs. 1, 2). The males attach ribbon-shaped or spherical presenceof eggs in the abdomenof femalesand sperm 103 0045259 - 104 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 11, NO. 1, 1991 240 - February 1984 210 180 - 24( C) July 1983 21C 150 120 - 18c 0- 15t 0- 90- 60 - 12C 0- 30 910- 6( 0- 0 - 240 March 1984 3( 0- ' 210 10- 180 241 0- August 1983 211 150 18( 120 15( 90 121 60 0- 9( 30 61 0 3; 570 0- >. 540 Zz 24( 0- September 1983 510 211 480 18l 450 15! 420 12 0- 390 0 9 360 !O 6 330 -I 3 300 LL 270 2424 o - October 1983 240 Rpril 1984 21 0- 210 18 0- 180 15 0 - LU 150 121! 0- 120 9 0 LJ 90 6 60 3 30 24 0 480 21 LO0 - 450 18 420 15 0 - 390 WJ 12 tO 360 9 0 - 330 6 ;O 0 O - 300, 310 C,) 270 24 10 - DeceMber 1983 240 nay 1984 21 .O 210 18 180 s0 - 15 150 20 - 12 120 990 - 90 6i0 _ 60 3 30 0 24 40 - January 1984 240 June 1984 2J LO 210 11 10 - 180 - SO 150 121;20 120 90 - 90 60 - 60 30 30 0 . .. 0 3 4 S 6 7 8 9 10 1 12 13 14 15 16 17 18 19 3 4 5 6 7 8 9 10 14 12 13 14 15 16 17 18 19 CARAPACE LENGTH (MM) Fig. 1. Size-frequency distributions of females of Emerita portoricensis (individuals/15.48 m2) at all sites for 1983-1985. 0045260 SASTRE: GROWTH AND SURVIVAL IN EMERITA PORTORICENSIS 105 240 July 1984 210 180 150 120 90 60 30 1984 August 24( 0 - January 1985 21C 18C - 15( 120 - 12( 0 90 - 92 0 60 6C0 - 30 31 - 0 240 SepteMber 1984 0 24( - February 1985 210 211 00- 180 z 18( 0 - 150 151 0 - 120 121 90 _ 9( 60 6 0 30 3 C~ O 0 - 540 24 510 21 480 18 - 0 450 15 420 12 ;00 -r 390 9 360 6 lO - 330 3 10 4 300 270 24 t0 Apri 1 1985 21 LO 240 October 1984 I-m 210 18 0 - 180 15 O 150 12 - 9 0 - 120 30 -J 6 390 60 330 30 mG...~...,~ 27 ?o 0 240 HoveMber 1984 24 (In 210 21 180 18 150 15 50 - 120 12 90 9 60 620 30 330 0 0 240 Decemnber 1984 24 40- June 1985 210 21 LO 180 150 15 50- 0 120 12 20 90 90- 60 60 30 30 0 O 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 CARAPACE LENGTH (MM) . 0045261 106 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 11, NO. 1, 1991 360 July 1983 320 280- 240- 360 - February 1984 200- 320 160 - 280 120 240 80 200 40- 160 0 . L 480 120 440 _ 80 400- 40 360 August 1983 0 360 March 1984 320 - 320 280 - 280 240 - 240 200- 200 160 160 120- 120 80 80 40- 0 40 0 0 360 - eptenber 1983 600 z 320 560 280- 520 240 - 480 200- 440 160 - 400 120 360 April 1984 80 320 40- 280 U- 0 240 LLI 360 October 1983 200 320 160 280- 120 240- 80 200- 40 160 - 0 120 880 LLI 80 840 40 - (I) 800 36( o'D L November 1983 760 32t 720 28C 680 1 - 24C D - 640 600 20C 0 - 16C O - 560 12C O - 520 8(C 480 4( 440 400 36C 30 1.December 1983 360 Hay 1984 (n 320 32C 28C o - 280 m 24C o - 240 20( 3 - 200 160 16? o - 3 121 120 81 80 4( 40 0 0 - 361 0 - January 1984 360 June 1984 0 321 320 28( O0 - 280 - 241 240 0 - 20( 200 16 0- 160 121 0 120 8 80 0 - 4 0 - 40 0 3 4 5 6 7 8 9 10 11 3 4 5 6 7 8 9 10 11 CARAPACE LENGTH (MM) Fig. 2. Size-frequency distributions of males of Emerita portoricensis (individuals/15.48 m2) at all sites for 1983-1985. 0045262 0 SASTRE: GROWTH AND SURVIVAL IN EMERITA PORTORICENSIS 107 600 560 520 480 440 400 360 320 280 240 200 160 361 0 January 1985 120 321 0. 80 28( 0 40 24( 0 0 20( D 360 August 1984 16( 0 320 12( 280 8( D 240 4( 0: 200 0 0. 160 361 0 February 1985 D- 0 120 32( 0 80 281 0 z 40 24( 0 0 201 LU 360 Septexber 1984 16( 0 320 12( 0 0. 280 8( 0 240 4( UI 200 OC 160 361 120 32( LLJ 80 28( 40 24( 0 201 0 - 680 16( o - LU 640 121 0 600 81 560 4(41 520 480 36( 0 - April 1985 w 440 321 0- 400 28( 360 October 1984 24( O 320 201 280 16( 240 121 o - 0 200 8( 0 - 0 - 160 41 120 O 80 48( 40 441 40( C') 0 360 - oveber- 1984 361 D - May 1985 320 32( o_ - m 280 28( 240 24( o 200 201 160 16( 120 12( 80 8( 40 4( 0 360 December 1984 36( 0O - _ June 1985 320 32( 0 - 280 281 0 240 24( 0 200 20( 160 161 120 121 80 8 40 4 un - I, m 3 4 5 6 7 8 9 101o 1 3 4 5 6 i 8 9 10 11 CARAPACE LENGTH (MM) 0045263 108 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 11, NO. 1, 1991 Percentagesurvival was calculatedfor each size class as: (S,,+/S) x 100. A three-wayANOVA test was used to evaluatedif- 560 - O \ ferences in percentagesurvival among age, months, W 40-40 - and sexes (Sokal and Rohlf, 1981). Regressionanalysis was used to determinepossible Z 30 - relationshipsbetween monthly growth and carapace W 20 - lengthfor each sex.