1 SEX RATIOS OF TWO SPECIES OF SPIDER CRABS, LIBINIA DUBIA H. MILNE EDWARDS, 1834 AND L. EMARGINATA LEACH, 1815, IN THE AREA OF GREAT BAY, NEW JERSEY BY SUSAN BETH O BRIEN 1,2 ), MATTHEW LANDAU 1,3 ) and KENNETH W. ABLE 2 ) 1 ) Marine Science Program, Richard Stockton College, Pomona, NJ 08240, U.S.A. 2 ) Marine Field Station, Institute of Marine and Coastal Science, Rutgers University, 132 Great Bay Blvd., Tuckerton, NJ 08087, U.S.A. ABSTRACT Over a 7 year period, Libinia emarginata and L. dubia were sampled in the Great Bay estuary (New Jersey) and the waters immediately adjacent to it, using a beam trawl and an otter trawl. At collection sites where L. emarginata was common, the average male : female ratio was 2.5 : 1. Males were also significantly larger than females. RÉSUMÉ Pendant une période de 7 ans, Libinia emarginata et L. dubia on été échantillonnés dans l estuaire de Great Bay (New Jersey) et dans les eaux adjacentes, en utilisant un chalut à perche et un chalut à panneaux. Là où L. emarginata était commun, la ratio moyenne mâles : femelles était 2,5 : 1. Les mâles étaient aussi, de façon significative, plus grands que les femelles. INTRODUCTION Two species of spider crabs (family Majidae), Libinia dubia H. Milne Edwards, 1834 and L. emarginata Leach, 1815, are commonly found in the temperate coastal waters of the western Atlantic, including our study area, Great Bay, New Jersey. While spider crabs of the genus Libinia are of no marketable value and therefore not commercially harvested, they may be periodically abundant in certain locations (Rathbun, 1925; Hildebrand, 1954; Musick & McEachran, 1972; Johnson, 1985; DeGoursey & Auster, 1992), suggesting that they may have a considerable impact on local ecosystems. 3 ) Corresponding author. Koninklijke Brill NV, Leiden, 1999 Crustaceana 72 (2)
2 188 S. B. O BRIEN, M. LANDAU & K. W. ABLE However, little is known about the life history of Libinia. Much of the published literature about their distribution and habits comes from more general examinations of crustaceans in specific areas (Hildebrand, 1954; Williams, 1965; Musick & McEachran, 1972; Johnson, 1985; Ropes, 1989), although there have been some scattered natural history studies specifically on Libinia spp. (Rathbun, 1925; Aldrich, 1974; DeGoursey & Auster, 1992). O Brien et al. (1995) found that both species seemed to prefer a sand mix bottom type with no salinity preference. L. emarginata and L. dubia were common in the study area during the fall and spring (water temperatures 10 to 20 o C), but both species were somewhat scarce in the summer and rare in the winter, suggesting that both high and low temperatures precipitated an exodus from Great Bay, or the crabs buried themselves in the substrate and were unavailable to the sampling gear. Winget et al. (1974), Wenner & Wenner (1989), and DeGoursey & Auster (1992) found that, at least at times, there were more males in their study populations than females. The aim of this study is to confirm the reported unbalanced sex ratios in a relatively confined estuary. MATERIALS AND METHODS Trawl survey data, from specific locations throughout Great Bay and adjacent waters, were collected for seven years, , by Rutgers University Marine Field Station in Tuckerton, New Jersey; Libinia were collected from 16 of the stations (fig. 1). Most of the stations were sampled monthly, weather and other conditions permitting. Collections were made with a 1 m beam trawl (epibenthic sled,1 m wide, 3 mm mesh) or a 4.9 m wide otter trawl (19 mm mesh wings, 6.3 mm mesh liner). Catch data were normalized to consider the differences in trawl size and the number of minutes the trawl was in the water; this catch per unit effort (CPUE) was measured as: CPUE =(crabs per net meter-minute) 100 Libinia collected during samplings were identified to species by counting median carapace spines (Williams, 1965). Carapace widths (CW) of up to twenty randomly selected crabs per tow were measured to the nearest millimeter, and sex was determined. The sampling periods covered the four (unequal) seasons: winter (December and January, there was no sampling in February), spring (March, April, May, and June), summer (July, August, and September), and fall (October and November) (see also O Brien et al., 1995).
3 SEX RATIOS OF SPIDER CRABS IN GREAT BAY 189 Fig. 1. Map of the sampling stations in Great Bay, New Jersey, and in adjacent waters where Libinia were taken. RESULTS Libinia emarginata was the more consistently collected species across seasons and sites (77.4% of the 1201 spider crabs identified). Table I gives the CPUE and CW values, as a function of season, for each sex of L. dubia and L. emarginata. The CPUE for both species was relatively low during the winter, greater in the summer, and peaked during spring and fall. L. emarginata were abundant (defined here as having a mean seasonal CPUE > 50) at: sites 5, 6, 7, 8, 9, and 27 during the spring; sites 4, 5, and 6 during the summer; and sites 5, 7, 8, 9, and 10 during the fall. Of these 14 season-sites, the male : female ratio was 1.0 at one site, but this ratio was > 1.0 in eleven of the remaining thirteen sites, which is significant (p = 0.011, one-tailed binomial test; Zar, 1996). The two sites which were not in the group had a male : female ratio of 0.91 and Of the eleven sites dominated by males, six had male : female ratios > 2.0 (range was ). The average male : female ratio for all 14 sites was 2.5.
4 190 S. B. O BRIEN, M. LANDAU & K. W. ABLE TABLE I The CPUE and mean CW (mm) ± standard deviation, for both species of spider crab collected during Libinia emarginata Libinia dubia males females males females Spring CPUE Spring mean CW ± SD 62.0 ± ± ± ± 18.9 Summer CPUE Summer mean CW ± SD 60.8 ± ± ± ± 22.1 Fall CPUE Fall mean CW ± SD 64.4 ± ± ± ± 16.4 Winter CPUE Winter mean CW ± SD 85.3 ± ± Adult L. dubia were common only at site 7 during the spring (CPUE = 56), with a male : female ratio of 1.33; this difference in the sexes was not significant (p >0.05, Wilcoxon test for paired data; Siegal, 1956). Since adult L. dubia were not abundant at any other site during the spring, or at any site during any other season, no average sex ratio could be determined. Male L. dubia were slightly larger than females (fig. 2), but the difference was not found to be significant (p >0.05, Mann-Whitney U-test; Zar, 1996). However, the L. emarginata males were significantly larger than the females (p <0.05). The CW of L. dubia males measured up to 75 mm, and males had a greater CW size range than females. Male L. emarginata, which measured up to 88 mm in CW, also had a greater CW size range than females (fig. 2). DISCUSSION Libinia emarginata and L. dubia were most common in the fall and spring seasons. There were large declines in the catch per unit effort in the summer and winter. Most sites had a peak abundance during May and October. Winget et al. (1974), using lobster traps, found in Delaware Bay a rapid decline in L. emarginata abundance in late November, with no spider crabs caught from late February to late March, but relatively high numbers of crabs caught during the summer. They concluded that L. emarginata possibly undergo a short period of dormancy during winter months or migrate to deeper waters. The stresses of summer, especially high temperatures and low dissolved oxygen levels, may be less extreme in Delaware Bay than in Great Bay, because Delaware Bay is deeper and better flushed. This difference may explain the absence of summer crabs that was recorded in our study, but not observed in Delaware Bay.
5 SEX RATIOS OF SPIDER CRABS IN GREAT BAY 191 Fig. 2. Mean season-combined CW distribution for both species of Libinia as a function of the crab s sex. DeGoursey & Auster (1992) observed that L. emarginata in winter migrated into deeper waters and aggregated in dense patches where they remained buried in the substrate until spring. Since few spider crabs were collected during the winter in our study, the possibility exists that L. dubia and L. emarginata are moving offshore and burying. If the crabs are moving to deeper waters, the location must be beyond station 1, a site representative of shallow coastal environments (fig. 1). We found an average male : female ratio of 2.5 at sites where L. emarginata were common. Wenner & Wenner (1989) collected Libinia in trawls and found males significantly outnumbered females during the winter when the male : female ratio was DeGoursey & Auster (1992) used direct observation and video taping to observe mating aggregations of L. emarginata; they found a male : female ratio of 1.1, and that males were significantly larger than the females they
6 192 S. B. O BRIEN, M. LANDAU & K. W. ABLE were paired with. Our male : female ratio value is similar to the 3.28 that Winget et al. (1974) found. Our results are also similar to their observation that males were, on average, about 15% larger than females. One of their suggested possible explanations was that there might be a difference in their ability to capture crabs of different sexes. However, the trawls used in our study, in contrast to the lobster traps used by Winget et al. (1974), would make differential capture of the sexes less likely to be an artifact of the collection method. ACKNOWLEDGEMENTS The authors would like to thank Dr. Paul Jivoff for his helpful comments. REFERENCES ALDRICH, J. C., Allometric studies on energy relationships in the spider crab Libinia emarginata (Leach). Biol. Bull., Woods Hole, 147: DEGOURSEY, R. E.& P. J. AUSTER, A mating aggregation of the spider crab (Libinia emarginata). Journ. northw. Atlantic Fish. Sci., 13: HILDEBRAND, H. H., A study of the fauna of the brown shrimp (Penaeus aztecus Ives) grounds in the western Gulf of Mexico. Publ. Inst. mar. Sci., Univ. Texas, 3: JOHNSON, D. F., The distribution of brachyuran crustacean megalopae in the waters of the York River, lower Chesapeake bay and adjacent shelf: implications for recruitment. Estuarine coast. Shelf Sci., 20: MUSICK, J. A.& J. D. MCEACHRAN, Autumn and winter occurrence of decapod crustaceans in Chesapeake Bight, U.S.A. Crustaceana, 22: O BRIEN, S. B., M. Landau & K. Able, Seasonal and spatial distribution of two species of spider crab in Great Bay, New Jersey. American Zool., 35: 67A. RATHBUN, M. J., The spider crabs of America. Bull. U.S. natn. Museum, 129: ROPES, J. W., The food habitats of five crab species at Pettaquamscutt River, Rhode Island. Fish. Bull., U.S., 87: SIEGAL, S., Nonparametric statistics for the behavioral sciences: (McGraw-Hill, New York). WENNER, E.L.& C.A. WENNER, Seasonal composition and abundance of decapod and stomatopod crustaceans from coastal habitats, southeastern United States. Fish. Bull., U.S., 87: WILLIAMS, A. B., Marine decapod crustaceans of the Carolinas. Fish. Bull., U.S., 65 (1): WINGET, R. R., D. MAURER & H. SEYMOUR, Occurrence, size, composition, and sex ratio of the rock crab, Cancer irroratus Say and the spider crab, Libinia emarginata Leach in Delaware Bay. Journ. nat. Hist., London, 8: ZAR, H.H., Biostatistical analysis: (3rd ed.; Prentice Hall, Upper Saddle River, New Jersey). First received 3 February Final version accepted 19 May 1998.