LEBER AND GREENING: COMMUNITY STUDIES IN SEAGRASS MEADOWS 



Several crustaceans emerge from the substratum 

 and forage at night in grassbeds, including pink 

 shrimp, Pendens duorarum, some majid crabs 

 (notably Pitho and adult Libinia at our site), and 

 alpheid and processid shrimps (Fuss 1964; Fuss and 

 Ogren 1966; Hughes 1968; Kikuchi and Peres 1977; 

 Saloman 1979; Greening and Livingston 1982; Leber 

 1983). Emergence of nocturnal organisms from the 

 substratum after dark would explain some of the 

 variability between day and night collections of in- 

 vertebrates. Higher densities of diurnally active 

 animals in night samples may be due to nocturnal 

 vertical migrations up grass-blades. Animals located 

 near the tips of blades are clearly more vulnerable 

 to capture by either device; even the scrape misses 

 individuals trapped between grass-blades and 

 substratum by the crossbar, an event less likely to 

 occur to an individual near a blade tip. Fishes were 

 probably less abundant in daytime trawl collections 

 because of avoidance reactions to the clearly visible 

 net. 



Emergence and vertical migration do not account 

 for all of the increases in invertebrate abundance in 

 night samples. The case of the arrow shrimp, 

 Tozeuma carolinense, is interesting in this regard. 

 We expected no day-night sampling differences for 

 Tozeuma with either device, based on evidence that 

 Tozeuma inhabit the region near tips of grass-blades, 

 both during the day and at night (Main in press). As 

 expected, Tozeuma were collected in roughly equal 

 numbers in both day and night scrape samples. 

 However, almost an order of magnitude more 

 Tozeuma were taken in night trawl samples than dur- 

 ing daytime collections (Leber and Greening, unpubl. 

 data). It appears that Tozeuma may be capable of 

 avoiding the trawl, which is highly visible during the 

 day. These shrimp have keen vision in daylight and 

 are capable of rapid movement (up to 30 cm) via a 

 caridoid escape response (Main in press). They need 

 only move down blades, closer to the substratum, to 

 avoid the trawl net. 



This study suggests that many demersal fishes and 

 epibenthic invertebrates may be more important 

 numerically in seagrass communities than indicated 

 by collections made with an otter trawl. Species 

 ranks and relative abundances of these organisms 

 determined from trawl collections in seagrass beds 

 should be interpreted with care Whereas trawl col- 

 lections may be satisfactory for monthly or year-to- 

 year comparisons of single species abundances 

 within a seagrass habitat, application of such data 

 to examination of predatory-prey relationships (e.g, 

 energy flow and optimal-diet models) or other biotic 

 interactions in grassbeds may lead to erroneous 



interpretations. The combined approach of day-night 

 sampling with both an otter trawl (for water-column 

 fishes) and a crab scrape (for demersal organisms) 

 is recommended for seagrass studies. 



ACKNOWLEDGMENTS 



We thank B. J. Freeman, J. Gerritsen, R. Howard, 

 C. Koenig, F G. Lewis, K. Main, G. Morrison, and 

 J. Ryan for comments and reviews of earlier drafts 

 of this manuscript. R. J. Livingston provided 

 technical support and M. Babineau provided help 

 with the graphics. 



LITERATURE CITED 



DUGAN, P. J. 



1983. Seasonal and geographic distribution of seven decapod 

 crustaceans in Apalachee Bay, Florida. Contrib. Mar. Sci. 

 26:65-79. 

 Dugan, P. J., and R. J. Livingston. 



1982. Long-term variation of macroinvertebrate assemblages 

 in Apalachee Bay, Florida. Estuarine Coast. Shelf Sci. 14: 

 391-403. 

 Fuss, C. M., Jr. 



1964. Observations on burrowing behavior of the pink shrimp, 

 Penaeus duorarum Burkenroad. Bull. Mar. Sci. Gulf Caribb. 

 14(l):62-73. 

 Fuss, C. M., and L. H. Ogren. 



1966. Factors affecting activity and burrowing habits of the 

 pink shrimp, Penaeus duorarum Burkenroad. Biol. Bull. 

 (Woods Hole) 130:170-191. 

 Gore, R. H., E. E. Gallaher, L. E. Scotto, and K. A. Wilson. 



1981. Studies on decapod Crustacea for the Indian River 

 region of Florida. XI. Community composition of structure, 

 biomass and species areal relationships of seagrass and drift 

 algae-associated macrocrustaceans. Estuarine Coastal Shelf 

 Sci. 12:485-508. 



Greening, H. S., and R. J. Livingston. 



1982. Diel variation in the structure of seagrass-associated 

 epibenthic macroinvertebrate communities. Mar. Ecol. 

 Prog. Ser. 7:147-156. 



Heck, K. L., Jr. 



1976. Community structure and the effects of pollution in sea- 

 grass meadows and adjacent habitats. Mar. Biol. 35:345-357. 



1977. Comparative species richness, composition, and abun- 

 dance of invertebrates in Caribbean seagrass (Thalassia 

 testudinum) meadows (Panama). Mar. Biol. 41:335-348. 



1979. Some determinants of composition and abundance of 

 motile macroinvertebrate species in tropical and temperate 

 turtlegrass (Thalassia testudinum) meadows. J. Biogeogr. 

 6:183-200. 



Heck, K. L., Jr., and R. J. Orth. 



1980. Structural components of eelgrass (Zostera marina) 

 meadows in the lower Chesapeake Bay-decapod Crustacea. 

 Estuaries 3:289-295. 



Heck, K. L., Jr., and G. S. Wetstone. 



1977. Habitat complexity and invertebrate species richness 

 and abundance in tropical seagrass meadows. J. Biogeogr. 

 4:135-142. 

 Hooks, T. A., K. L. Heck, and R. J. Livingston. 



1976. An inshore marine invertebrate community: structure 



449 



