Stoner and Sandt: Habitat quality for Strombus gigas in seagrass meadows 



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fluence on site differences. Despite these limitations, 

 high mortality rates and low or negative growth rates 

 provide clear evidence that five of the sites without 

 resident conch (except site C2) were poor habitats for 

 juvenile queen conch. 



Stoner and Waite (1990) showed that 1- and 2-year- 

 old conch actively select areas with intermediate 

 seagrass biomass over areas with high and low biomass. 

 Additionally, it was shown that seagrass shoot densi- 

 ty and biomass were good predictors of juvenile conch 

 distribution within nursery habitats. Among the sites 

 which were known to serve as natural habitats for 

 queen conch (CI, C3, Nl) there was a direct relation- 

 ship between seagrass biomass and habitat quality, as 

 measured by conch growth. On the larger scale of this 

 study, however, seagrass density was not a good pre- 

 dictor for conch success, and other variables— probably 

 trophic factors— need to be examined. 



Adult and subadult queen conch are generally con- 

 sidered to be herbivores (Robertson 1961, Randall 

 1964, Hesse 1976), but recent data on juvenile conch 

 living in seagrass beds show that seagrass detritus is 

 consumed in large quantities, and macroalgae such as 

 Batophora oerstedi and Laurencia poitei are primary 

 sources of nutrition (Stoner and Waite 1991). Field 

 experiments have shown that juvenile conch can have 

 a major influence on detritus and other characteristics 

 of the benthic environment, with much of the benthic 

 productivity being removed by grazing (Stoner 

 1989a, b). The observed depletion of detrital biomass 

 followed by decreasing conch growth rates at the 

 moderate biomass site LI suggests trophic limitation 

 at that station. 



Abundance of macrophytes does not necessarily pro- 

 vide superior habitat. Site L2 had a thick accumulation 

 of detritus and high seagrass biomass, but conch 

 transplanted to that site had high mortality and low 

 growth rates. Randall (1964) noted that young conch 

 may not be able to move readily through dense seagrass 

 stands, and are rarely found in that habitat type. 



No attempt is made here to equate mortality rates 

 in the transplants with natural mortality because of 

 potential density-dependent effects on predation rate, 

 partial exclusion of predators by cages, and uncertainty 

 in the sources of mortality. Observations of potential 

 queen conch predators in the enclosures were rare. The 

 tulip snail Fasciolaria tulipa was seen attacking conch 

 three different times, and in a few instances the giant 

 hermit crab Petrochirus diogenes was found in empty 

 shells; both of these species are known predators on 

 queen conch (Randall 1964, Jory 1982). The apple 

 murex Murex pomum and hermit crabs Paguristes sp. 

 were observed occasionally in the enclosures, but both 



are thought to be scavengers (Jory and Iversen 1983, 

 Iversen et al. 1986). Lack of broken shells, low body 

 condition, and low shell growth rates lead us to con- 

 clude that much of the mortality in "poor" habitats was 

 a result of inadequate food resources. On the other 

 hand, large differences in mortality rate between the 

 two enclosures at two of the sites suggest that heavy 

 predation may have occurred in some cages. 



The fact that one site with no resident juvenile conch 

 (C2) produced high growth rates and survivorship sug- 

 gests that queen conch do not occupy all suitable 

 habitats. Several explanations are plausible: Pelagic 

 larvae may not be dispersed to the area, settlement 

 may not occur, or early-juvenile stages may suffer high 

 mortality rates at the site. It is also possible that, 

 historically, site C2 has been a nursery site for queen 

 conch. Macroscopic conch shell fragments at the site 

 were found in a density of 3.0 fragments/m 2 ; this is 

 lower than the density at site CI (6.8/m 2 ), but con- 

 siderably higher than that in other non-conch areas 

 (e.g., density at LI was 0.05 fragments/m 2 ) (Stoner 

 and Yoshioka, unpubl. data). 



Presence or absence of juvenile queen conch in 

 seagrass meadows appears to be mediated by site- 

 specific characteristics. Habitat quality, as measured 

 by growth rate, was related directly to seagrass 

 biomass in natural conch sites; however, the hypothesis 

 that habitats with similar depth, sediments, and 

 macrophyte biomass have equivalent qualities as 

 nurseries for juvenile conch is clearly rejected. Until 

 the distributional ecology of queen conch is understood, 

 small-scale transplanting will be a valuable tool in 

 manipulations designed to test the significance of other 

 environmental variables, and as a means of testing 

 habitats where enhancement of stocks is proposed. 



Acknowledgments 



This research was supported by a grant from the Na- 

 tional Undersea Research Program of the National 

 Oceanic and Atmospheric Administration, U.S. Depart- 

 ment of Commerce. N. Christie, K. McCarthy, and 

 E. Wishinski helped with the fieldwork, and P. Berg- 

 man conducted the sediment and macrophyte analyses. 

 B.L. Olla provided help in design of the experiment and 

 through discussion of the results. J. Chaplin, L.S. Mar- 

 shall, J.M. Waite, R.I. Wicklund, and two anonymous 

 reviewers provided helpful comments on the 

 manuscript. 



