Sandt and Stoner: Ecology of earlyjuvenile Strombus gigas 



523 



Early juvenile conch were not present in seagrass 

 zones where 1- and 2-year-old conch were most abun- 

 dant; this observation suggests that the distribution 

 pattern was related to an ontogenetic shift in habitat 

 rather than differential survivorship. Conch between 

 35 and 54 mm probably moved from sand to vegetated 

 habitats to exploit higher algal concentrations. Stom- 

 ach analysis indicated that algae are the primary di- 

 etary components for early juvenile conch at Neighbor 

 Cay, similar to the diets of older individuals (Stoner 

 and Waite, 1991). In sandy substrates, these foods prob- 

 ably become limiting as conch grow in size and food 

 intake. This conclusion is supported by the results of 

 our enclosure experiment which showed low growth 

 for 37-49 mm conch held in the sand habitat. Growth 

 rates were much higher in seagrass (0.11 mm/day) and 

 rocky habitats (0.09 mm/day) than in sand (0.01mm/ 

 day), and were similar to the rate (0.09 mm/day) ob- 

 served in an earlier study of 1-year-old juveniles en- 

 closed in seagrass at the same site (Stoner and Sandt, 

 1991). 



Although earlyjuvenile conch were normally buried 

 during the day in the shallowest subtidal habitat and 

 in sparse seagrass at Neighbor Cay, we have observed 

 12-25 mm conch on the surface in deeper seagrass beds 

 with high shoot densities near Lee Stocking Island 

 (Stoner, unpubl. data). Similar mention of epifaunal 

 habits in early juveniles was made by Brownell (1977). 

 It seems likely, therefore, that queen conch have dif- 

 ferent strategies of defense depending upon habitat. 

 Conch recruiting to shallow sand habitats bury them- 

 selves in the substratum for protection from diurnal 

 predators, and demonstrate strong, diurnal rhythms. 

 Conversely, small conch inhabiting relatively dense 

 seagrass beds find shelter among the seagrass blades. 

 Burrowing would be inhibited by rhizome mats and 

 anoxic sediments typical of seagrass beds with high 

 biomass. 



Although early juvenile conch appear to have con- 

 siderable behavioral flexibility in avoiding predators, 

 two lines of evidence suggest that early juveniles be- 

 come less vulnerable to at least a certain suite of preda- 

 tors by the time they reach 50 or 60 mm SL. First, 

 queen conch of approximately 60 mm have been the 

 smallest individuals typically found on the surface dur- 

 ing the day. Second, resistence to shell crushing is 

 relatively low and constant up to 55 mm, then increases 

 rapidly with conch size (Jory and Iversen, 1988). Thus, 

 conch begin epifaunal existence once they are past the 

 stage of highest vulnerability. As shown in our enclo- 

 sure experiments, shallow sand bars probably become 

 inadequate for juvenile conch by the time they reach 

 50 or 60 mm. Higher food intake, therefore, probably 

 necessitates the move to vegetated habitats. However, 



as observed at Neighbor Cay, mortality can be very 

 high during the ontogenetic habitat shift. 



The proportion of early juvenile conch inhabiting 

 sand bars and seagrass habitats is unknown. Our con- 

 clusions on the importance of the sand or seagrass 

 interface, or both, to conch growth and survival is sup- 

 ported by two lines of circumstantial evidence. First, 

 Stoner and Waite (1990) reported increasing length 

 frequency of 1+ year class queen conch from sand bars 

 to adjacent seagrass habitats. Data reported here show 

 that the age-specific trend continues into the 0+ year 

 class and supports the significance of ontogenetic shift. 

 Second, large-scale distributional patterns in 1+ year 

 class queen conch in the Exuma Cays show that nearly 

 all major nurseries are located in seagrass meadows 

 immediately adjacent to coarse-grained, shallow sand 

 bars or beaches (Stoner et al., 1993). The compromise 

 between protection offered by infaunal existence and 

 food availability on the sediment surface appears to be 

 an important mechanism regulating the distribution 

 and abundance of juvenile queen conch in the vicinity 

 of Lee Stocking Island. 



Successful stock enhancement with Strombus gigas 

 will depend upon releasing young-of-the-year juveniles 

 into suitable habitats (Siddall, 1983). Hatchery pro- 

 duction of juvenile conch is now relatively well refined 

 (Davis et al., 1987; Davis 1993), but release proce- 

 dures are not (Stoner, unpubl. data). Given the high 

 vulnerability of conch less than 50-60 mm, extreme 

 care must be taken to insure placement of small conch 

 for stock enhancement in habitats with adequate food 

 and shelter from predators. Sediment characteristics 

 will be particularly important in the site selection be- 

 cause of the normal burrowing activity of early juve- 

 nile stages. Sediments should have high porosity and 

 be well oxygenated. Additionally, releases would best 

 be made at night when the conch are normally on the 

 sediment surface. It is clear from this study that the 

 behavior and habitat requirements of early juveniles 

 are different from those of 1-year-old conch and provi- 

 sion must be made for ontogenetic habitat shifts. Sand 

 bars close to seagrass meadows probably provide the 

 optimal habitat for early juvenile conch in the 

 Bahamas. 



Acknowledgments 



This research was supported by a grant from the Un- 

 dersea Research Program of NOAA, U.S. Department 

 of Commerce. We are grateful to R.I. Wicklund for 

 introducing us to the Neighbor Cay site, to B.L. Olla 

 for providing recommendations on rhythm experiments, 

 and to C. Kendall for suggestions on measurements of 



