Sandt and Stoner: Ecology of earlyjuvenile Strombus gigas 



521 



Table 2 



Daily growth rate in shell length and body condition factor 1 of early 

 juvenile conch (37^19 mm shell length) held in enclosures on sand, 

 seagrass, and rock habitats. Values are means ±SD. Two enclosures 

 with eight conch each were used in the analysis. Differences among 

 treatments were different for both growth rate (ANOVA on log- 

 transformed data, F = 769.35, P < 0.001 1 and condition factor luntrans- 

 formed data, F = 12.82, P = 0.034). Letter codes indicate mean values 

 which were not significantly different I Tukey's multiple comparison 

 test, P> 0.05). 



Treatment 

 Sand habitat Seagrass habitat Rock habitat 



Growth rate 



(mm/day) 0.012 ± 0.001" 



Condition factor 



(g/mm) 0.027 ± 0.001" 



0.112 ± 0.010 1 ' 0.094 ± 0.002 f ' 



0.042 ± 0.002 h 0.039 ± 0.005" b 



'Condition factor is defined as the wet weight of soft tissue divided by 

 shell length of an individual. 



P < 0.001), and Tukey's multiple range test indicated 

 that conch in sand grew at a rate lower than those in 

 seagrass and rock habitats (P < 0.001). Growth rates 

 (P = 0.124) for conch in seagrass and rock were the 

 same. 



Condition factor was also lower in the sand treat- 

 ment (0.027), than in seagrass (0.042) and rock habi- 

 tats (0.039) (Table 2). The habitat effects were signifi- 

 cant (ANOVA, F = 12.82, P = 0.034). Differences in 

 mean condition factor in seagrass and sand treatments 

 were significant (Tukey's test, P = 0.035), but the dif- 

 ferences were not significant in sand and 

 rock (P = 0.063 ) or seagrass and rock (P = 

 0.648). 



Stomach contents of conch from the three 

 different habitats were primarily algae and 

 sand (Table 3). Conch enclosed in the rocky 

 zone had a lower percentage of sand (26.5%) 

 in their stomachs than conch in the other 

 two habitats (48.6-58.2%); however, algae 

 made up over 87% of the organic constitu- 

 ents of the stomach contents in all cases. 

 Detritus particles, identified as T. 

 testudinum, represented a small percentage 

 of stomach contents of conch held in 

 seagrass and rock habitats (Table 3). Fora- 

 minifera and Gastropoda found in the stom- 

 achs were small forms (<lmm), probably 

 taken incidentally with sand particles. The 

 only polychaete found was Spirorbis sp., a 

 small, sessile, tube-builder probably taken 

 with epiphytic algae. Non-sand weight per 

 stomach was highest for the conch in the 

 rock habitat (Table 3). 



Burrowing experiments 



Fewer than 30% of the early juveniles observed 

 in the field were found on the surface during 

 the day, whereas almost 100% were found on 

 the surface at night (Fig. 4). Detailed observa- 

 tions near crepuscular hours showed that conch 

 buried themselves before sunrise and surfaced 

 before sunset. There was no indication of tidal 

 rhythm on burrowing during our study. 



Juvenile conch demonstrated lower percent- 

 ages of burrowing in the laboratory than in 

 the wild; but a marked rhythm in burrowing 

 was observed during the four days of acclima- 

 tion (Fig. 5). After light manipulations began, 

 conch exposed to natural cycles of light (12 

 hours: 12 hours) continued to emerge from the 

 sediment in darkness hours (Fig. 5). Conch 

 held in constant darkness demonstrated a 

 much reduced frequency of burying themselves 

 but this pattern became more erratic by the 

 end of the experiment. Conversely, animals 

 held in constant light continued to show natural 

 rhythms of activity (i.e., burying themselves and sur- 

 facing on the light-entrained schedule). 



Discussion 



Queen conch less than 50 mm SL have rarely been 

 observed on the sediment surface. Iversen et al. ( 1986) 

 concluded that conch spend their first year of life bur- 

 ied in the sediment, after which time they are prima- 



