522 



Fishery Bulletin 91(3), 1993 



10 12 14 16 18 

 Time (hours) 



Figure 4 



Percentage of early juvenile conch on the sediment surface at 

 Neighbor Cay, Bahamas, at different times of day. Local time 

 of nautical twilight (NT), sunrise, and sunset are shown. Ob- 

 servations were made between 17 January and 28 February 

 1989. State of tide is indicated for the individual times of 

 observation. 



rily epifaunal. Our observations of burrowing rhythms 

 in 35-54 mm queen conch at Neighbor Cay corrobo- 

 rate Randall's (1964) conclusion that early juvenile 

 queen conch emerge from the sediment at night, but 

 the extent to which smaller conch remain buried is 

 unclear. Because early juvenile conch were never ob- 



served between the summer spawning season and the 

 January discovery of 35-mm conch, we hypothesized 

 that burrowing progresses from full-time activity in 

 conch less than approximately 25 or 30 mm until near 

 continuous epifaunal existence in conch at approxi- 

 mately 1 year of age. 



Early juvenile conch lack long spines and strong 

 shells characteristic of older juveniles, and burrowing 

 in shallow waters is probably a predator avoidance 

 mechanism (Iversen et al., 1986, 1989). Laboratory ex- 

 periments have shown that shell strength in queen 

 conch increases rapidly with length above 55 mm ( Jory 

 and Iversen, 1988), similar to the size at which conch 

 at Neighbor Cay became apparent on the sediment 

 surface. Shell structure may also explain the typical 

 age-specific type IV survival patterns observed in 

 Strombus gigas and S. costatus (Appeldoorn, 1984). 

 The significance of shell size and spination in provid- 

 ing protection from predators has been shown experi- 

 mentally for other gastropods (Palmer, 1979). 



Conch burrowing may also vary with age-related 

 morphological constraints or with microhabitat fea- 

 tures. For instance, burrowing abilities in queen conch 

 may be reduced with increase in size and associated 

 apical spination (Marshall, 1988). Also, low sediment 

 cohesiveness probably facilitates burial. This may ex- 

 plain the high density of early juveniles in the surf 

 berm at Neighbor Cay. 



Natural light cycles 



Modified tight cycles 



12:00 12.00 1200 



Time of day (hours) 



12:00 



Figure 5 



Frequency of burrowing in early juvenile queen conch held in the laboratory. Four days of 

 natural light cycle (12 hours light:12 hours dark) were followed by 4-day periods of modi- 

 fied light cycle. The modified light treatments were A = continued natural light cycle, B = 

 constant light, and C = constant dark. Values shown are the means for three separate runs 

 (see text for explanation of methods). 



