Sandt and Stoner Ecology of early juvenile Strombus gigas 



519 



Enclosure experiment 



Habitat-specific growth rates were examined experi- 

 mentally in covered enclosures. Circular cages, 1.5 m 

 in diameter (1.77 m-) were constructed with polyethyl- 

 ene mesh (1 x 1cm) wired to a reinforcement bar 

 driven into the sediment. Two cages were built in zone 

 B, near transect 6 in bare sand, two were placed in the 

 seagrass zone E, and two were built in the rocky zone 

 between transect 11 and 12 (Fig. 2). The mesh was 

 pushed into the sediment approximately 5.0 cm to pre- 

 vent escape of conch. Tops of the cages were covered 

 with the same plastic mesh to exclude predators. 



Conch between 37 and 49 mm SL were collected in 

 zone B. After removing all visible invertebrates from 

 the 6 cages, 8 individually tagged and measured conch 

 were randomly assigned to each cage. This yielded a 

 density of 4.5 conch/nr, near the highest natural den- 

 sities of similar size class conch in the field. 



The experiment was initiated on 3 February 1989, 

 examined after 19 and 41 days for cage damage and 

 conch loss, and terminated after 63 days. Growth rates 

 were expressed in mm shell length per day. 



At the end of the enclosure experiment, conch were 

 collected and frozen for measurement of body condi- 

 tion factor and stomach analysis. After thawing, the 

 animals were drawn from their shells and rinsed to 

 remove feces and mucus. All undamaged conch were 

 blotted lightly and weighed (0.1 g). Condition factor 

 was expressed as the ratio of soft tissue weight:shell 

 length (g/mm). No significant sexual dimorphism oc- 

 curs in queen conch until the gonads begin to develop 

 at approximately 2 years of age; therefore, sex of the 

 experimental animals was not considered. 



Stomach contents, removed from the conch and pre- 

 served in a 70% solution of ethanol mixed with dilute 

 rose bengal, were quantified by the gravimetric sieve- 

 fractionation method of Carr and Adams (1972). Pooled 

 contents from the stomachs of conch from each experi- 

 mental treatment were washed through a series of 

 four sieves of decreasing mesh size (425, 250, 150, and 

 75 urn, and each sieve fraction was examined under a 

 dissecting microscope (20-40 x). Food items were 

 heavily macerated but easily classified into general 

 taxonomic categories; these included algae, detritus, 

 and small invertebrates such as foraminiferans, gas- 

 tropods, and polychaetes. The proportion of each food 

 type (as well as sand) in each sieve fraction was esti- 

 mated by identifying and counting the individual par- 

 ticles. After examination and identification, each sieve 

 fraction was dried for 12 hours at 80°C. Dry weights 

 of the fractions were summed and the relative impor- 

 tance of the different food categories were reported as 

 the percentage of total dry weight. 



Analysis of variance was used to test the signifi- 

 cance of differences in growth and condition factor 

 among the habitat treatments. Because multiple mea- 

 surements from individual enclosures are pseudo- 

 replicates (Hurlbert, 1984), mean values from the rep- 

 licate cages were used in the analyses. Growth rates 

 were log 10 -transformed to produce homogeneity in vari- 

 ance (Box test, F = 0.511, P = 0.608). Transformation 

 was unnecessary for condition data. Tukey's test was 

 used for multiple comparisons. 



Burrowing in the field 



Early juvenile conch were first observed at Neighbor 

 Cay late in the day (17:00 hours) on 8 January 1989, 

 but none were visible the following morning. Assum- 

 ing daytime burial, a survey of the population was 

 initiated to examine diel periodicity in burrowing be- 

 havior. On 17 January, 64 early juveniles were tagged 

 with vinyl orange spaghetti tags (Floy Tag & Manu- 

 facturing, Inc. ) tied to the shell spire and released in 

 zone B of the study site. A 4-cm free end was left on 

 the tag so that buried conch could be easily seen and 

 counted. During the survey, all early juvenile conch 

 were tagged when observed (500 total). 



Surveys for buried conch were made on 22 dates 

 between 17 January and 28 February 1989. Between 

 17 January and 7 February, surveys were made by 

 two divers swimming parallel to the beach. All tagged 

 conch were counted and their behavior was recorded 

 as either on the surface, or buried. Because massive 

 mortality due to predation on early juveniles occurred 

 during the first week of February, subsequent obser- 

 vations were made on the tagged conch held in experi- 

 mental enclosures (see previous section). Over the 

 course of the study the beach was surveyed at nearly 

 all times of day and night and at different stages of 

 the tide. Twice in January, surveys were made each 

 half-hour during each of the transitions from dark to 

 light and from light to dark. This permitted observa- 

 tions on the precise time of emergence and burrowing 

 relative to times of twilight, sunrise, and sunset. Tim- 

 ing on 53 surveys was such that observations were 

 made at least once or twice every half hour through 

 day and night. 



Burrowing in the laboratory 



The role of light stimulus on burrowing behavior of 

 early juvenile conch was examined by subjecting them 

 to different light-dark cycles in laboratory aquaria. Be- 

 cause most of the early juveniles found in the field had 

 been manipulated during previous experiments, we 



