BULLETIN OF MARINE SCIENCE, 51(2): 250-266, 1992 CORAL REEF PAPER REPRODUCTION AND LARVAL DEVELOPMENT OF THE WEST INDIAN TOPSHELL, CITTARIUM PICA (TROCHIDAE), IN THE BAHAMAS Lori J. Bell ABSTRACT The reproduction and larval development of the West Indian topshell, Cittarium pica (Linnaeus), was studied in the southern Exuma Cays, Bahamas, between June 1990 and June 1991. Topshells held in outdoor water tables spawned unpredictably, possibly in response to simulated low and high tides and the moon phase. Eight of nine spontaneous spawnings occurred within 4 days of either the full or new moon between 20 June and 4 November. Larvae of C. pica were lecithotrophic with a relatively short larval life of 3.5-4.5 d, when reared at 26.5-27.5·C. Emergence from the egg membrane occurred at the trochophore stage. At this stage the larval shell covered about two-thirds to three-fourths of the posterior end of the larvae, with the velum not yet developed. Settlement could be induced by providing a bacterial/algal film substratum. Oocyte diameters from histological slides of ovaries were used to determine reproductive seasonality. Females began to mature in July. A gradual increase in oocyte diameters occurred over the summer until they peaked in late September and early October, followed by a significant decline. Oocyte size was variable between October and January, after which the mean size was consistently small with little variability. The data suggest a natural spawning period in early October. Mean oocyte diameter was highly cor- related with seawater temperature. An influx of juvenile C. pica (1-2 mm shell width) was observed in January. Using growth rates of laboratory-reared juvenile C. pica, a spawning date corresponding to the significant decline in oocyte diameter in early October was ex- trapolated. The West Indian topshell, Cittarium pica (Linnaeus, 1758) (Trochidae), is an intertidal gastropod found along rocky shores in the Bahamas and Caribbean, south to Trinidad (Abbott, 1974). It is very rare in south Florida (Abbott, 1976) and became extinct in Bermuda in comparatively recent times (probably early 1800's, Verrill, 1901). It is an important food item, second only to the queen conch, Strombus gigas, in value among Caribbean gastropods (Randall, 1964; Flores and Talarico, 1981), and it has been overexploited in many of the dense1y- populated areas of the West Indies (e.g., Cayman Islands, P. Bush, pers. comm.; Virgin Islands, R. Boulon, pers. comm.; Venezuela, Flores and Talarico, 1981). The biology of C. pica was first studied in the Virgin Islands by Randall (1964). More recently Castell (1987) and Debrot (1990a; 1990b) studied various aspects of its biology. Aside from seasonal trends in gonad indices (Castell, 1987; Debrot, 1990a), little is known of the reproductive activity of C. pica. Trochids are found in tropical and temperate waters and the reproductive cycles of temperate intertidal trochids have been fairly well documented (Fretter and Graham, 1977; Garwood and Kendall, 1985; Lasiak, 1987). Colman and Tyler (1988) have provided the first insight into the reproduction of a deep-sea (990- 2,450 m) trochid. Spawning in the laboratory, as well as larval development, has been described for several temperate species (Desai, 1966; Underwood, 1972; Holyoak, 1988a; 1988b). Much less is known about reproduction of tropical trochid gastropods. Spawning and larval development offive tropical species have been described from the Pacific and Red Sea (Gohar and Eisawy, 1963; Duch, 1969; Eisawy, 1970; Heslinga, 1981; Hulings, 1986), although little has been reported of their reproductive periodicity (Randall, 1964; Heslinga and Hillmann, 1981). 250 BELL: TOPSHELL REPRODUCTION AND LARVAL DEVELOPMENT 251 The objectives of the present study were to describe and illustrate the larval development of C. pica and to examine aspects of its reproductive periodicity. This information could be applied to management and possible mariculture of the species. MATERIALS AND METHODS This study was conducted from June 1990 through June 1991 at Lee Stocking Island (23°46'N, 76006'W) in the southern Exuma Cays, Bahamas. The islands in this area are uninhabited, but topshell harvesting does occur. The intertidal habitat in the Exumas is described by Debrot (1990b). The tides in the Exuma Cays are semi-diurnal with a tidal range of approximately 1.5 m. Daylength for the study area was calculated (The Astronomical Almanac, 1990). Seawater temperature was monitored from May 1989 to February 1991 using a recording thermograph (Ryan Tempmentor, Ryan Instru- ments), located on the bank at a depth of 2.8 m. From June to November 1990, weekly collections of approximately 75 adult C. pica were made from islands within 3 km of Lee Stocking Island. Spontaneous spawnings could be observed when holding animals for I week in outdoor water tables equipped with flow-through running seawater. Alternating low-tide and flood-tide environments were simulated in these tanks as follows: animals were maintained dry for periods typically ranging from 3-36 h then flooded with fresh seawater, and maintained as such for 1-2 days. These dry/wet cycles were continuously repeated for all collections, and included all moon phases of each month during the 5-month period. All water tables were continuously drained through 125 !tm mesh nets that were checked twice daily for egg release. Animals were returned to their collection site after I week. Larval development of C. pica was described from eggs spontaneously spawned or from newly- released eggs that were fertilized in small glass dishes. Fertilized eggs were maintained indoors in fresh seawater in glass or plastic containers, without aeration, at an ambient temperature of 26.5-27 .5°C. Specimens were preserved in 5% buffered formalin in seawater, and drawings oflive specimens, noting live coloration, were made. Hatching, used herein, will refer to the emergence of larvae from the egg membrane. Once the larvae began to crawl they were placed in biofouled petri dishes, containing a bacterial/algal film, to induce metamorphosis. The dishes had been hung outdoors in seawater for 1- 2 weeks. A growth equation was obtained from regular measurements of reared juveniles maintained in heavily fouled dishes in the laboratory through mid-November, when summer field work was completed. Drawings of preserved embryos and larvae were made using a camera lucida attachment on a Wild compound microscope. Measurements of specimens were made using a calibrated ocular micrometer. Developmental times are described as min or h post-fertilization. A settlement experiment was conducted to determine the effect of unfouled and biofouled substra- turns on the occurrence and timing of metamorphosis. Five sterile and five biofouled dishes were used for the two treatments. Each dish was filled with I !tm-filtered natural seawater, stocked with 20 2-day-old post-torsional veliger larvae and maintained at an ambient temperature of 26.5-27.5°C. The water in each dish was changed daily and the number of metamorphosed and non metamorphosed larvae, as well as survival, were scored for 5 days, until mean survival was less than 50% for each treatment. Results were analyzed comparing the number of metamorphosed individuals in each treat- ment on day 7 using the Mann-Whitney U test (Sokal and Rohlf, 1981). Beginning July 1990, weekly collections of 10 or more C. pica were made on or near days of the moon phases for histological examination of the ovaries. These weekly collections continued through the new moon in November. Samples of approximately 30 animals were then collected monthly on or near the new moon from November 1990 through June 1991. A total of 27 collections were made during the 12-month period. Specimens were processed in the laboratory on the day of collection. Shell length and width measurements (Debrot, 1990a) were taken for each animal and shells were cracked and animals sexed, when possible, upon the basis of gonad color. Ovaries were green and testes a cream color. Gonads were preserved in 10% buffered formalin in seawater for a minimum of one month, transferred to 35% ethanol for one week, and then placed in 70% ethanol for storage. Collections were made at numerous locations in an attempt to avoid greatly reducing the population of C. pica at anyone island. Five females between 40-55 mm shell width, from each of the 27 collections, were randomly selected for histological analysis. A cross-section of the visceral coil (gonad and digestive gland) was taken from each specimen just behind the gastric cecum, embedded in paraffin, sectioned at 7 !tm and stained with hematoxylin and alcoholic eosin. A minimum of 50 oocytes per animal was measured at 100 x magnification using a computer video image analysis system (Java, Jandel Scientific). Only pre-vitellogenic and vitellogenic oocytes in which the nucleolus was visible were measured. The area of each cell was measured and then converted to 252 BULLETIN OF MARINE SCIENCE, VOL. 51, NO.2, 1992 a theoretical diameter, as if the cell was perfectly round, Relative size-frequency histograms of oocyte diameters were averaged for the five animals of each collection. Data on oocyte sizes were analyzed following Grant and Tyler (1983), A Kruskal-Wallis test was performed to compare mean oocyte sizes while a Kolmogorov-Smirnov two-sample test for goodness of fit was used to compare sequential distributions of oocyte sizes among samples. Recruitment was studied by monitoring the appearance of juveniles, 1-2 mm width, into the population at four sites of moderate wave exposure from July 1990 to May 1991. This monitoring was done monthly from July to November 1990, and bimonthly thereafter. Each site was marked at 4-m intervals along a 20-m transect using underwater epoxy. All ::520-mm-wide C. pica found within each transect between the high-water and low-water spring tide marks were collected.