STUDIES ON THE REPRODUCTIVE BIOLOGY OF THE GENUS CERITHIUM (GASTROPODA: PROSOBRANCHIA) IN THE WESTERN ATLANTIC RICHARD S. (JOSEPH R.) HOUBRICKl Department of Biology, University of South Florida, Tampa, Florida 33620 ABSTRACT The reproductive biology of western Atlantic members of the genus Cerithium (C. muscarum, C. variabile, C. eburneum, C. floridanum, C. litteratum, and C. auricoma) is investigated. Pairing, spawning, descrip- tions of egg masses and early development are considered. Spermatophores and their method of transfer are described for C. muscarum. Eggs are laid by Cerithillm species in gelatinous strings or coils attached to a substratum as a tangled mass. There are two types of development in the genus in Florida: The first and most common is indirect development involving many eggs, rapid cleavage, a short encapsulated period, and emergence of free-swimming veligers. The second pattern, seen in C. mllscarum and C. variabile, in- volves direct development, fewer and larger eggs, and a longer period of encapsulation, with the young hatching only when metamorphosed as small crawling snails. Spawning and incubation periods, early developmental rates, and mea- surements of ova and larvae are established, and comparisons made with species from other parts of the world. INTRODUCTION Knowledge of reproduction, development, and larval ecology of marine subtropical prosobranchs is sparse compared with that of north temperate species. In recent years our understanding of tropical species has increased considerably, but few members of the family Cerithiidae have been inves- tigated in any detail. Although many species of the genus Cerithium inhabit Florida and the Caribbean region, there are few published records of their reproductive anatomy, breeding habits, or early life histories. Reproductive anatomy of the genus Cerithium has been discussed by Sunderbrink (1929), Risbec (1943, 1955), Johansson (1947, 1953, 1956), Marcus & Marcus (1964 ), and Houbrick (1971). A summary of spawning and develop- mental information on the genus, from a worldwide point of view, is pre- sented in Table 1. Lebour (1945) described the eggs and larvae of C. ferrugineum from Bermuda, but it is unclear with which species she dealt, because C. ferrugineum is a synonym of C. variabile, and the egg masses and larvae depicted by her are definitely not those of C. variabile. Marcus 1 Present address: Supervisor for Invertebrates, Smithsonian Oceanographic Sorting Center, Smith- sonian Institution, Washington, D. C. 20560. 876 Bulletin of Marine Science [23(4) TABLE 1 SPAWNING AND DEVELOPMENTAL STUDIES ON THE GENUS Cerithium (x = published material) Locale of study Authority C. auricoma Schwengel, 1940 x x x Florida D'Asaro (1970) C. lit/eratum (Born, 1778) x x x Florida D' Asaro (1970) C. {toridanum Morch, 1876 x x x Florida Torrance (1969) C. algicola C. B. Adams, 1848 x x x Jamaica Davis (1967) C. variabile C. B. Adams, 1845 x x x Florida Raeihle (1968) x x x Florida Houbrick (1970) C. atratum Born, 1778 x x x Brazil Marcus & Marcus (1964) C. ferrugineum Say, 1792 x x Bermuda Lebour (1945) C. stercusmuscarum Valenciennes, 1833 x x Baja California Wolfson (1969) C.morus Lamarck, 1822 x x x India Natarajan (1958) C. vulgatum Bruguiere, 1789 x Italy LoBianco (1888) C. ponticum Milaschewitsch, 1909 x Black Sea Chukhchin (1960) C. nodulosum Bruguiere, 1792 x x x Eniwetok Houbrick (1971) & Marcus (1964) described the eggs and veliger larva of C. atrattlm, but the taxonomic position of this species is unclear. Breeding habits and larval development of closely related genera in the family Cerithiidae have likewise received little attention. Records include those of the genus Bittium (Thorson, 1946; Fretter & Graham, 1962; Mur- ray, 1969), and related genera in the Cerithiaceae: Cerithidea (Habe, 1955; Natarajan, 1958; Panikkar & Aiyar, 1939; Vohra 1970), and Ceri- thiopsis (Lebour, 1933, 1945). 1973] Houbrick: Reproductive Biology of Cerithium 877 The spawn masses for members of the genus Cerithium are described in the literature and in this study as tangled masses or flat coils deposited in gelatinous strings attached to the substratum. The ova of species discussed herein are typically telolecithal and exhibit holoblastic cleavage. In contrast with archaeogastropods and neogastropods, most larvae of mesogastropods tend to have indirect development and a long pelagic life. This is true of most of the species of Cerithium hitherto investigated. Their mode of reproduction and development is indicative of a typical pattern which in- volves many eggs, rapid cleavage and attainment of the veliger stage, and emergence as planktotrophic larvae. Since free-swimming larvae are hard to rear under laboratory conditions, most species of Cerithium are difficult subjects for complete life-history studies. Two of the species in this study, C. muscarum Say, 1832, and C. variabile C. B. Adams, 1845, were found to have direct development. The account which follows will first concern itself with these two species. Subsequently, the species with indirect development (C. eburneum Bruguiere, 1792, C. floridanum Morch, 1876, C. litteratum [Born, 1778], C. auricoma Schwen- gel, 1940) will be treated. MATERIALS AND METHODS This study is based upon material collected in Florida. Observations were made in the field and laboratory from September 1968 through May 1971. Monthly samples and observations were taken at four field sta- tions, but other areas in the state were visited less regularly. The four main stations in Florida are located at Port Everglades (lat. 26° 6' N, long. 80° 4' W), Bear Cut (lat. 25° 44' N, long. 80° 8' W), Dunedin (lat. 28° l' N, long. 82° 47' W), and Mullet Key (lat. 27" 35' N, long. 82° 44' W). The first two stations are located on the lower east coast of Florida, where the environment and fauna are largely tropical and Caribbean in nature, while the latter two are located along the central Gulf coast of the state, where the marine fauna and environment are subtropical and largely Carolinian in composition. For life-history studies, various species of Cerithium were collected alive and maintained at a temperature of 25° C in aquaria of sea water with a salinity of 34i!c. Snails were fed on local algae from the Tampa Bay region and the detritus collected from grass beds. When snails spawned in the laboratory, eggs and larvae were reared at room temperature in finger bowls. Artificial sea water was used and was changed on a daily basis. No aeration was provided. Free-swimming veliger larvae were fed with dense cultures of Phaeodactylum tricornutum and CycloteUa nana, but despite such at- tempts no suitable food was found for the larvae. Living gametes and larvae were studied under oil or lower powers with a phase-contrast compound microscope, and drawings were made with the aid of a camera lucida. Eggs 878 Bulletin of Marine Science [23(4) and veligers were fixed in 60· C Bouin's solution for 3 to 4 minutes and transferred to 90 per cent ethanol before storage in 70 per cent ethanol, according to the method of Fretter (pers. commun.). Lynch's precipitation method of Grenacher's alcholic borax-carmine technique (Davenport, 1960) was used for whole mounts of larvae and eggs. Smears of spermatozoa were made using both the dry-smear technique and staining under the coverslip with Ehrlich's hematoxylin as suggested by Franzen (1956). Some sperm were fixed using Carnoy's fixative, but with poor results. Living spermato- zoa were stained with Janus green B in sea-water solution and observed under the compound microscope. Eggs and larvae were measured with an ocular micrometer and photomicrographs were taken of various develop- mental stages. Microscopic examinations of gonadal smears were made to determine sex ratios during the breeding season. EXPLANATION OF LETTERING aI, albumen Ih, larval heart bm, basal membrane m, mouth ct, ctcnidium me, mantle edge dg, digestive gland mp, midpiece e, eye 0, operculum em, embryo s, stomach elm, external limiting membrane sh, shell f, foot st, statocyst hc, hyaline capsule t, tentacle j, jelly v, velum jc, jelly capsule z, zygote REPRODUCTION, EGGS, AND LARVAE OF SPECIES OF Cerithium WITH DIRECT DEVELOPMENT 1. Cerithium muscarum Say, 1832 A population of C. muscarum from Mullet Key, Florida, was sampled monthly from April 1969 to February 1971. This population occurs just at, and below, the low-tide mark; extremely low tides will expose the entire population. C. muscarum is dioecious, with no apparent sexual dimorphism. A sex ratio of 1: 1.23 (39 per cent females; 48 per cent males) occurs during the middle of the breeding season. There is no evidence of sex reversal or hermaphroditism. Neuter specimens, due to parasitic castration by trema- todes, made up 13 per cent of the population. The breeding Season takes place from January through July, with more egg-laying activity occurring in the latter months. Gametogenesis.-Early in the reproductive season, females have cream- colored ovaries filled with oocytes and ova. The circular to oval ova average 1973] Houbrick: Reproductive Biology of Cerithium 879 800/.1.in diameter. Oocytes are smaller (700/.1.), and are distinguished from the ova by the presence of a germinal vesicle. During the early part of the reproductive period, winter maturation of eggs results in limited spawning, since many ova disintegrate within the ovary and are absorbed. During April, ova were observed moving down the ovarian duct into the open pallial gonadal duct, and sperm masses containing both eupyrene and apyrene sperm were seen in the female's sperm-collecting gutter. Only eupyrene sperm were found in the receptaculum seminis where they were oriented with their heads embedded in the receptaculum wall. In ripe males, the testes are bright orange and the seminal vesicles are packed with both eupyrene and apyrene sperm. The former were 40 /.I. in length and the latter 35/.1.. Eupyrene sperm are typical in form, having small heads, very long middle pieces and short flagella.