ECOLOGICAL AND SYSTEMATIC NOTES ON CAECIDAE FROM ST. CROIX, U. S. VIRGIN ISLANDS! DONALD R. MOORE University of Miami, Rosenstiel School of Marine and Atmospheric Science ABSTRACT A survey of the shallow marine fauna at St. Croix, U.S. Virgin Islands, was made in the summers of 1969 and 1970. Sediment samples were collected by diving, and micromollusks were picked from them. The Cae- cidae were studied from shallow-bay sediments and a quantitative study was made at three slightly deeper stations in and around the coral reefs. Seven species of Caecidae, 268 specimens, were found in 176 cc of sedi- ment from the three stations in deeper water. An eighth species was rare in the area of shallower water. The eight species, Caecum condylum Moore, C. subvolutum Folin, C. lineicinctum Folin, C. regulare Carpenter, C. textile Folin, C. imbricatum Carpenter, C. (Meioceras) nitidum Stimpson, and C. (M.) cornucopiae (Carpenter), are all poorly known. The first three species do not live in back-reef or lagoonal areas, and so were supposed to be extremely rare. However, these three species comprised 55 per cent of the Caecidae from the three stations in deeper water. They live in an environment that has been little sampled, so have been seldom collected. Distribution for all of the species is tropical, although C. nitidum and C. imbricatum are found in the northern Gulf of Mexico as well. INTRODUCTION Two brief surveys of the shallow marine environment of northeastern $1. Croix were undertaken by Dr. H. Gray Multer, Dr. Wayne D. Bock, and myself in 1969 and 1970. Dr. Donald Marszalek and Cynthia Moore also participated in the 1970 survey. Observations and colIections were made on the reef, on grassy bottom, on open sandy bottom and along the rocky shore. In addition to some macroinvertebrates, sediment samples were collected at selected sites in depths ranging from one to about 20 m. The area surveyed (Figs. 1 and 2) forms a triangle, with the east end of Buck Island at the apex, and Tague Bay and Cottongarden Point re- spectively occupying the west and east ends of the base line. Sediment samples were obtained by diving during 1969; in 1970, the samples were collected by diving and by the use of a small clamshell bottom sampler in two transects from Buck Island to S1. Croix. A series of samples taken in shallow water inside the reef were picked and sorted during 1969. In this area, some 27 species of micromollusks ranging in size from 0.8 to around 5.0 mm were identified, and several additional species are still 1Contribution No. 1562 from the University of Miami, Rosenstiel School of Marine and Atmo- spheric Science, and Contribution No. 4 from the West Indies Laboratory, Fairleigh Dickinson University. 882 Bulletin of Marine Science [22(4) FIGURE 1. Map of St. Croix, U. S. Virgin Islands, showing fringing reefs en- closing the shallow bays. The reefs are concentrated around the eastern two- thirds of the island. unrecognized. For 1970, it was decided to study the fauna in somewhat deeper water around, and in front of, the reef. Two samples from 12 m and one from about 14 m were picked and the micromol1usks studied. Allor most of each sample was picked in order to obtain as many species as possible. This sediment was almost entirely calcareous in origin, and was derived from algae and the shelled inverte- brates living in the area. The important elements were mollusks, Foram- inifera, the alga Halimeda, coral fragments, crustacean exoskeletons, and, more rarely, sponge spicules, brachiopods, and echinoderm fragments. A few of these animals were living in situ, but most were evidently living on hard bottom close by. After the death of these animals, water move- ment carries the shells a few meters to the low places, where they accumu- late as calcareous sediment. At first, it was planned to contrast the micro molluscan faunas living in the fore reef and back reef areas. Too many systematic problems have, however, caused a revision of these plans. Groups of species will be reported on as their taxonomy is worked out, and will be reported in a series of short papers. This is the first of the series. One 12-m sample came from the open bottom in front of the reef stretching across Tague Bay. The second 12-m sample came from the eastern end of Buck Island about 2.5 km north of the first locality. The third locality was about 200 m southeast of the second, and was taken from a depth of about 14 m. The two Buck Island localities were close to towering mounds of coral (Montastrea, Diploria, and Acropora), while the Tague Bay locality was at the foot of a long gradual reef slope. All of the species of Caecidae here considered are very poorly known, so they will be discussed in some detail. The first species was only described in 1972] Moore: Caecidae from St. Croix 883 BUCK is. ST. eR.OI X FIGURE 2. Map of the eastern end of St. Croix showing Tague Bay and Buck Island. The three deeper water stations are marked (see text for localities). 1969 from two specimens, hence its discovery in some abundance at St. Croix is of interest. SYSTEMATIC AND ECOLOGICAL NOTES Caecum candylum Moore, 1969 Fig. 12 Caecum condylum Moore, 1969, Nautilus, 83(1): 26-28, one fig. Type-Locality.-Payardi Island, near Colon, on the Atlantic side of Panama. Material.-Tague Bay: 5 specimens; Buck Island, 12 m: 2 specimens; Buck Island, 14 m: 27 specimens. Description.- The teleoconch is cylindrical and slightly curved for most of its length. The anterior part is more strongly curved and there is a broad, rounded swelling near the aperture. The septum is slightly de- pressed and the low, somewhat pointed mucro is angled to the right. Sculp- 884 Bulletin of Marine Science [22(4) FIGURES3-5.-3, Caecum lineicinctum Folin, length 2.44 mm, seen from the right side. This specimen has been bored by a small carnivorous gastropod.- 4, C. subvolutum Folin, length 2.18 mm.-5, C. lineicinctw/1, another view, to show sculpture at the anterior end of the shell. ture consists of approximately 100 low, close-set annular ridges which are all about the same width. The shells are almost transparent when fresh, and bear two or three diffuse brown bands. The operculum was not observed. Remarks.-None of the specimens of C. condylum was mature and in good condition. The specimens were either short second-stage, immature third-stage, or broken mature specimens. Since all of the adults were broken, this suggests that some enemy, possibly a crab, is a heavy predator on C. condylum. All of the other species of Caecum were represented by adults in good condition. As I had predicted, even second-stage specimens of C. condylum were easy to recognize, since no other western Atlantic species has a shell with so many close-set ridges. 1972] Moore: Caecidae from St. Croix 885 Fresh specimens of C. condylum are shining and almost transparent; the brown bands are quite striking on these individuals. None of the specimens was alive when collected, for the species probably lives on the hard substrate adjacent to the sandy patch from which the sediment was collected. Samples of shallow-water sediment inshore from the reef did not contain specimens of C. condylum. Apparently, this is a true coral- reef species which shuns the shallow reef flat area, but which is abundant at depths of more than 10m. The maximum depth probably coincides with the greatest depth of coral reef development, or around 40 m in the Caribbean. The distribution of C. condylum will probably take in the entire tropical West Indian area when proper collecting is done. The pres- ent record establishes the species 15° farther east than in the original paper. Caecum subvolutum Folin, 1874 Fig. 4 Caecum subvolutum Folin, 1874, Fonds de la Mer, 2: 277, pI. 10, figs. 6-7. Type-Locality.-In 100 fm (183 m), west of Barbados, a few miles north of Bridgetown; Hassler Expedition. Material.-Tague Bay: 11 specimens; Buck Island, 12 m: 34 specimens; Buck Island, 14 m: 47 specimens. Description.-The shell is slender, smooth, and cylindrical for most of its length. A rather thick brown periostracum is present on fresh specimens. Curvature of the shell is moderate, but stronger at the anterior end. There is a low varix at the aperture, bearing several minute annular ridges. The septum is slightly recessed and bears a strong flattened mucro. A large adult measured 2.25 mm long and 0.4 mm in diameter at the middle of the shell. There is no color, although the shell turns white sometime after the death of the animal. Remarks.-At first glance, this species might be mistaken for C. condy- lum. C. subvolutum, however, has a much stronger mucro projecting at a higher angle, is smooth or nearly smooth except on the varix, and docs not have bands of color. The holotype is in the Folin Collection at the Laboratoire de Malaco- logie in Paris. The mucro in Folin's drawing is exaggerated. The type itself was damaged and repaired near the aperture, but several small annular ridges are visible on the varix. The specimen was in all probability carried down from shallow water and did not live where it was collected. It was not a fresh specimen and showed signs of wear. The species apparently ranges from Barbados to Florida. Two worn specimens that are apparently this species were collected off Miami in 162 fm by the University of Miami ship GERDA.