The Systematic Position of the Family Caecidae (Mollusca: Gastropoda)

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THE SYSTEIVIATIC POSITION OF THE FAMILY l CAECIDAE (MOLLUSCA: GASTROPODA ) DONALD R. MOORE Institute of Marine Science, University of Miami

ABSTRACT The Family Caecidae has been placed near the Vermetidae in the superfamily Cerithiacea since Clark published his views in 1855. Close examination of the animals shows the caecids to have a close relationship to the superfamily Rissoacea. Animals of the Vitrinellidae were most similar to the Caecidae, and the shells of the Ctiloceratidae showed affinity to both the Caecidae and the Vitrinellidae. No close relationship between the Caecidae and Vermetidae or other Cerithiacea could be demonstrated, and both the Caecidae and Ctiloceratidae are transferred from the Cerithiacea to the Rissoacea. The genus Caecum was long a puzzle to malacologists, for the minute curved shell is unlike any other mollusk. Some authors ignored its closed posterior end, and included it in the Scaphopoda; others fancied a relationship with the pteropods or the paleozooic Ortho- ceras, a genus of large cephalopods. It was not until 1849, when William Clark published his observations, that anything was known of the animal and its habits. Clark showed that it was an active bottom dwelling gastropod closely resembling other prosobranchs of the area in which he worked. Clark (1855) thought that Caecum was very like Vermetus, an animal that he apparently never saw alive, and was even inclined to synonymize the two genera. He decided, however, that there was enough difference between them to maintain them as separate genera. He erected a new family, the Vermetidae, and included Turritella as well. This was notwithstanding the fact that the families Vermetidae, Caecidae and Turritellidae had been established a number of years before. Malacologists since 1855 have usually placed the Caecidae close to the Vermetidae although there was considerable variation in the arrangement of families. No one seriously questioned Clark's decision, and Thiele (1928-1935) placed the Caecidae in the superfamily Cerithiacea where it followed the Vermetidae. The animals in this superfamily usually have shells with many whorls and a high spire. There is a rather deep pallial cavity with a well developed ctenidium, and there is no male copulatory organ. It appears that the Cerithiacea is a rather heterogeneous assemblage of families, and may prove to be a fruitful field of investigation in the future. Since Clark's researches, three investigators have published further

IContribution No. 426 from The Marine Laboratory, University of Miami. This study was supported by National Science Foundation grant G-14521. 696 Bulletin of Marine Science of the Gulf and Caribbean [J 2 (4) information on the animal of Caecum (Plate, 1896; Vayssiere, 1930; and Gotze, 1938). The study by Gatze on Caecum glabrum was the most detailed, and forms the basis of our knowledge concerning the biology of the family. She found that C. glabrum had ciliated tentacles, and that the male had a large, well-developed penis. She compared this last structure with the penis found in the Hydrobidae. She did not find a gill, and the Caecidae have been considered ever since to lack this organ (Yonge, 1947, 1960). The most recent paper discussing the classification of the Caecidae and related animals is that of Iredale and Laseron (1957). They established a family, the Ctiloceratidae, for the reception of the genus Ctiloceras Watson and for five new genera. In this family, the coiled larval shell straightens out as in the Caecidae. After the forma- tion of a varix, however, the adult shell is coiled much as in the Vitrinellidae. Because of the appearance of the juvenile portion of the shell, they related the family to the Caecidae and added two subfamilies for intermediate forms. No observations were made on the animals, and ,even the operculum was unknown to them. The writer became especially interested in the Caecidae while working on the sea grass communities of Biscayne Bay, Florida. At one locality a total of 1312 Caecum pulchellum Stimpson was collected from an area of one tenth of a square meter of Thallassia testudinum Konig (Moore, 1961). Live specimens were observed at that time, and other species have also been observed from time to time. Other small gastropods were also studied under the micro- scope and comparative notes were made. A number of small cerithiaceans collected from the grass beds had two characters in common. The tentacles were smooth and not ciliated, and there was usually a tendency to form a short, incomplete siphon. The various species of Caecum, on the other hand, had cephalic tentacles tipped with long, stiff, sensory cilia, and were provided with tracts of motile cilia. The tracts of cilia were in constant motion while the animal was extended, and provided a steady current of water past the head and body. There was no indication of a siphon in any of the species of Caecum. The differences in the anterior part of the body of Caecum and such small cerithiaceans as Seila adamsi Lea, Bittium varium Pfeiffer, and Triphora nigrocincta C. B. Adams, seemed to indicate that there was no close relationship. Caecum was also compared with several small rissoaceans. A1vania auberiana d'Orbigny, a small' species about 2 mm long, was very 1962] Moore: Systematic Position of Caecidae 697 similar in appearance to Caecum. The stiff sensory cilia were not confined to the tips of the tentacles, however, but were scattered along the entire length of the tentacles. The motile cilia were confined to a tract on the under side of the tentacle and thus were not as effective in propelling a water current as those observed in Caecum. However, the appearance and coloration of the animal was very similar to that observed in the species of Caecum. Other rissoids were examined to see if ciliated tentacles were

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FIGURE 1. A. Diagram of major anatomical features of a generalized caecid; osphradium (os), gill filaments (gf), penis (pe), and distal end of intestine (int) shown as if shell were transparent; posterior end of foot omitted.-B. Diagram of major anatomical features of a vitrineIlid, Parviturboides interruptus (C. B. Adams). 698 Bulletin of Marine Science of the Gulf and Caribbean [12 (4) a characteristic of the various genera. A mphithalamus vallei Aguayo and J aume, with a shell 1 mm in length, was the smallest species observed. It had two short, paddle-shaped tentacles without stiff cilia, but the ciliated tract on the under side was present. Rissoina chesneli Michaud was also examined, and an extremely fine tract of small cilia was found on the underside of the tentacles. There were no stiff cilia. The presence of a respiratory current set up by tentacular ciliation in the Caecidae was thought to be the result of the loss of the gill and its powerful cilia. Since the rissoids had somewhat reduced tentacular ciliation, it was considered likely that the gill was reduced but not altogether lost. A series of the thr,ee species mentioned above were removed from the shell and the mantle cavity opened. The gill filaments w,ere finger-like projections well provided with cilia. There were about thirty of these filaments in Rissoina chesneli, thirteeen in Alvania auberiana, and seven in Amphithalamus vallei. Several specimens of Caecum were also investigated, and contrary to reports in the literature, a gill was found close to the large, oval osphradium. There were seven finger-like filaments in both C. pulchellum and C. nebulosum. In this respect, both species were similar to A mphithalamus vallei. (Fig. 1, A.) Observation of the pallial organs also showed that the male copulatory organ in two species, Meioceras nitidum and Caecum sp., was similar to that found in Amphithalamus vallei. In all three species this organ is long and slender, and is coiled counterclockwise. In Caecum pulchellum, however, it is a very complex structure which looks something like a hand, and terminates in five separate projections. In Alvania auberiana and Rissoina chesneli this organ differs from that found in the preceeding species. At this point, F. M. Bayer and Robert Work collected seven living specimens of Parviturboides interruptus (c. B. Adams), a species belonging to the Virtrinellidae. The cephalic tentacles were almost identical to those of the Caecidae. In addition to the stiff cilia on the tips and tracts of motile cilia, there was a series of low rounded swellings on the proximal posterior border of the left tentacle. This structure, whose significance is totally unknown, is a characteristic feature of the Caecidae. (Fig. 1, B.) At least one species of Vitrinellidae, then, has the same structure of the head region as in the Caeciae. Figures of living vitrinellids have been published by Pilsbry and McGinty (1945). Although their 1962] Moore: Systematic Position of Caecidae 699

FIGURE 2. Transcopia clathrata (Hedley), a ctiloceratid.-A. Lateral view.- B. Dorsal view. figures do not show much detail, it is apparent that the animals of several different genera are very similar in appearance. The stiff cilia at the ends of the tentacles are shown, but not the tracts of motile cilia or the swellings on the left tentacle. Pilsbry (] 953) placed the vitrinellids in the superfamily Rissoacea, a decision with which the author emphatically concurs. The Vitrinellidae also agree with the Caecidae in having a round, spiral operculum, and a short, truncate foot. The families differ, however, in that the vitrinellids, like the rissoids Alvania and Rissoina, have pallial tentacles well provided with stiff cilia like those found in A lvania. The Caecidae do not have pallial tentacles, but this is probably so because the aperture of Caecum is narrow and circular. There is no room for pallial tentacles except around the head where they are unnecessary. From the foregoing account it seems that the Caecidae are members of the Rissoacea and that they are rather closely related to the Vitrinellidae. However, there is another family to which they seem even more closely related. The family Ctiloceratidae lredale and Laseron (1957) was established for a group of odd little Australian sheIls in which the protoconch is at first tightly coiled, then nearly straight and conical. It terminates in a varix, and the mature sheIl is coiled in a depressed spiral. The adult sheIls thus look very much like vitrinellids except for the unusual protoconch. The animal and operculum are unknown, but the shell characters would place the family in close proximity to both the Caecidae and VitrineIlidae. (Fig. 2.) 700 Bulletin of Marine Science of the Gulf and Caribbean [12 (4) The Caecidae, then, appear to be Rissoacea with an uncoiled shell. Any resemblance to the Vermetidae is superficial, and is based on the circular operculum, the uncoiled shell, and internal septa. How- ever, the protoconch of the vermetids is quite different from that of the Caecidae, and their irregular, uncoiled shell is the result of a sessile, filter-feeding life. The circular operculum and internal septa are found in a numb.er of gastropods with long, slender, tubular shells. The Caecidae are active bottom crawlers, and septa are formed to enable the animal to discard the hindering juvenile portion of the shell. The Caecidae are thus considered to have no close affinity to the Vermetidae or any of the Cerithiacea, and are hereby transferred, along with the Ctiloceratidae, to the superfamily Rissoacea. (Fig. 3.)

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FIGURE 3. Probable relationship of the families Caecidae, Ctiloceratidae, and Vitrinellidae. 1962] Moore: Systematic Position of Caecidae 701 REFERENCES CLARK, W. 1849. On the animals of Caecum trachea and C. glabrum. Ann. Mag. nat. Hist., ser. 2, 4: 180-184. 1855. British Marine Testaceous Mollusca. London, John van Voorst. 1-536. GOTZE, E. 1938. Bau und leben von Caecum glabrum (Montagu) Zool. Jb. (Syst.), 71: 55-122. IREDALE, T. AND C. F. LASERON 1957. The systematic status of Ctiloceras and some comparative genera. Proc. R. Zool. Soc. N. S. W. 1955-1956: 97-109. MOORE, D. R. 1961. The molluscan sea grass communities in Biscayne Bay. Ann. Rept. Amer. malac. Union, 1961: 20 (abstract). PILSBRY, H. A. 1953. Vitrinellidae, in Olsson, A. A., A. Harbison, W. G. Fargo and H. A. PUsbry. Pliocene mollusca of southern Florida. Monogr. Acad. nat. Sci. Philadelphia, 8: 1-457, 65 pIs. PILSBRY, H. A. AND T. MCGINTY 1945. Cyclostrematidae and Vitrinellidae of Florida. Nautilus 59 (1): 1-13. PLATE, 1896. Ueber den habitus und die kriechweise von Caecum auriculatum De Folin. S. B. Ges. naturf. Fr. Berl., 130-133. VAYSSIERE, A. 1930. btude zoologique et anatomique sur quelque gasteropodes pro so- branches provenant des campagnes scientific du Prince Albert 1aT de Monaco. Result. Camp. sci. Monaco, 80: 1-26. YONGE, C. M. 1947. The pallial organs in the aspidobranch gastropoda and their evolution throughout the Mollusca. Phil. Trans. Roy. Soc. London. ser. B., 232 (591): 443-518. 1960. General characters of Mollusca. In: Moore, R. C. (Ed.), Treatise on invertebrate paleontology. Part I, Mollusca, 1: 3-36.