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Comprehensive Description

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Description

"Bulbus benthicolus n.sp. (Figs 242, 243, 273)

 

MATERIAL EXAMINED: Eltanin Stns 472, 1511, 1514,1957,1964,1969.

 

DISTRIBUTION: Known only from two widely separate deep-water basins, the western boundary of the Scotia Sea in the South Antilles Basin and in the extreme southeastern extension of the eastern Indian-Antarctic Basin in 2836-3470 m. The known distribution mirrors the sporadic nature of the sampling of these deep-water areas rather than a real distribution.

 

DESCRIPTION: Shell of medium size, globose, thin, narrowly umbilicate. Height greater than the diameter. Spire of medium height. Apex worn in all specimens but whorls 4 ½-5. Suture quite deeply channelled. Whorl outline curving rapidly from the sutural channel, then considerably constricted giving a flattened outline to the upper third of the whorl, then broadly rounded, resulting in a very characteristic outline in adult shells. Epidermis thin, light brown, easily abraded. Surface sculptured with fine, irregular, wavy, spiral striae varying considerably in strength, but finer and more wavy on the sides of the subsutural channel. On the holotype these striae number about 50 at the close of the penultimate whorl. In other specimens they are finer and less regular, and may become almost obsolete across the base of the last whorl. The only other sculpture consists of irregular growth lines. Aperture ovate, outer lip curving rather widely from suture, outline flattened across upper third, thence evenly curving. Inner lip a rather solid glaze on the parietal wall. Columella relatively heavily thickened, reflected broadly adapically to half cover the narrow, open umbilicus.

 

Operculum thin, horny, light brown, the same size as the aperture. Animal able to contract totally within the shell. Central tooth of radula with a single relatively short central cusp and two basal cusps, lateral with a single massive cusp, marginals simple.

 

MEASUREMENTS: See Table 12.

 

HOLOTYPE: from Eltanin Stn 1511 in National Museum of Natural History (USNM 860114).

 

PARATYPES: from Eltanin Stns 1511 (one, USNM 860115), 195 7 (one, USNM 860116) and 1969 (one, USNM 860117) in National Museum of Natural History and from Stn 1511 (one, MF.56604) in National Museum of New Zealand.

 

REMARKS: The specimens from Eltanin Stns 472 and 1514 are smaller than the rest of the series and have the last whorl evenly rounded. In all other respects they match the larger shells and are considered conspecific.

 

This new southern deep-water species is extremely close in shell characters to Choristes carpenteri Dall, 1896. Marincovich (1977) gave localities for carpenteri ranging from the Gulf of Panama (c. 13°N) to off Central Oregon (c. 44°N) in depths of 2500-3436 m. Dr J. H. McLean of the Los Angeles County Museum of Natural History kindly made authentic specimens of carpenteri available for comparison. The new species differs in the whorl outline at least in adult shells particularly in the subsutural flattening of the whorls, in the presence of consistent, obvious, fine, wavy, spiral striae, and in a narrower umbilicus. The radulae are essentially similar apart from the markedly elongate lateral tooth in carpenteri.

 

The generic placing of B. benthicolus n.sp. raises a number of difficulties. The form of the radula is closest to that of the New Zealand genus Globisinum and the shell sculpture is reminiscent also of this genus, some of the Cenozoic fossil species of which are umbilicate. The only described living species of Globisinum is G. drewi (Murdoch, 1899), which lacks an operculum and has an expanded rather spongy foot that cannot be withdrawn inside the shell. Some operculate antarctic species that have radulae similar to Globisinum have been placed in Bulbus in the present study. However, none of these species is umbilicate. Since some of the New Zealand fossil species of Globisinum are umbilicate, the presence or absence of an umbilicus cannot be considered onits own as a generic character, and there seems no reason why umbilicate and imperforate species cannot be included in Bulbus. The other genus in which benthicolus could possibly be accommodated is Falsilunatia, which has an essentially similar radula. Perhaps benthicolus, powelli and carpenteri should be associated together in a new genus since the radulae in all three species are similar, all are umbilicate, and all have large, fragile, horny opercula. However, the shell texture of powelli is very different and it is probably best keptseparate. Separating the umbilicate forms would also divorce them from such species as the northern smithi Brown, coani Marincovich, glacialis Thorson and the southern scotianus n.sp. In the meantime until the relationships of Bulbus, Acrybia and Choristes can be elucidated, benthicolus seems best placed in Bulbus. The thin shell of benthicolus, which partly influences its comparison with Bulbus is of course only to be expected in naticids from such depths."

 

(Dell, 1990: 158-159)

 

Table 12. Measurements and shell ratios of species of Bulbus.

                                                                                                                                              
 

 
H (mm)D (mm)D/H ratio
B. carcellesi
Eltanin Stn 944 (holotype)31.329.092
“ (paratype)30.228.092
“ (paratype)26.324.091
Eltanin Stn 97614.512.385
Hero Stn 47024.720.884
Off Bahia Blanco28.526.385
20.017.788
B. scotianus
Eltanin Stn 435 (holotype)27.625.191
“ (paratype)22.319.185
Eltanin Stn 410 (paratype)23.219.885
“ “24.720.984
“ “20.218.391
18.916.889
B. benthicolus
Eltanin Stn 1515 (holotype)23.020.690
“ (paratype)19.117.994
Eltanin Stn 1957 (paratype)23.221.492
Eltanin Stn 1969 (paratype)25.923.189
B. carpenteri
44° 40.9’N, 125° 35,3’W24.322.191

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© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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