The deep-sea Buccinoidea (Gastropoda: Neogastropoda) of the Scotia Sea and adjacent abyssal plains and trenches

Four new genera and species of buccinoidean gastropods, S/HW;»cr/nmn s/p/f/imiff/r new genus, new species: Dn^M/wof/on/ff.s /o/z/f/Nf/c new genus, new species; Afff^zn/wcr/HMMi cof/icdnf/f new genus, new spi-cies; and (x/ifionro n/rWw new genus, new species, are described from the Scotia tectonic plate and adjacent alwssal plains. Only fW/w/f/oMH? (V;fM^»& "Uncle, 1912, TTtMn/no /W/m r//;f/w/n)/f/ Clarke, 1961 and 71 m/jf/vwrnmi Lus, 1993, had prcviouslv been reported from aliyssal depths off Antarctica. The latter two species were proposed in the genus Th)Himr7. sulmciaicutly shown to belong to the family Muricidar. Therefore, a new genus. /^isf/mmf#im is proposed for those abyssal and hadal buccinoidean species. Analyses of the taxonomic placement, geographical and l)athymctric distribution, and diversity o( the 29 buccinoidean genera prcsently known from Antarctica and the Magellanic Province have shown that the abyssal (>2200 m) buccinoidean fauna of the region shares no genera with the sublittoral or bathyal faunas. None of the six aliyssal genera conform readily to the sublamilies rcpnvicutcd b\ the sublittoral or lialhval faunas. (Credible sister taxa and likely origins (or some abyssal genera occur on the adjacent continental slope. For others, closest relatives may IxIbund on ahysail plains beyond the Antarctic convergence. Generic diversity decreases with increasing depth for both the bathyal and abyssal buccinoidean (annas, while bathvmetrie range tends to increase. For abyssal bueeinoideans, maximum generic diversity occurs l)etween 2600 and 32(X) meters. The proportion of monotypic genera in the Antarctic and Magellanic Provinces is extraordinarily high (48.3%), and may be an artefact of low sampling density exacerbated by difficulties in differentiating closely related species. Neither gigantism nor dwarfism is evident in the abyssal buccinoidean fauna. Rather, the range in sixes narrows with increasing depth. Genera inhabitiug the base of the continental slope are smaller than those of either the upper slope or continental rise. In the abyssal zone, maximum shell size is reached near the boundary of the continental rise and ahys&il plain, and subsequently decreases with increasing depth.