Aplacophora as a Tethyan Slope Taxon: Evidence from the Pacific

Recent aplacophoran collections from the Pacific yield information on global distributions of this class of Mollusca. A former Tethyan or pre-Tethyan distribution is suggested by the disjunct occurrence of five species which form a subgroup within the genus Faleidens, two from the western Atlantic and three from the western Pacific; by three species of the genus Prochaetoderma with a similar disjunct distribution; and by the broadly distributed family Pararrhopalidae. High diversity of species and less than 40% numerical dominance by any one species typify samples from slope stations off southeast Australia, whereas low diversity and dominance greater than 60% typify the shelf in Bass Strait; the latter may reflect the disruptive effects of Pleistocene regression of sea level. Three species pairs with one of each pair occurring on the slope and the other on the shelf may indicate that shelf species evolved from slope species. A new genus with eight species, seven from slope and abyssal hydrothermal vents, is comprised of six species from slope depth and two from abyssal depths; the abyssal species have character states derived from the shallower species. The evidence from present-day distributions of species, genera, and families indicates that the Aplacophora were already widespread on continental slopes by Tethyan times.