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Aplacophora As a Tethyan Slope Taxon: Evidence from the Pacific

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Aplacophora As a Tethyan Slope Taxon: Evidence from the Pacific BULLETIN OF MARINE SCIENCE, 47(1): 50-61,1990 APLACOPHORA AS A TETHYAN SLOPE TAXON: EVIDENCE FROM THE PACIFIC Amelie H. Scheltema ABSTRACT 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 Fa/eidens, 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 gl~nuswith 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 stat(:s 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. Aplacophoran mollusks are primarily deep-water forms and only with the ad- vent of precision depth fmders and adequate collecting gear- box cores, grabs, and epibenthic sleds used in conjunction with small-meshed screens-have they been collected in numbers that represent their actual occurrence. The numbers of specimens and species in samples taken at the turn of the century mostly with coarse-meshed trawls can be contrasted with those now being taken (Table I). For instance, the SIBOGAExpedition to the East Indies in 1899-1900 collected 66 aplacophoran specimens and about 13 species from 28 dredgings between 22 m and over 3,000 m (Nierstrasz, 1902; Stork, 1941), whereas a single epibenthic sled sample taken in 1986 from the continental slope at 400 m off southeast Australia yielded 536 specimens and 22 species. In 1906, the ALBATROSSdredged 11 specimens of a single: species from slope depths at 868 m in the Aleutians (Heath, 1911), but in 1970, 31 specimens of another species were taken in a 114- m2 spade box core from hadal depths at 7,298 m in the Aleutian Trench, an astounding density of l24'm-2 (Scheltema, 1985). Notwithstanding the inadequacies of sampling by earlier dredging efforts, the data that have accumulated since the CHALLENGERtook a single specimen off the Philippines in 1875 (Selenka, 1885) indicate a rich and diverse aplacophoran fauna in the Pacific from depths of 10 m to over 7,000 m, even though fewer than 3,600 specimens exist in collections or have been described from the entire Pacific. More than one-third ofthese have come from recent surveys off southeast Australia, and more than 700 were taken from areas of hydrothermal vents. Aplacophora have been collected from a wide variety of habitats: from the shelf beneath the cold waters of the north Pacific and from the tropical Great Barrier Reef and New Caledonia; from vent regions; from sea mounts; from the vast abyssal, oligotrophic regions of the central north Pacific; interstitially from sand; from shallow areas with .cyclic physical disturbance and from slope, abyssal and 50 SCHELTEMA: APLACOPHORA, A TETHYAN SLOPE TAXON 51 • • • Figure I. Pacific Ocean locations from which aplacophorans have been collected. Large solid circles: areas recently surveyed using epibenthic sleds and quantitative gear or, in the case of hadal depths greater than 5,000 m, by a single box-core collection; all samples sieved through fine-mesh screens; squares: localities represented by a single or a few non-quantitative samples; diamonds: hydrothermal vent locations sampled from a submarine; small open circles: sea mounts (guyots) sampled from a submarine (cf. Tables I, 3). hadal regions of great stability (Fig. 1; Table 1; and unpublished data). Species vary in size from the large, stout Neomenia yamamotoi (100 mm by 35 mm; Baba, 1975) and the long, cylindrical Epimenia verrucosa (300 mm by 10 mm; Baba, 1939) to numerous species less than 5 mm in length. Patterns of distribution are discernible within this great diversity of morphology and habitat. This paper suggests what some ofthese patterns are and the evidence for them: (1) many Aplacophora at slope depths (between 200 and 3,000 m) have distributions that are at least as old as Tethys; (2) new species have evolved on the continental shelf (subtidal to 200 m) and in the abyss (3,000 to 5,000 m) from colonization upward and downward from the slope respectively; (3) the high diversities on the slope and low diversities on the continental shelf reflect dis- turbance of the shelf fauna by Pleistocene regression of sea level. MATERIALS Specimens examined were from the Pacific surveys and expeditions marked by section marks in Table I. Several samples from the western Atlantic taken by Woods Hole Oceanographic Institution ships also provided information: R.Y. CHAIN Cruise 58 Stns 96 (136+ specimens) and 105 (105 specimens), and R.Y. KNORR Cruises 25 Stns 295, 297 (66 specimens) and 35 Stn 346 (190+ specimens). Other western Atlantic samples used were from Duke University (1965) Stns 351, 2361, 2732, and 3417 (14 specimens); and from the Instituto de Biologia, Universidade Federal do Rio de Janeiro (9 specimens). 52 BULLETINOF MARINE SCIENCE,VOL. 47, NO. I, 1990 Table I. Major expeditions and surveys which have collected Aplacophora in the Pacific (cf. Fig. 1)* Specimens Speciesi: Survey/expeditiont Date Location/depth m (No.) (No.) Authors, if published SIBOGA Exp. 1899-1900 East Indies 66 -13 Nierstrasz, 1902; 18-3,088 Stork, 1941 ALBATROSS Exp. 1891,1902 off Hawaii 18 7 Heath, 1911 9-275 ALBATROSS Exp. 1892, 1906 No. Pacific, Kuriles, 17 3 Heath, 1911 Alaska 122-868 ALBATROSS Exp. 1897,1904 off so. California 199 10 Heath, 1911 38-3,953 ALBATROSS Exp. 1903 So. Alaska (panhan- 22 3 Heath, 1911 die) + "several" 90-527 ALBATROSS Exp. 1906 off Japan 18 7 Heath, 1911 185-413 Allan Hancock 1952-1957 off so. California 283 14 Schwabl, 1963 Pacific Exp. 33-1,102 VEMA Exp.; 1958-1961, off Central, So. 11 3 Salvini-Plawen, U.S. Antarc- 1964 America 1972, 1978 tic Res. Pgm. 626-5,834 U.S. Antarctic 1964-1966 So. Pacific 39 9 Salvini-Plawen, Res. Pgm. 146-3,694 1978 Univ. Oregon§ 1962-1967 off Oregon 276 -12 unpubl. 100-2,900 SSR Academy 1970 Peter the Great Bay 42 4 Ivanov, 1984 Nauk 120 SEVENTOW 1970 Aleutian Trench 31 Scheltema, 1985 7,298 DOMES Project 1975-1976 mid-Pacific 3 4,985-5,117 Scheltema, 1985 So. California§ 1976 off Los Angeles, 143+ 10+ unpubl. Bight Pgm. California SEATECH§ 1981 off British Columbia 205 3 unpubl. 107-175 DSRV ALVIN 1981-1982 Panama Basin 59 -5 Scheltema, 1985 4,000 (part) Bass Str. Sur- 1980-1983 Bass Strait 382 -25 Scheltema, 1989 vey,§ Mus. <200 (part) Victoria CSIRO-FR5- 1986 Slope off SE Austra- 937 >40 Scheitema, 1989 86§ lia (part) 363-1,850 • Singlespeciesor specimensno! includedin the table but indicated on Figure I are describedin Selenka(1885: Philippines),Thiele (1897; 1902: Torres Strait. n.w. Australia, East Indies), Ponder (1970: off New Zealand), Baba (1940, 1975: Japan), Okuda (1943: Japan), Osorio and Tarifeiio (1976: Chile),Morse (1979: PugetSound, interstitial).Speciesto be describedhave been taken from sea mounts in the eastern Pacific(13 specimens, 7 spp.), from Panama Bay (25 specimens, 1 sPp.),from New Caledonia (4 specimens,2 spp.),and from the Great BarrierReef(J specimens,2 spp.). Most oflhe more than 700 specimensand 14 speciesfrom hydrotherrnal vents are listed in Table 3. t CSIRO= AustralianCommonwealthScientificand IndustrialResearchOrganization;SEVENTOW= R.V.Melville Cruise70, Scripps Institution of Oceanography;DOMES was a project under the National Oceanographicand Atmospheric Administration, U.S. De- pal1mentof Commerce;SEATECH wasa project under the Canadian Depal1mentof Fisheriesand Oceans;So.CaliforniaBightPgm. was under Minerals Management Servicl~, U.S. Department of Interior. * Speciesknown or suspectedto be synonyms in Slboga, Albalross, and Allan Hancock samples are not included. The same species may OCCurin more than one collection. § Collectionswith unpublishedspeciesexaminedfor the present paper. SCHELTEMA: APLACOPHORA, A TETHYAN SLOPE TAXON 53 TAXONOMIC CHARACTERS The main characters used to discriminate species and higher taxa in both apla- cophoran subclasses Chaetodermomorpha and Neomeniomorpha are body shape and proportions, position of spicules in the integumental cuticle, and spicule and radula morphology; in addition, the anatomy of the reproductive system, including the morphology of copulatory spicules, and type of salivary glands are taxonomic characters used in differentiating the hermaphroditic Neomeniomorpha. APLACOPHORA AS A TETHYAN FAUNA OF THE SLOPE The present-day distribution of aplacophorans has existed at least since Tethys, before the Atlantic and Pacific were isolated from each other. The evidence comes both from families with widespread slope distributions and from genera with disjunct Atlantic and Pacific distributions on slopes at mid-latitudes. The evidence that this ancient aplacophoran fauna existed primarily at slope depths, just as it does today, is the greater diversity of slope over continental shelf species, probably reflecting the stable environment of the former. Speciation appears to have oc- curred vertically in both upward and downward directions from the slope. Evidence for Tethyan distribution". - Two genera in the subclass Chaetodermo- morpha, the burrowing aplacophorans (Fig. 2A), and one family in the Neomenio- morpha, the creeping, cnidarian-eating aplacophorans (Fig. SA, B), are presented here as examples of Tethyan groups.
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