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The Aplacophora: History, Taxonomy, Phylogeny, Biogeography, and Ecology

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Doctoral thesis

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The Aplacophora are a spicule-covered, vermiform class of Mollusca important in understanding molluscan evolution and the biogeography and ecology of the deep sea. They first became known to scientists from dredgings made in the waters around Scandinavia in 1844. There are about 300 species described worldwide from all oceans at depths between 10 to over 9000 m with many species still undescribed. Most Aplacophora are less than 5 mm in length, ranging from broad and bulky to narrow and threadlike. There are two taxa one has a narrow foot upon which they creep, the other has lost the foot and burrows through the sediment. The uniqueness of Aplacophora lies in an external specialized vermiform shape while retaining an internal organization that appears to be primitive. It is proposed that they arose through a process of paedomorphosis which could account for 1 the small size, vermiform shape, and expression of primitive characters 2 a plesiomorphic metamery in Mollusca 3 a plesiomorphic large pericardium in the primitive classes and 4less developed characters in the Aplacophora of a chemically simple cuticle, radula with two teeth per row, small posterior mantle cavity, lack of kidneys, a foot as a nonmuscular ciliated ridge, and joined gonads and pericardium. The classes of molluscs are placed in the sister groups Aculifera and Conchifera. Distributions of aplacophoran genera indicate a pre-Tethyan deep sea radiation. Some species are dominant forms at some localities. Data from deep-sea experimental boxes indicates that one species becomes sexually mature within one year at 1500 m.

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  • Biological Oceanography

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