The deep-sea isopods: A biogeographic and phylogenetic overview

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Abstract
Isopod crustaceans are among the most numerous and diverse macrofaunal taxa in the deep sea. In contrast to the situation in shallow water, the Asellota encompasses the majority of the deep-sea fauna. Taxa within the Asellota exhibit marked depth zonation. Its most primitive superfamily is limited to fresh water. The rest are marine, where with increasing depth the incidence of more specialized taxa increases. Within its most advanced superfamily, Janiroidea, the most primitive families are primarily restricted to shallow water, while the more specialized families dominate in the deep sea. The most notable exception is the occurrence of these specialized families in shallow water at high latitudes. On the basis of this pattern, it has been suggested that the deep-sea fauna is a result of invasion from shallow water, primarily at high latitudes. The present paper argues that, on the contrary, the specialized deep-sea families evolved in situ, and that their presence at shallow high latitudes is the result of subsequent emergence. Several lines of evidence support this conclusion. Their primary diversity is in the deep sea. In some families, the most primitive genera or their progenitors in other families live in the abyss. Finally, the deep-sea families all lack eyes, regardless of locality, whereas the more primitive families that dominate in shallow water have them, except where they are found in the deep sea. The only simple and consistent explanation for these facts is a deep-water origin for the deep-sea families.