Evolution and systematics

The class Sorberacea was erected in 1975 by the French ascidiologists Claude and Fran├žoise Monniot and Fran├žoise Gail for several highly adapted deepwater species belonging to four genera (Oligotrema, Hexacrobylus, Gasterascidia, and Sorbera) united in the family Hexacrobylidae. The characters used to distinguish the Sorberacea from class Ascidiacea were the absence of the branchial sac, the presence of dorsal nervous cord, a very superficial position of the nervous ganglion, the histology of the digestive tract, and some other features.

The taxonomic position of Hexacrobylidae was not certain, but the family was most often considered as related to the ascidian family Molgulidae, or placed into a separate order, Aspiraculata, within the Ascidiacea. However, other tax-onomists, including the Australian ascidiologist Patricia Kott, doubted whether the Hexacrobylidae should be removed from the class Ascidiacea. Kott considered that the characters used to distinguish Sorberacea and Ascidiacea could not be confirmed as true differences between Hexacrobylidae and the rest of the Ascidiacea. In particular, although the branchial sac is strongly reduced in Hexacrobylidae, it is not absent, and the neural complex in Hexacrobylidae is in the same position beneath the epidermis as in other ascidians. In addition, the structure and histology of the digestive tract varies in other ascidiaceans.

There are also problems in defining genera and species in the group. Of the four nominal genera mentioned above, Gasterascidia, Sorbera, and Hexacrobylus are now treated as synonyms of Oligotrema, and all species assigned previously to

Hexacrobylus (apart from the type species of this genus) now belong to Asajirus. Thus the family Hexacrobylidae contains only two valid genera: Oligotrema and Asajirus. Oligotrema contains five species, and Asajirus (formerly known as Hexac-robylus), contains 12 nominal species, six of which are most probably synonyms of widely distributed Asajirus indicus.

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