HEXACROBYLIDAE Seeliger, 1906

Kott, Patricia, 2009, Taxonomic revision of Ascidiacea (Tunicata) from the upper continental slope off north-western Australia, Journal of Natural History 43 (31 - 32), pp. 1947-1986 : 1982-1983

Family HEXACROBYLIDAE Seeliger, 1906

Type genus: Hexacrobylus Sluiter, 1905 (< Oligotrema Bourne, 1903 ).

The family Hexacrobylidae , with its type genus Hexacrobylus Sluiter, 1905 was erected by Seeliger, 1906. However Kott (1989) established that Sluiter’s genus is a junior synonym of Oligotrema Bourne, 1903 (type species O. psammites ) and H. indicus Oka, 1913 , wrongly assigned to Sluiter’s genus, has become the type species of the genus Asajirus Kott, 1989 .

The family, reviewed most recently by Kott (1992), contains stolidobranch ascidians related most closely to the Molgulidae with a hermaphrodite gonad on each side of the body (consisting of tight clusters of branching testis follicles around the proximal end of a long tubular ovary) and a large molgulid-like kidney on the right side of the body. The genera in this family are characterized by remarkable adaptations that appear to be associated with a predatory rather than filter feeding habit. They have a large buccal cavity in front of the incurrent aperture that is produced from a forward-projecting fold of the body wall. Six pinnate arms arranged around its anterior margin protect this cavity. These arms are muscular extensions of the body wall and should not be regarded as homologous with the endodermal branchial tentacles found universally in the Ascidiacea. Body musculature is strong but localized, consisting of circular siphonal sphincters and a few strong longitudinal bands usually confined to the anterior part of the body that project out into the branchial arms. The mucus secreting endostyle has not been detected. The pharynx is much reduced to a thick ring of tissue around the gut at the base of the buccal cavity. The wall of the pharynx is perforated by ciliated stigmata that connect it with the two anterior horns of the atrial cavity. These openings sometimes are depressed into simple concavities or occasionally a complex three-dimensional network of interconnected spaces. These spaces should not be regarded as tubes or tubules as C. Monniot and F. Monniot (1990) have termed them and homologous spaces can be seen in the elaborate complex of overlapping internal branchial vessels lining the pharynx of certain species of the Molgulidae ( Kott 1989) . It appears that the incurrent flow of water that could be generated by these small and limited perforated areas would be for irrigation and possible aeration rather than filter feeding. The atrial opening, relatively small in comparison with the large branchial aperture, appears to be adequate when compared with the likely current flow through the restricted passages from the pharynx to the peribranchial cavity. Large, possibly glandular diverticulae of the gut that could be digestive pouches occur in the genus Asajirus (which also has lost its branchial tentacles), supporting the view that species in this family are carnivorous. The two closely related genera in this family are also distinguished by the position of the atrial siphon, which is postero-dorsal and directed posteriorly in Oligotrema Bourne, 1903 , and mid-dorsal and directed anteriorly in Asajirus Kott, 1989 . Both genera are known from relatively few species, the former from four species and the latter from three.

Two Oligotrema species , O. psammites (the only species from this family represented in this collection) and O. lyra Monniot C. and F., 1973, and one Asajirus species , A. indicus (Oka, 1913) , have unusually extensive depth and geographic ranges from about 600 m to 5000 m in the Indian, Pacific and Atlantic Oceans. Of these species only O. lyra has not been recorded from Australian waters ( Kott 1989, 1992; also see later).