Hypsistozoa distomoides ( Herdman, 1899 )

Kott, Patricia, 2007, Taxonomic affinities of three stalked colonial species of the Ascidiacea (Tunicata) from the central coast of New South Wales and indications of a trans-Tasman connection, Journal of Natural History 41 (9 - 12), pp. 633-645 : 639-642

Hypsistozoa distomoides ( Herdman, 1899)

Figures 1C–H , 2 , 3B

Amaroucium distomoides Herdman 1899, p. 72 (holotype: AM G2106 ).

Aplidium distomoides: Kott 1963, p. 92 (reference only).

Distaplia distomoides: Kott 1972a, p. 170 ; 1972b, p. 243

Hypsistozoa distomoides: Kott 1990, p. 134 .

Distribution

Previously recorded (see Kott 1990). South Australia (Great Australian Bight, Spencer Gulf, Yorke Peninsula); New South Wales (Jervis Bay, Port Kembla, Port Jackson ) .

New record. New South Wales (Port Stephens, QM G308880 G328032 G328076 G328092).

Description

The newly reported colonies are narrow (up to 1 cm diameter and 7 cm long) with a short thick stalk at the base. Zooids are crowded in circular, oval or elongate double-row systems. Crustacean commensals are present in cup-shaped depressions on the surface of the colony.

Zooids have the usual large sessile atrial apertures exposing the four rows of stigmata directly to the common cloacal cavity. The tip of the large anterior atrial lip is pointed or straight, sometimes with a fringe of up to five pointed lobes. The branchial sac has four rows of 12 stigmata in each half row. Each row is crossed by a parastigmatic vessel. The gut loop is characteristic of the family, with a narrow oesophagus and a rounded distal loop. The stomach is short, horizontal, with eight shallow rounded folds that flatten distally where the stomach narrows. The duodenal area is a narrow, cylindrical tube with a distinct rectal valve separating it from the long rectum that occupies the pole of the loop and the whole of the ascending limb. A large, conspicuous reservoir of the gastrointestinal gland is in the centre of the gut loop.

A transparent thick posterior abdominal vegetative stolon containing an epicardial sac extends behind the abdomen. It is not in vegetative phase in the newly recorded specimens.

Gonads often are not present. Male follicles but neither ova nor embryos are in one colony (QM G328032) collected in June. Immature larvae are present in re-examined colonies collected from Port Kembla in September and October (see Kott 1990) and in newly recorded colonies collected in July (QM G328092), August (QM G 328076) and September (QM G308880). The larvae have an almost spherical trunk up to 0.09 mm diameter with a tail, three triradially arranged adhesive organs (each a cone of adhesive cells in a shallow ectodermal cup at the end of a large oval melon-like protuberance), otolith and ocellus, endodermal tubes, ectotrophic membranes, stomach and rectum and a thick cylindrical vegetative stolon of about 0.25 mm diameter that projects forwards to the base of the adhesive organs on the left side of the larval trunk. However, in the best developed Australian larvae (QM G308880) the pharynx is not differentiated and stigmata were not detected, precocious buds are not present and the stolon has not developed to the U-shape reported in the more advanced larvae in the New Zealand species ( Brewin 1956). The larval test, generated by the ectoderm beneath the ectotrophic membranes, can be seen forming the vanes of the tail but was not detected elsewhere. The larval tail is wound nearly all of the way around the trunk, terminating just in front of the sense organs.

Remarks

As well as documenting the histology of development of embryos and larvae, Brewin (1956) has given a timetable of the annual cycle of reproduction and growth of the New Zealand Hypsistozoa fasmeriana . Having produced eggs in June and sperm from May to July, colonies are incubating embryos and larvae in their brood pouches from June to October. Zooid replication and colony growth occurs from November to April/May (summer/ autumn) and gonads are not present during those months. Brewin (1956) found that the egg, fertilised in the brood pouch, had developed to maturity and was freed from the parent zooid after five and a half months.

Newly recorded specimens of H. distomoides have immature larvae in the brood pouches from July to August and previously they have been reported from September and October ( Kott 1990). Testis follicles are present in southern Australian populations in March, April and May ( Kott 1990) and in newly recorded material in June. Populations disappeared during the summer months from estuarine rocky reef locations with strong tidal currents at Halifax (Port Stephens). The Australian species appears to have a reproductive timetable similar to the New Zealand one and the larval organs that are present are very much like those in the New Zealand species at comparable stages of development. In particular, the stout, conspicuous, 0.25 mm diameter vegetative stolon resembles the 0.235 mm diameter stolon of H. fasmeriana ( Brewin 1959) . The size of ova of H. distomoides is not known, possibly because few specimens have been collected in midwinter. Similarly there are no records of the species in late spring when it could be expected that the pharynx of maturing larvae would be perforated and a long vegetative stolon would be actively replicating as described for the New Zealand species. Although the blastozooids that Brewin (1959) reported in the more advanced larvae have not been reported in the Australian species, it is probable that the long larval development from late winter through spring (over five months) that Brewin found in the New Zealand material could occur also in the Australian populations. This could explain the fact that other unique aspects of the larvae (including the ectotrophic membranes and endodermal tubes) now known to occur in the Australian species were not detected previously. It appears from the newly recorded specimen that the unique larval development that Brewin (1956, 1959) described is characteristic of the genus rather than being exclusive to the New Zealand species.

Although the similarity detected in the early larvae probably persists through to maturity, differences in the number of stomach folds ( Kott 1990) and the size of the larval trunks are significant and justify the separation of the Australian and the New Zealand species. The stomach has 14–18 folds in H. fasmeriana and only eight broad folds in the present species. The larval trunk of H. fasmeriana is apparently twice the length of the Australian species at the same stage of development. Larval trunks of the former species 1.8 mm long ( Brewin 1956 Figure 8E) are at the same stage of development as those of the newly recorded Australian specimens with the trunk 0.9 mm long. In the New Zealand species the average length of the larval trunk is, in well developed larvae, 2.9 mm (see Brewin 1956, p. 448).

The further study of the Australian species H. distomoides and its natural history (including its larval development) is well justified in view of its unique larvae otherwise known only in the New Zealand species.

Figures

Figure 1. Euclavella claviformis (QM G308883): (A) colony; (B) developmental sequence of embryos in the distal part of the oviduct forming a collar around the top of the oesophageal neck. Hypsistozoa distomoides (QM G308880): (C) colony; (D) zooid; (E) zooid (with denticles on the atrial lip); (F) larva (left side with ectotrophic membranes pulled away from the trunk); (G) larva (with ectotrophic membranes intact); (H) larva (right side with ectotrophic membranes pulled away from the trunk). Scale bars: A, B, 10 mm; C–G, 0.2 mm.

Figure 2. Hypsistozoa distomoides (QM G308880) postero-dorsal vertical section of larval trunk (semidiagramatic): e, endodermal tube; nc, nerve cord; o, section through oesophagus; ec, ectotrophic membrane; s, stomach wall; oe, oesophagus; h, haemocoele; le, larval ectoderm; n, notochord; r, rectum. Scale bar, 0.1 mm.

Figure 3. In situ colour images: (A) Euclavella claviformis; (B) Hypsistozoa distomoides colony lying flat on the sea floor at low slack tide; (C) Sigillina cyanea from high-energy location, showing tip of colony worn by abrasion on sea floor sediments. The possibly commensal nudibranch Nembrotha sp. is associated with the colony.