Rhopalaea crassa (Herdman, 1880)

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 : 1953-1955

Rhopalaea crassa (Herdman, 1880)

Ecteinsascidia crassa Herdman 1880, p. 723 .

Rhopalaea crassa: Kott 1992, p. 623 and synonymy; Kott 2003, p. 1613 and synonymy. Monniot F. and C. 2001, p. 297.

Distribution

Previously recorded ( Kott 1992; Monniot F and Monniot C 2001): Western Australia (Cockburn Sound, Houtman’s Abrolhos); Queensland (Mooloolaba, Heron Is. and north to Lizard Is.); Papua New Guinea; Indonesia: Philippines; Mariana Is.; the South China Sea; Japan; Sri Lanka; Mozambique. New record: CSIRO 05/07 western Australia (Station 082-173, Imperieuse, 18.46 S 120.15 E, 80 m, Sled tow, 19.6.07, QM G328561).

The new record, though deeper than earlier ones (which mostly are from reefal habitats), is on the continental shelf.

Description

The newly recorded specimen is 2.5 cm long, with firm opaque test over the whole zooid as formerly described for mature specimens of this species. The thoracic test is smooth on the surface but abdominal test is uneven and debris adheres to it. Each of the six-lobed apertures, inconspicuous in the thick, firm test, is on a short siphon. Both apertures are at the anterior end of the zooid. The thorax of the zooid is contracted (obscuring details of the branchial sac) and only wide parallel longitudinal muscle bands can be detected along each side. The abdomen is firmly embedded in the thick abdominal test and muscles were not detected on it. The rectum of this specimen, projecting well forward to the atrial aperture, is filled with mud.

Remarks

As previously observed ( Kott 1990), the species displays great diversity associated principally with its age and the state of contraction of its thoracic muscles. The consistency of the thoracic test certainly varies with age, being delicate, flexible and transparent in younger zooids, but becoming thick, firm, inflexible and opaque like the abdominal test as individuals mature and possibly as thoraces regenerate ( Kott 1990). Colour of the zooids also appears to be variable, from clear and colourless to pale and even dark blue and the test also varies in colour, some more mature specimens being yellow. It is possible that there is a cline in colour from south to north, colours being more intense toward the northern part of the range.

F. Monniot and C. Monniot (2001; p. 295) define the Indo-west Pacific range of this species (from Fiji to the western Indian Ocean). This range is not unusual for tropical species. Nevertheless, these authors doubt the variability reported for the species, believing that “characters of several species have been mixed up”. However, size and proportions of different parts of the body are affected by growth and/or muscle contraction, as are the numbers; arrangement and course of the longitudinal thoracic muscle bands and these characters do not constitute the reliable differences that would distinguish these species from one another ( Kott 1990).

Key to species of Diazona from the Indo-west Pacific

1. Colonies stalked.............................................. 2 Colonies not stalked........................................... 3

2. Colony mushroom-shaped............ D. fungia Monniot F and C, 2001 Colony not mushroom-shaped.... D. pedunculata Monniot F. and C, 2001

3. Surface with network of raised ridges.. D. labyrinthea Monniot F. and C., 1996 Surface without network of raised ridges........................... 4

4. Thoraces project from surface of colony............................ 5 Thoraces do not project from surface of colony...................... 6

5. Terminal lobes of branching colonies containing 2–10 thoraces.......................................... D. formosa Monniot F. and C., 1996 Thoraces project independently from surface........................... D. textura Monniot C., 1987 (> D. tenera Monniot F. and C., 1996, 2001)

6. Longitudinal muscles extend onto posterior abdominal stolon; colonies massive...................................................... 7 Longitudinal muscles do not extend posterior to the abdomen; colonies not massive......................... D. angulata Monniot F. and C., 1996

7. Colonies globular........................ D. chinensis (Tokioka, 1955) Colonies not globular............... D. carnosa Monniot F. and C., 1996