Genus Okamia n.gen.
This genus consists of compound Styelidae with a branchial sac without folds and with a smaller number (about five to 10) of internal longitudinal vessels on each side; with gonads, mostly, if not all, of a single sex; with ovaries lying posterior to the testes, which are present on both sides of the body, the ovaries sometimes being confined to the left side of the body.
Note: Since Michaelsen's revision of the compound Styelidae (1904) the classification of this group has been based mainly on the presence or absence of longitudinal folds in the branchial sac and the structure and manner of arrangement of the gonads. In Michaelsen's original diagnosis of the genus Metandrocarpa (1904, p. 69), in which he has placed M. thilenii, a New Zealand species, the portion referring to the gonads reads thus: “Geschlechtsorgane in eingeschlechtlichen Polycarpen jederseits ventral neben der Medianlinie, in der vorderen Partie weibliche, in der hinteren Partie männliche.” However, when in 1922 Michaelsen amended this genus to house two New Zealand forms, M. thilenii Mich, and M. photostigmatica Mich., he describes the gonads thus (1922, p. 416): “Polycarpe meist sämtlich eingeschlechtlich, jederseits männliche und weibliche, selten gelentlich zwei Polycarpe verschiedenen Geschlechts mehr oder weniger innig zu einem Zwitterorgan verwachsen.” The most important living worker in this group, Dr. Willard Van Name (1945, p. 241) interprets Michaelsen's amendment thus: “The gonads are mostly, if not all of them, each of a single sex, and are present on both sides of the body, the female only in the anterior part, the male in the posterior part. The male gonads consist each of a single testis,” though from Michaelsen's explanation of the amendment it is apparent that he considered the significant point of the diagnosis to lie not in the particular arrangement of the polycarps of different sexes (female anterior, male posterior), but in that male and female are present on both sides of the body.
The author has no doubt but that the specimens collected from Little Passage, Hauraki Gulf, and described below are identical with Michaelsen's Metandrocarpa thilenii. Thirty zooids from eight different colonies exhibited the gonad arrangement depicted in Text Fig. 3, the ovaries lying on the left side of the body on the arc of a circle posterior to the arc of the testes on that side. Three zooids, however, showed the gonad arrangement depicted by Michaelsen (1922, p. 459) in which the arc of the ovaries is slightly more extensive and one or two ovaries lie on the other side of the mid-ventral line. From these facts it is obvious that the presence of male and female gonads on both sides of the body is the exception rather than the rule in this species, and hence the species cannot be fitted easily into Michaelsen's genus Metandrocarpa even with the amended diagnosis, and the fact that the ovaries lie posterior to the testes removes it from the genus Metandrocarpa as interpreted by Van Name (1945). For these reasons and also because of the fact that the diagnosis of the genus Metandrocarpa seems to be completely unsatisfactory, it seems desirable that the species M. thilenii Mich. should be lifted from the genus Metandrocarpa
and placed in a new genus, as diagnosed above and which the author proposes to call Okamia.
Synonymy and Literature: 1922, Metandrocarpa thilenii, Michaelsen, Vidensk. Medd. naturh. Foren., bd. 73, p. 457.
Colonies of this species were very common on the undersurfaces and sides of rocks, mussels, etc., on the mainland side of Little Passage, which lies between Wanganui Island and the Coromandel mainland. The colonies form large, irregularly shaped mats up to twelve inches in the longest diameter. The zooids vary in colour from dull red to slate blue in different colonies and the siphonal linings are in all cases a brilliant red. The colony has a very thin basal membrane not more than 1 mm. in thickness. The zooids are joined by the basal membrane and occasionally by the lower third as well. They are packed very closely together (Plate 9, Fig. 1) and often the lower half of a zooid may be pushed into a square shape. Young zooids develop from any region of the lower third of the adult zooid.
Text Fig. 3—Okamia thilenii. Dissection showing body opened from the ventral surface, pharynx removed. × 7.5.
Individuals measure up to 10 mm. in length, 3 mm. in breadth, and 3 mm. in depth, and the siphonal apertures, which are situated at equal distances from the centre of the top surface, are from 1·5 to 2 mm. apart and 0·4 to 0·5 mm. in height. The test is thin, 0·1 to 0·3 mm. in thickness, and in colonies growing in a region washed by a strong current is free from sand grains or incrustations, though in specimens from still backwaters it may be quite heavily impregnated with fine particles of sand. The mantle wall is a dark dull red in colour and is well supplied with fine muscle fibres. On each side of the wall of the pharynx there are ten longitudinal vessels separated by few or many stigmata according to their position in the pharynx, there being usually nine in the mesh next the endostyle, two in the mesh next the dorsal lamina and four in the intervening meshes. The stigmata are five to six times as long as they are wide and are usually crossed by a parastigmatic vessel. The branchial tentacles are thirty-two to forty-eight in number and of one or two orders of size. The dorsal tubercle is a small round cushion and the dorsal lamina a plain straight membrane. The gut loop lies on the left side of the body and
consists of a short narrow oesophagus; a wide stomach bearing twenty to twenty-two longitudinal folds arranged on a slight spiral and with a hook-shaped gastric caecum; and a long smooth-walled intestine which takes a sharp turn close to the stomach and then runs straight up towards the atrial aperture. The anal aperture is smooth and two-lipped. Small pigmented endocarps are present on both sides of the mantle wall and number from ten to fifteen on the right and from eight to ten on the left. Atrial tentacles are present. They are very small and extremely numerous.
The gonads are of one sex, the testes being light-coloured, pear-shaped structures with well-marked gonoducts, and the ovaries being brownish-red rounded structures. The ovaries are arranged in a single row along the arc of a circle and lie towards the posterior end of the left side of the body. The arc of the ovaries may extend just slightly past the mid-ventral line. Testes are present on each side of the body, where they are arranged in a single row along the arc of a circle, the row on the left side being always anterior to the row of ovaries. Ovaries are from five to ten in number in the Coromandel specimens, and testes five to eight in number on the right side of the body, nine to thirteen in number on the left side of the body. The eggs ripen in January. They are bright orange in colour and extremely large (0·5 mm. in diameter) compared with the size of the zooid. They undergo development within the mantle cavity and become large orange tadpoles heavily pigmented at the anterior end (Plate 9, Fig. 8). The largest measured 3·5 mm. in length and 0·5 mm. in width in the head region.
Distribution: In New Zealand—Tauranga (Thil. legacy, Berlin Museum), New Plymouth (Michaelsen), Hauraki Gulf.
Remarks: Type specimens of this form are not available, but the author has no doubt but that the specimens described above belong to Metandrocarpa thilenii Mich., which the author proposes to place in the newly constructed genus Okamia. The gonad count given by Michaelsen slightly exceeds that of the Coromandel specimens, but in a group showing a wide range of variability such a minor difference may easily be due to local variation.