Family Brissidae

Brissopsis L. Agassiz, 1840

Brissopsis oldhami Alcock

• Brissopsis oldhami Alcock, 1893. J. Asiatic Soc. Bengal, 62, p. 174, Pl. 8 (7–8).
• Brissopsis zealandiae Mortensen Th., 1921. Vid. Medd. dansk naturh. For., 73, p. 193, Pl. 6, 33–34.

Material Examined: About 300 specimens from the following stations: 550 fathoms, Cook Strait, VUZ Station 101, about 30 specimens; 430 fathoms, Cook Strait, VUZ Station 97, 5 specimens; 435 fathoms, Cook Strait, VUZ Station 77, very abundant—about 200 specimens brought back to land; 400 fathoms, Cook Strait, VUZ Station 87, several specimens; 380 fathoms, Cook Strait, VUZ Station 96, 40 specimens; 380 fathoms, Cook Strait, VUZ Station 100, several specimens; 250–350 fathoms, Cook Strait, VUZ Station 53, 2 juvenile forms; 200–250 fathoms, Cook Strait, VUZ Station 10, 9 specimens; 50–200 fathoms, Cook Strait, VUZ Station 54, 2 juvenile specimens; 330 fathoms, Station 41, Chatham Islands 1954 Expedition, 23 juvenile specimens; 260 fathoms, Station 52, Chatham Rise, Chatham Islands 1954 Expedition, 3 juvenile specimens.

Remarks: Until now the genus Brissopsis has been recorded from New Zealand on only two occasions, one as Brissopsis luzonica, collected by the Challenger at Station 168, 1,100 fathoms, off Cape Kidnappers (Agassiz, 1881, p. 189), the other by Mortensen (1921), as Brissopsis zealandiae, at a depth of 75 metres, off Bare Island (holotype and one paratype). The very rich material now at our disposal makes it desirable to reassess the systematic status of the earlier records.

First, as regards "Brissopsis luzonica", it can be stated that this probably rests upon an error of identification. Following the recent discovery of large numbers of Brissopsis oldhami Alcock at Cook Strait abyssal stations, with no sign of other species, it appeared possible that the Challenger record might have been based on the latter species. Accordingly I forwarded two Cook Strait specimens to Miss A. M. Clark at the British Museum of Natural History and asked if they could be compared with the Challenger specimens of "B. luzonica" from Station 168. Miss Clark kindly located the Challenger material. There proved to be only a single specimen which, Miss Clark writes (personal communication 28/11/1957) "is very small and very broken. The apical system remains in one piece and although there are only 6 or 7 pore-pairs in the posterior petals they appear to form a straight, rather than sinuous page 39line, the two petals diverging. I think you can say it is not luzonica". The condition described by Miss Clark recalls that found in the young stage of B. oldhami.

Brissopsis zealandiae was established by Mortensen (1921) on the basis of two small specimens, said to be distinguished from B. oldhami in having 5 (instead of 4) ambulacral plates cut by the subanal fasciole, the posterior petals more divergent, an elevated periproct, smaller size, and small genital pores; the photograph of the holotype also indicates a more compact test, with steeper anterior and posterior faces, than in B. oldhami. After examining the very large sample of Brissopsis oldhami from Cook Strait, including specimens of a wide range of size, I have come to the following conclusions. (1) In most specimens it is difficult to decide exactly how many plates enter the subanal fasciole, but in those specimens where the outlines are quite distinct, the character is variable—either 4 or 5 ambulacral plates are involved with about equal frequency, and sometimes 4 plates plus a minute portion of a fifth plate. (2) The posterior petals are divergent in the size-range corresponding to that of Mortensen's types, but they become successively less so with increasing size (i.e., age). (3) The periproct may be elevated or not, according to the specimen. (4) There is no sharp distinction in size—the large specimens corresponding to Alcock's form intergrade imperceptibly with the smaller stages corresponding to Mortensen's. From VUZ Station 77 every stage from 10 mm to 80 mm length form one homogeneous population. (5) The size of the genital pores varies, though they are always relatively large in adult stages. In the small stages corresponding to Mortensen's material the pores are sometimes very small, but this scarcely means anything more than that they are juvenile stages of a large species. (6) Shape—here there appeared to be a hiatus between my material and Mortensen's type. However. Mortensen (1951, p. 397) mentioned that a second specimen was sent to the Dominion Museum. As this was not figured by him, I applied to Dr. R. K. Dell for permission to examine it. To my surprise it proves to have precisely the same shape as that normal for all my young stages of oldhami, and I am now convinced that Mortensen's holotype was abnormal in that one regard. Also the subanal region of the syntype had not been denuded, and it is difficult to distinguish reliably how many plates enter the fasciole unless this is done. It is therefore clear that Mortensen's original description must have been based mainly on the holotype only. In view of the evidence here assembled I conclude that Brissopsis zealandiae is a synonym of B. oldhami Alcock, and that all New Zealand specimens of Brissopsis so far seen are to be referred to the latter species. Photographs of the new material will be included in the official report of the Chatham Islands 1954 Expedition.