Art. VII.—On the Changes in Form of a Parasitic Isopod (Nerocila).
[Read before the Otago Institute, 13th October, 1890.]
Cilonera macleayii, Leach, MS.; White, in Dieffenb. Voy. N.Z., vol. ii., p. 268. [See Miers, Zool. H.M.S.” Alert,” p. 301.]
Nerocila imbricata, List Crust. Brit. Mus., p. 108, sinedescr. Miers, Cat. N.Z. Crust., p. 107.
Nerocila novæ-zelandiæ, Schiödte & Meinert, Naturhistorisk Tidsskrifft, ser. iii., vol. xiii., p. 70, pl. v., figs. 10, 11.
Nerocila macleayii, Thomson and Chilton, Trans. N.Z. Inst., vol. xviii., p. 155.
Nerocila macleayii, Thomson, Trans. N.Z. Inst., vol. xxi., p. 263.
This species was given, under the name Nerocila imbricata, in Miers's “Catalogue of the New Zealand Crustacea” (1876), as being represented in the collections of the British Museum by specimens from New Zealand. Up to the time of publishing our “Critical List of the Crustacea Malacostraca of New Zealand” in 1885 neither Mr. Thomson nor myself had met with any specimens, and consequently the note, “We do not know this species,” was added in the list after the name of the species. Very soon, however, after the publication of this list both of us got specimens, and it appears that the species is not an uncommon parasite on several fish, especially on the west coast of New Zealand.
Young forms on which the incubatory pouch has not yet been developed differ very much from the adult female in the proportions of the body, the prominence of the epimera, &c., so that I did not recognise them as belonging to the same species until I got from the Dunedin Museum a bottle con-
taining four specimens, of which one was an undeveloped female, and another a fully-developed female with large incubatory pouch, and the other two also females with pouches developed, but in other respects intermediate stages between the first and second. It is the first form that has been described by Schiödte and Meinert as Nerocila novæ-zelandiæ, and is by them spoken of as “virgo.” Their detailed description applies very closely to specimens of this stage, so that I have no hesitation in giving their name as a synonym. These authors have given the differences between the “virgo” and the adult female in other species of Nerocila, but the material at their command evidently did not enable them to do this for this particular species; and, as their work is not easily accessible to students in New Zealand,* I propose to give here a description of the “virgo,” and afterwards to compare it with the fully-developed female.
The body is narrow-oval, about three times as long as broad. The head is large, suborbicular, broader than long, slightly immersed in the first segment of the thorax, front rounded or with very slight indication of an angle in the centre. The eyes are of fair size and quite distinct, sub-pentagonal or subhexagonal in shape. The 1st pair of antennæ are somewhat compressed; those of the 2nd pair much more slender, terete, 11-jointed. The posterior angles of the first five thoracic segments are not produced, and are rounded or obtuse; those of the 6th and 7th segments are obtuse and very slightly produced backwards. The first three pairs of epimera are rectangular, overlapping, rounded posteriorly; the last three are somewhat acute posteriorly; the 5th more produced than the 4th, and the 6th equally more so than the 5th. Last pair of thoracic legs a little longer and more slender than the others, and with a few spines on the meros, carpus, and propodos. Abdomen exposed—that is, not covered by the thorax; much longer than broad; 1st segment shorter than the others; 2nd, 3rd, and 4th subequal; 5th a little longer: pleural portions of the 1st and 2nd segments only slightly produced. Telson cordate, as long as the other segments of the abdomen together, about as broad as long, somewhat sharply rounded at the extremity. Uropoda with the basal joint expanded distally, and slightly produced at the inner distal angle; inner branch broad and flat, widening a little distally, obliquely truncate, a very small tooth at the inner angle, the truncate extremity slightly sinuate, outer angle reaching a little beyond the extremity of the abdomen; outer branch much narrower and somewhat longer, widest about the middle, flat, extremity subacute.
[Footnote] * I am indebted to Mr. Thomson for the loan of his copy.
Colour indistinct, yellowish or olivaceous, sometimes with an indistinct darker line down the middle, and one much more indistinct down each side.
Length, 0.78in.; breadth, 0.28in.
There are no marks indicative of sex on the immature form I have described and figured (see Pl. XI., fig. 2); but it appears to be a slightly younger form than the “virgo” described by Schiödte and Meinert under the name Nerocila novæ-zelandiæ.
The adult female (see Pl. XI., fig. 1, a, b, c) is much broader in proportion, being only about twice as long as broad; the eyes are very indistinct; the 6th and 7th thoracic segments have the postero-lateral angles acute and much more produced, and the last three pairs of epimera are acutely produced backward, but not beyond the posterior margin of the segments to which they belong. The pleural portions of the first two segments of the abdomen are largely developed and flat; that of the 2nd extends backwards as far as the base of the uropoda. The uropoda are more slender than in the immature forms, and have the basal joint acutely produced rather more than half-way along the inner edge of the inner branch, which is obliquely truncate, and has the outer angle very acute; the outer branch is narrower and rounder than in the immature forms, does not widen at all in the middle, but gradually narrows from the base to the extremity, which is acute.
The colour is olivaceous, and is usually darker and more uniform than in the immature forms.
Length, 1.3in.; greatest breadth, 0.65in.
From the Otago University Museum I obtained four specimens, of which the first was an immature form closely resembling the one I have already described, the length being 0.9in., and the breadth 0.3in. The second specimen was a female with eggs in the brood-pouch, but evidently not so fully grown as the mature form described above; the length was 1.05in., and the greatest breadth was 0.45in., so that the body is narrower than in the fully-developed female; and the epimera, posterior angles of the thoracic segments, and pleural portions of the first two segments of the abdomen were also less developed. The third specimen was also a female bearing eggs, and was larger and more developed in all the respects mentioned than the second specimen, being 1.15in. long, and 0.5in. in its greatest breadth. The fourth specimen was a fully-developed female precisely similar to the form described above, but was slightly larger, being 1.4in. long and 0.7in. broad.
This species seems to be widely distributed in New Zealand, and to be found on several kinds of fish, though, unfortunately, I am not able to give any precise information as
to its hosts. I have seen specimens from Lyttelton, Dunedin, and Greymouth, those from the latter locality being forwarded to me by Mr. R. Helms. Mr. Thomson also records the species from the west coast of the South Island. Schiödte and Meinert, had specimens from Melbourne also.
It is hazardous to venture opinions on the affinities of different species from descriptions alone; but, so far as I can tell from those of the species described by Schiödte and Meinert, the one that comes nearest to our species is N. fluviatilis from the Rio Plata—a species which is perhaps the same as one found at the Falkland Islands.
Description of Plate XI.
Fig. 1. Nerocila macleayii, mature female: a, dorsal view; b, side view, limbs omitted; c, uropoda.
Fig. 2. Nerocila macleayii, immature form: a, dorsal view; b, side view, limbs omitted; c, uropoda.
Note.—Figs. 1, a, and 1, b, and 2, a, and 2, b, are twice the natural size; figs. 1, c, and 2, c, are more enlarged.