Corophium crassicorne Bruz.
Corophium contractum Stimpson, 1855, P. Ac. Philad., vol. 7, p. 383. C. contractum G. M. Thomson, 1880, p. 6; 1881, p. 220, pl. 8, fig. 9: Thomson and Chilton, 1886, p. 142. C. crassicorne Thomson and Chilton, 1886, p. 142; Sars, 1894, p. 615, pl. 220; Stebbing, 1906, p. 690.? C. bonellii Sars, 1894, p. 616, pl. 221, fig. 1; Stebbing, 1906, p. 691; Walker, 1914, p. 559.
In 1880 Mr. G. M. Thomson (1880, p. 6) obtained by the dredge in Dunedin Harbour two specimens of a species of Corophium which he identified as C. contractum Stimpson, a species described from Japan. Both Mr. Thomson's specimens were stated to be adult females. In a paper published in the following year (1881, p. 220) he repeated the observations and description which he had given of his specimens, and added a figure
of the whole animal. Shortly after this I collected in Lyttelton Harbour specimens that agreed with the description given by Mr. Thomson, and I therefore identified them as C. contractum. At the same time, and in association with these specimens, I collected others similar in most characters but differing in the form of the second antenna. These specimens appeared to be closely similar to the descriptions and figures given of C. crassicorne Bruz. in Spence Bate's Catalogue of the Amphipoda in the British Museum and in Bate and Westwood's British Sessile-eyed Crustacea, and were accordingly named C. crassicorne. Since the specimens identified as C. crassicorne were associated with those identified as C. contractum and apparently were males—at any rate, not bearing eggs—I concluded from the general resemblance between the two that they were male and female of the one species. As C. crassicorne was recorded from Europe, I looked up the works mentioned above to see if there was any mention of a form similar to C. contractum to represent the female of C. crassicorne in Europe, and found that C. bonellii Milne-Edwards appeared to be very similar to the New Zealand specimens I had identified as C. contractum, and I concluded therefore that it was probably the female of C. crassicorne. On writing to the Rev. T. R. R. Stebbing asking for information as to whether this conclusion was correct or not, he replied that some authorities considered C. crassicorne and C. bonellii to be male and female of the one species, while others, including Sars, considered them as distinct species.
In view of this difference of opinion, and in the absence of specimens from Europe, or sufficiently detailed descriptions to investigate the matter fully, the question was for the time left an open one, and in the list of the Crustacea Malacostraca of New Zealand, published in 1896 by Mr. G. M. Thomson and myself, the two species C. contractum Stimpson and C. crassicorne Bruz. were included with the following note after the last-named: “This species is taken along with C. contractum, and it is probable that they are only male and female of the same species. C. bonellii (Milne-Edwards) is probably the same as C. contractum.—C. C.” (1886, p. 142).
For various reasons I was unable to give further attention to this particular question for many years, though on several occasions when specimens of Corophium were collected at different parts of the New Zealand coast both forms—i.e., “C. contractum Stimpson” and “C. crassicorne Bruz.”—were taken together, thus fully confirming my opinion that these were male and female of the same species, whatever might be the case with the C. crassicorne Bruz. and C. bonellii in Europe.
In the meantime many important works on the Amphipoda have been published which contain more or less direct evidence on the point at issue: e.g., Sars in his great work on the Amphipoda of Norway in 1894 still keeps the two species separate, and describes forms which he considers to be male and female of C. crassicorne, the female form being different from the specimens which he refers to C. bonellii. Of this latter species he describes no male, saying, “It is very strange that I have never met with males of this form, though I have collected the species in several different places. Perhaps the sexual difference is so very slight as to escape attention” (1894, p. 617). In Das Tierreich Amphipoda, Stebbing (1906, p. 690), apparently following Sars, describes male and female forms of C. crassicorne, and considers C. bonellii a separate species, of which only the female is known.
I do not propose to go into the history of the various opinions that have been expressed as to the relation of C. crassicorne Bruz. and
C. bonellii M.-Edw. It is evidently a difficult question, and probably will not be thoroughly settled till we know more of the life-history and sexual differences of these animals. The latest discussion with which I am acquainted is given in a paper by Walker (1914, p. 559), where he points out that C. acherusicum Costa is a synonym of C. bonellii, and in which he regards this species as distinct from C. crassicorne Bruz. He had previously (1909, p. 343) recorded C. bonellii from the Indian Ocean, but at that time had evidently been in considerable doubt about the identification, for in the copy of his paper forwarded to me he had altered the printed name C. bonellii to C. crassicorne. In 1914 he says the name C. bonellii should be left as printed.
I shall content myself with a statement of the facts of the New Zealand species as they appear to me. The male specimens have the very large stout second antennae corresponding precisely with the figures given by Sars for C. crassicorne Bruz., and in other points the animals appear to agree closely with his description and figures except for the slight difference in the third uropod which is mentioned below. The female specimens also seem to agree closely with the description he gives for the female of C. crassicorne, though there appears to be some variation in the second antenna, the number of spines on which does not always agree precisely with the figure, and in some specimens these appendages agree more closely with his figure of C. bonellii. These two forms have been constantly found together in New Zealand, and I feel certain that they must be looked upon as male and female of the one species. Doubtless, as in other species, the adult characters of the second antenna in the male are only gradually attained, and the immature stages more or less closely resemble the female form. In an attempt to settle the question I got specimens some years ago, through the kindness of Mrs. Sexton, Plymouth, from the Dutch coast, sent by Dr. Hoek as “C. crassicorne,” and others from the laboratory at Plymouth labelled “C. bonellii.” The Plymouth specimens were apparently all females—at any rate. I have not found an adult male among them; but those from the Dutch coast contained both males and females, the males agreeing closely with Sars's description of C. crassicorne. After careful comparison of both sexes of these specimens with the New Zealand forms I have failed to distinguish any character that I consider of specific importance, and I am therefore labelling and recording the New Zealand specimens as C. crassicorne Bruz. I have also specimens from Port Jackson, New South Wales, agreeing minutely with the New Zealand forms.
Sars says that that C. bonellii is distinguished by (1) the absence of a rostrum, (2) the rounded lateral angle of the head (not sharply acute as in C. crassicorne), and (3) the character of the second antenna of the female. In all the specimens that I have examined for this particular point—viz., from New Zealand, “C. crassicorne” from the Dutch coast, and “C. bonellii” from Plymouth—the rostrum is present. The lateral angle of the head is, as Walker states, difficult to see, but as far as I can make out it varies, in some cases being somewhat rounded, as described by Sars for C. bonellii, and in others more acute. With regard to the third point, as already stated, I find considerable variation in the antennae of the females, and the New Zealand forms agree, some with the figure given by Sars for C. crassicorne, others with that for C. bonellii.
The only point in which the New Zealand specimens differ from the European ones that I have examined appears to be in the third uropods, which are slightly broader both in the peduncle and in the ramus, and have the two rami usually directed slightly towards the median line, instead of
projecting directly backwards as shown by Sars for C. crassicorne. The difference is, however, not great, although it is easy to make considerable difference in the figure, and the general appearance of the end of the pleon is very near to that figured by Sars for C. bonellii.*
Although the fully adult males and females in this species appear to be readily distinguished from one another by the characters of the second antenna, it is probable that the sexual relations are not always quite so simple. For example, I have a specimen, now mounted permanently as a micro-slide, in which the second antennae are stout and have on the under-surface a stout tooth which corresponds to the tooth found in the adult male, though not so pronounced; this specimen I should without hesitation consider as an immature male, but unfortunately on the appendages of the peraeon there are brood-pouches similar to those in the female. In the two species C. spinicorne Stimpson and C. salmonis Stimpson from the Pacific, which were redescribed in 1908 by Bradley, the adult females, as figured by him, have the characters of the second antennae of the adult male, though these are not developed to quite the same extent.
It is well known that C. crassicorne, like other species of the genus, is frequently found in brackish and sometimes even in perfectly fresh water. As far as I am aware, the New Zealand species has been taken in salt water only, though the allied form Paracorophium excavatum Thomson is found in brackish and fresh water. Stebbing has described from the brackish water of Lake Negombo, in Ceylon, a species, C. triaeonyx, which appears to me to be very close to the New Zealand forms, but differs in having the third uropods much less broadened. Similarly, in 1912, Wundsch described C. devium from fresh water near Berlin, a species which, from his figures, seems to agree very closely with Stebbing's species in the characters of the terminal uropods.
[Footnote] * Stebbing (1914, p. 372) records Corophium cylindricus (Say) from the Falkland Islands, saying, “The figures and description of the female supplied by Dr. S. J. Holmes leave no doubt that Mr. Vallentin's specimens belong to this species.” He quotes C. cylindricus Paulmier (1905, p. 167, fig. 37) as a synonym, and suggests that C. quadriceps Dana (2 mm. long) from Rio de Janeiro, and C. contractum Stimpson, 1855, from Japan, and the specimens from New Zealand recorded under this name by G. M. Thomson also belong to the same species. He gives no description of the Falkland Islands specimens except that they measure only 3 mm., as compared with 3–4 mm. given by Holmes, and 5 mm. by Paulmier, “probably with reference to a male specimen which he figures in full.” I agree with Stebbing that the Falkland Island specimens are probably the same as those from New Zealand, but I do not know why he assigns them to C. cylindricus rather than to C. crassicorne. In Das Tierreich Amphipoda (1906, p. 692) he classes C. cylindricus among the “obscure” species, but in the appendix (p. 740) gives references to the description and figures given by Paulmier and Holmes.
[Footnote] I can find nothing in Holmes's description and figures inconsistent with the supposition that the species he describes is the same as the European C. crassicorne, and certainly the figures he gives of the second antenna both of male and female apply well to the New Zealand forms that I have referred to C. crassicorne. Similarly, the description and the figure of the male given by Paulmier apply equally well to the New Zealand forms. Neither Paulmier nor Holmes makes any reference to or comparison with other species.
[Footnote] Barnard (1916, p. 272) records C. acherusicum Costa from Durban Bay. Stebbing (1906, p. 692) give this species among the “obscure” species, with the remark, “perhaps identical with C. bonellii.” Walker (1914, p. 559), after comparing specimens of each, definitely united C. acherusicum with the older C. bonellii, to which he also referred C. crassicorne Hoeck (1879, p. 115).
[Footnote] It seems to me that these facts, which I had not paid special attention to when writing the remarks given above, show that all the forms to which these varied names have been given are so alike that they cannot be distinguished even by experts, and the conclusion I had already come to in the text receives additional confirmation.
It seems evident that a good deal more work must be devoted to the genus Corophium before the various problems indicated above can be solved. Probably we are dealing with a widely distributed form which is in the process of development but has not yet differentiated into distinct species, and some of the differences recorded may be associated with the character of the water in which it lives.
The telson appears to be practically the same in all the specimens—European, Australian, and New Zealand—that I have examined. It is broadly triangular, with the posterior margin truncate or slightly convex, and it bears on the dorsal surface, towards the posterior margin, two ridges diverging anteriorly and each bearing about four minute blunt spines projecting upwards. These ridges do not appear to be described or figured
Fig. 5.—Corophium crassicorne Bruz.
A. Telson with second and third uropoda.
B. Telson (more highly magnified).
by Sars or Stebbing, though they are indicated in Sars's figure of the telson of C. bonellii (1895, pl. 22, fig. 1, t), and apparently in that of C. affine (l.c., fig. 2, t). The telson shows different appearances according to its precise position when mounted. My specimens, which are all mounted permanently in Canada balsam, have become transparent enough to show the two ridges pretty clearly. In a specimen of C. triaeonyx Stebbing from Ceylon the terminal portion of the telson appears to have become doubled underneath, and consequently the two anterior spines extend clearly beyond the visible margin. In another specimen of the same species from Chilka Lake, however, the other spines could be clearly made out.