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Volume 59, 1928
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Fresh-water Fauna of New Zealand.
Nos. 7-8.

By Dr. V. Brehm, Biological Station, Lunz, Austria.
(Communicated by E. W. Bennett).

7. On a new Boeckellid from Lake Lyndon.

In the account of the fauna of Lake Lyndon, at p. 793 the occurrence of a Boeckella and of Bosmina hagmanni was recorded. The former was not further discussed, as I understood at the time that Mr. Bennett wished to deal with the members of this group. As, however, he has written to say that he does not intend to discuss any of the Lake Lyndon material, a description is here given of the new Boeckella from that locality; and the opportunity is also taken of reverting to the problems connected with the occurrence of Bosmina in the southern hemisphere, a matter which was also discussed in the former paper.

Boeckella hamata n. sp.

The Boeckella from Lake Lyndon is one of the small species of the genus, for the male is only 1.2 mm. and the female only 1.5 mm. long. The last thoracic segment of the female is produced into a long double lobe, whose outer and longer tip reaches almost to the end of the genital segment (Fig. 1). The antenna of the female does not reach right to the end of the furca. The fifth pair of feet is distinguished by having seven setae on the terminal joint of the outer branch. The inner branch is usually three-jointed, yet I found

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Fig. 1.—Boeckella hamata n. sp. Thoracic lobes of egg-bearing female, lateral view.

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in the same collection some egg-bearing specimens in which an articulation between the last and the penultimate segment of the inner branch was only indistinctly visible, or not at all; so that my attention was again forcibly drawn to the fact that the number of joints in the appendages is of trivial importance.

The structure of the modified male antenna is indicated by the accompanying figure (Fig. 2). In the fifth feet of the male (Fig. 3), both of the inner branches are one-jointed, and the second segment of the basal section of the left foot is neither toothed nor provided with a denticulate lamella. Since, moreover, the endopodite of the left fifth foot is shortened, this species would fall into group 16 of

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Fig 2.—Boeckella hamata n. sp. Outer portion of modified male antenna, represented In sections; m - m and n - n, corresponding points.

the subdivisions given by Marsh* in his artificial key for identification. This group includes B. rubra Smith and B. robusta Sars, of which the former is known from Tasmania and the latter from Australia. The Boeckella from Lake Lyndon, however, is not identical with either of these, and in spite of a certain resemblance to B. robusta constitutes a new species, which is especially distinguished by the structure of the fifth feet of the male (Fig. 3). Characteristic features of the left fifth foot of our species are:—(1) the inner branch is extremely short, but stouter (according to the figure given by Marsh) than that of B. robusta; (2) the end-claw is suddenly narrowed through approximately the last sixth of its length, so that in shape it reminds one of the end of the body of certain male Nematode worms, such as Oncholaimus. The right fifth foot shows more remarkable differences. The inner branch is not as narrow at the end as in B. robusta, but rounded, and furnished with a small latero-terminal

[Footnote] * A Synopsis of the species of Boeckella and Pseudoboeckella, etc.—Proc. U.S. Nat. Mus., vol. 64.

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point. The basal inner edge of the inner branch is produced into a long, pointed and hook-shaped process, on account of which the name hamata was chosen for the species. In the first joint of the outer

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Fig. 3.—Boeckella hamata n. sp. Fifth feet of male; a, left foot; b, right.

branch, the distal spine of the outer edge is longer than in B. robusta. The terminal claw bears basally, on the inner edge, a very small but nevertheless remarkable chitinous knob, and opposite this arises a chitinous lamella, which extends out obliquely and distinctly beyond the contour of the claw (Figs. 3b, 4). The end of the claw is suddenly narrowed, as described for the other foot.

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Fig. 4.—Boeckella hamata n. sp. Oblique view of terminal claw, whose basal part appears from this aspect to be broadened by the chitinous lamella.

If a phylogenetic comparison with B. robusta may be ventured upon, one may perhaps recognise in the New Zealand B. hamata a more primitive form, because in B. robusta the hook-shaped process on the basal inner edge on the inner branch of the right foot is reduced to a “short curved spine.”

In the material examined, the Boeckella was abundantly represented by both sexes, but immature forms predominated over the sexually mature.

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8. Bosmina hagmanni, some further remarks on.

As mentioned in the previous account of the plankton fauna of Lake Lyndon, abundant representatives of a Bosmina were present in the collections, and an investigation of this form led me to refer them to B. hagmanni; from the point of view of geographical distribution this is a result of some interest, for this species was previously regarded as a South American form. I also expressed the opinion on that occasion that Sars' B. meridionalis, described from Lake Wakatipu, New Zealand, might also belong to B. hagmanni. But another opinion as to the status of B. meridionalis has recently been given in a work which, unfortunately, I was unable to consult when writing my previous account; and this opens up the problem of Bosmina once more, and does so from a viewpoint which is in part a new one.

Burckhardt, in pt. 3 (Bosminidae) of his work on the zooplankton of the lakes of eastern and southern Asia,* has described a new Bosmina from China as B. fatalis, which is known with certainty only from the male, this circumstance providing the specific name bestowed upon it. Burckhardt's account (to translate his words) states that “of all the Bosminas known to me, none agrees as closely with B. fatalis as does the New Zealand B. meridionalis Sars. But neither Sars nor Brady found males of this species.…” There are in fact two gaps in our knowledge, which must be filled before this matter can be cleared up satisfactorily; in Burckhardt's account I find nothing concerning the position of the incisures on the mucro of B. fatalis, and to the present date the males of the New Zealand B. meridionalis remain unknown. There are thus two possibilities in attempting to assign the New Zealand form to its systematic-position; the males are either of the fatalis type, or they are of the coregoni type. Similarly, two possibilities present themselves in the case of Burckhardt's B. fatalis; this species has either dorsal or ventral incisures. These possibilities may be separately considered under the following four heads:—

1

If B. fatalis has ventral incisures on the mucro, Burckhardt's interpretation remains unchanged;
2

If B. fatalis has dorsal incisures, I would have to regard it as being the same as B. hagmanni. Since, however, I consider that this case may be excluded, I do not discuss it further here; the punctum saliens in this question arises again in connection with the fourth possibility below.
3

If B. hagmanni has males of the B. coregoni type, the views which I have previously expressed concerning B. hagmanni remain unaltered.
4

But if B. hagmanni has males of the fatalis type, I would regard the latter as B. hagmanni; and in my opinion it would then only remain to be determined whether all the collections of B. hagmanni have males of the fatalis type, or whether some have males of the fatalis type and other coregoni type; in other words, whether B. hagmanni is a general name including more than one species, these

[Footnote] * Revue d'Hydrobiologie, vol. 2, Aaru (Switzerland), 1924.

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  • species differing from one another in the structure of the male. From this viewpoint, the whole position, as far as the systematic nomenclature is concerned, depends on whether, in our attempts to distinguish species, we should attach chief importance to the incisures on the mucro, or whether we should rather stress the structure of the male abdomen. If any weight can be attached to a character which is ordinarily to be regarded as of subsidiary importance only, I would refer to the very characteristic geographical distribution of those Bosminas with dorsal incisures; for as far as is known at present, their distribution coincides with that of the Boeckellids, or perhaps more precisely with that of the marsupials, for Bosminas of the hagmanni type have hitherto been known from Australia and South America, but not from Africa. It appears then to me that the Bosminas with dorsal incisures on the mucro may constitute a group

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    Fig. 5.—Bosmina hagmanni from Lake Lyndon. Outline of a young female, illustrating; the dorsal incisures on the mucro. Abdomen shown on a larger scale, within the outline.

    characteristic of the southern hemisphere, and that in contrast with these, the forms with ventral incisures belong to the northern hemisphere. According to Burckhardt, there are then three northern-hemisphere species, B. coregoni, B. fatalis, and B. longirostris, which are in part further subdivided into subspecies. Whether the Bosminas of the southern hemisphere are similarly subdivided, and have likewise become differentiated into several species, will be known only when a sufficient number of collections of B. hagmanni from southern sources have been investigated.

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As the literature on this subject is very scattered, and the problems here discussed are, no doubt, thereby made more difficult for Australian and New Zealand zoologists, it may be worth quoting in this place the chief characters of the various types of Bosmina. The young female which is figured here also illustrates certain points; it shows distinct dorsal incisures on the mucro, (Fig. 5), and the two figures of the abdomen (Fig. 6) illustrate the remarkable differences in the contour of the anal region in the males of the coregoni and fatalis types respectively. Unfortunately, Burckhardt's figure has been indistinctly reproduced, so that the copy given here (Fig. 6b) no doubt contains some imperfections, though not such as to disguise the essential differences in question. The abdomen of the male of B. longirostris is very similar in outline to that of B. fatalis, but (as

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Fig. 6.—Bosmina hagmanni, abdomens of males; a, at B. coregoni; b, of B. fatalis.

indicated in the artificial key below) lacks the teeth found in that species. It is hoped that the reproduction of these figures may draw the attention of zoologists in the southern hemisphere to this matter, and that they will investigate anew at least the structure of the mucro and the form of the male in any collections of Bosmina available to them. And if some of the material from South America which the author hopes to discuss shortly in another article contains the requisite specimens for the elucidation of the problem, it will then become clear whether the systematic separation of these forms, as indicated in the following key, is justifiable. The key is a combination of Burckhardt's statements and my own, and I believe that it best indicates the present position of the systematics of Bosmina.

I. Mucro with dorsal incisures.
Southern Hemispere form		 B. hagmanni Stringelin.
II. Mucro with ventral incisures.
Predominant form of the northern hemisphere.
   A. Terminal claw of the male unarmed B. longirostris.
   B. Terminal claw of the male provided
with, a row of spinelets.
      a. Abdomen of the male suddenly narrowed in front of the terminal claw B. fatalis Burckhardt.
      b. Abdomen gradually decreasing in thickness B. coregoni.
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