Art. XII.—On a new Species of Daphnia.
[Read before the Otago Institute, 7th May, 1883.]
Plate XIII., figs. 6–9.
Last year Mr. Chas. Chilton sent me down specimens of a new Daphnia from South Canterbury, differing markedly from the form common about Dunedin, and which was described by me as D. obtusata in Trans. N.Z. Inst. vol. xi., p. 261.
It is singular that these two species of Daphnia and one of Chydorus should be the only representatives of the Cladocera hitherto found in New Zealand. While the general poverty of our fresh-water fauna may no doubt hold good in this case as in so many others, it is yet probable that a search in other parts of our islands will result in the discovery of other forms.
The species described now is very similar in general form to the common European D. pulex, and perhaps is only a variety of it. It differs however in several structural points, and is therefore raised provisionally to specific rank under the name D. similis.
The following is a brief description of the animal: Valves oval in form, not quite twice as long as broad, and produced posteriorly into a sharp stout spine, which is about one-seventh of the length of the carapace in the male, and about one-sixth in the female. The whole lower margins of the valves, as well as the spine, are fringed with short pectinate hairs.
The head in the male is nearly perpendicular in front, and ends in a blunt beak, which bears the anterior antennæ near its extremity: these are 1-jointed, and bear a single filament, which is flexible and serrated at its extremity. The whole length of these organs is nearly equal to the breadth of the head. In the female the front of the head protrudes considerably, and the beak is very acute. The anterior antennæ are represented by two very minute tubercles, destitute of setæ, which are situated on the underside, close to the extremity. These organs appear to be in a more rudimentary condition in our species than in any other hitherto described. The posterior antennæ are of normal form in both sexes, but are relatively shorter than the same organ in D. pulex, being only a little more than half as long as the carapace. All the joints are furnished with minute hairs.
The posterior portion of the body is dorsally produced into four rounded lobes, the upper one of which is the longest, while the lower bears two long setose spines or filaments. The abdomen in the male is much rounded at the anal orifice, and bears at that part on each side about 7 curved teeth, which are somewhat distant from the 2 .curved terminal hooks. In the female the abdomen is slightly rounded on the lower margin, and bears 10–12 curved teeth close up to the terminal hooks.
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My specimens included one male, 1/12th of an inch in length, and several females, the largest of which were about 1/9th of an inch long. When living, their colour, according to Mr. Chilton, was brick-red.
|Figs. 6–9.||Daphnia similis.|
|6. Adult female; 7. rostrum of same; 8. anterior antenna, male; 9. extremity of abdomen, male.|