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The New Zealand Species of Podocerus.
I have always been puzzled to distinguish the different species or forms of Podocerus found in New Zealand. P. cristatus (G. M. Thomson) was described by Mr. Thomson in 1879 under the name Cyrtophium cristatum, and it was easy to identify some of the forms found on the New Zealand coast with this species as described by him. In the same year Haswell independently described C. dentatum, an Australian form which Stebbing has—I think, rightly—considered to be the same as P. cristatus; in 1885 Haswell placed this species in the genus Dexiocerella.
The New Zealand species are common on the coast, frequently being found at the roots of seaweed, on the sea-squirt Boltenia, and on other organisms which they usually closely resemble in colour; as they are also somewhat sluggish in movement, they are sometimes hard to distinguish. They appear to show considerable variation in breadth of body, length of the appendages, colour, &c. Other specimens were at different times taken on sertularians, which they so closely resembled that on more than one occasion they were nearly being overlooked altogether, although it was known that they were frequently found on the hydroids. Some of these appeared at first sight narrower than the others, and it was thought they had probably formed a separate species. Closer examination, however, usually failed to reveal any striking or constant difference.
In 1885 Haswell had described another species, under the name Dexiocerella laevis, from Australian shores, differing from D. dentata mainly in having none of the segments dorsally produced.
In his Das Tierreich Amphipoda, published in 1906, Stebbing transferred these and other species to the genus Podocerus, recognizing ten species, two of them being P. laevis (Haswell) and P. cristatus (G. M. Thomson), the latter including Dexiocerella dentata Haswell. More recently I have examined and mounted numerous specimens, males and females, from different parts of Australia and New Zealand. Some of them it is easy to identify as P. laevis, and others as P. cristatus, in accordance with the descriptions given by Stebbing, and I think it is desirable to retain these two species distinct as a matter of convenience, though other forms are to a large extent transitional in one character or another. In P. laevis the body is usually smooth, without dorsal carination or with only slight indications of such. The gnathopoda are somewhat short and stout, in
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the first the carpus being short and having a distinct rounded lobe fringed with setae extending along nearly its whole length. The second gnathopod has the propod short and stout, fully half as broad as long, the palm is nearly straight, and in fully-grown specimens bears two or three somewhat blunted teeth, and is irregularly dentate near the base of the finger. In the peraeopoda the propods bear three or four stout setules, the strongest of them being on the proximal portion of the propod.
On the other hand, P. cristatus usually has the body more or less carinate on the posterior segments of the peraeon and of the pleon. The gnathopoda are more elongated, the carpus of the first being more slender and having the lobe extending only along part of its length. The second gnathopod in general resembles that of P. laevis, but is more elongated, being more than twice as long as broad. The peraeopoda have the setules on the propods less stout, and those that are present more evenly distributed along the margin.
Fig. 1.—Podocerus laevis (Haswell): male specimen from Cape Maria van Diemen. a, first gnathopod; b, second gnathopod; c, propod of peraeopod.
In many respects P. cristatus shows an approach towards P. danae Stebbing from Kerguelen Land, but in that species all the segments of the body, including the head, bear carinate teeth or processes, and the appendages, especially the second antennae and gnathopoda, are much more elongate. I have specimens from China and Japan that I feel inclined to consider a variety of P. danae, the only appreciable difference being that the carinal processes are rounded, instead of being pointed as
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in the Kerguelen Land specimen. P. brasiliensis Dana from the tropical Atlantic is in many respects similar to P. cristatus, but is described as having the body not carinate. I have only one specimen of P. variegatus Leach, the species commonly found in the North Atlantic, and this again shows very close approach both to P. laevis and P. cristatus, and it is not surprising that in 1893 Della Valle grouped most of these species under the name P. brasiliensis.
Fig. 2.—Podocerus cristatus (G. M. Thomson): male specimen from Cook Strait. a, first gnathopod; b, second gnathopod; c, propod of peraeopod.
Barnard (1916, p. 277) has recorded Podocerus cristatus (G. M. Thomson) from False Bay, South Africa.