Crangon antarcticus

"CRANGON ANTARCTICUS.

C. antarctiats Pfeffer, Jahrb. Hamburg. Wiss. Anst. iv. (1887), p. 45, pl. i., figs. 1-21; Coutière, C. R. Acad. Sci. Paris, cxxx. (1900), p. 1640 ; and Bull. Mus. Paris, vi. (1900), p. 240.

Description of females (not ovigerous).---Total length, 37.5-77 mm. General form slender. Surface of the carapace very uneven, with strongly marked ridges and hollows; in particular, a more or less sharply defined ridge runs backwards from the median dorsal spine for a distance equal to one-half the length of the carapace. The ridge running backwards from the antennal spine is continuous with that running forwards from the hepatic spine. Rostrum long, slender, compressed and acute, in one case nearly one and a half times as long as the eye-stalks. Abdomen long and slender, sixth somite generally more than one-sixth of total length of body. A pair of slender acute spines on hind margin of fifth somite dorso-laterally. Sixth somite with a strongly-marked double dorsal keel. Telson rounded at the tip, with a median spiniform point. Antennular peduncle slender, the distal end of first segment narrower than one-half the greatest diameter of the eye; outer lobe of first segment nearly flat, broadly ovate, produced anteriorly into a rather feeble spiniform point which does not reach distal end of segment. Antennal scale with outer margin straight, or, in smaller specimens, concave. Third maxillipeds extending to or slightly beyond end of scale. First legs extending a little beyond middle of terminal segment of third maxillipeds; hand from nearly four to nearly five times as long as broad, terminal tooth of palmar edge at about one-fourth of the length of the hand from distal end. Last pair of legs extending forward to the tip of the antennal scale. Endopod of first pleopod articulating with distal inner angle of peduncle.

Branchial system.—Five pleurobranchi on each side, on the last five thoracic somites; no arthro- or podo-branchiæ.

Remarks.—The ‘Discovery’ specimens differ from Dr. Pfeffer's description, and from a co-typical specimen with which I have compared them, in the more slender form of the body, due especially to the greater length of the sixth abdominal somite; in the greater length of the rostrum; in the shorter lobe on the basal segment of the antennule, reaching only to about the distal third of the segment, while in the typical form it reaches nearly to the end; and in the narrower "hand" of the first legs. But while each of the three well-preserved specimens in this collection differs from the co-type in all these points, they do so in varying degree. The differences are at least as important as some of those which have been regarded as of specific value by recent writers on the Crangonidm, but I do not think that they would justify us, at present, in separating the form inhabiting the area explored by the ‘Discovery’ from that found in the very distant region of South Georgia.

The following table gives some measurements, in millimetres, of the co-type of C. antarcticus as compared with the three most perfect specimens in the ‘Discovery’ collection. All the specimens appear to be females or immature males.

Total Length of Length of Length of Length Ratio Length

Length Carapace from Rostrum from Sixth Abdominal of to Breadth of

back of Orbit. back of Orbit, Somite Telson “Hand”

C. antarcticus, co-type 46.0 9.75 2.5 7.5 9.0 3.9

‘Discovery,’ January 27,1902 58.0 11.75 4.75 10.0 12.5 4.9

“ March 4,1904 77.0 17.0 13.0 15.0 —

“ January 22,1902 37.5 8.0 2.3 7.5 8.0 —

Dr. Pfeffer was the first to draw attention to the apparent "bipolarity" in the distribution of the genus Crangon. With the exception of the very imperfectly known C. capensis, Stimpson, from the Cape of Good Hope, C. antarcticus is the only species of the genus inhabiting the Southern Hemisphere, and is widely separated from all the other species, which are confined to the temperate and (if Sclerocrangon be included) the Arctic regions of the Atlantic and Pacific. The question has been discussed by Dr. Ortmann, who concludes that C. antarcticus is specially and closely related to the Californian C. franciscorum, Stimpson, and that its presence in the Southern Hemisphere is to be explained by migration from the North along the West coast of America, where the hydrographical conditions are such as to favour an inter­mixture of northern and southern faunas across the tropic zone.

With a view to testing this conclusion of Dr. Ortmann's, I have carefully compared the specimens of C. antarcticus with specimens of C. franciscorum in the Museum collection. The chief character on which Dr. Ortmann relies for linking the two species together is the presence of a pair of dorso-lateral spines on the hind margin of the fifth abdominal somite. This character is conspicuous and definite, but it may be doubted whether it is of great morphological importance. Prof. Sars figures a pair of spines of varying length in nearly the same position in all the larvæ of Crangonidw examined by him, and it seems likely that this larval character may have been retained independently in species not closely related. In other respects C. franciscorum differs considerably from the Antarctic species. The surface of the carapace is much less uneven, the various ridges and hollows being much less strongly marked. There is no ridge running backward from the median dorsal spine, and the ridge connecting the antennal and hepatic spines is interrupted by a groove. The pterygostomial spine is not compressed and expanded laterally as it is in C. antarcticus. The rostrum is shorter than the eye-stalks, depressed and hollowed on the dorsal surface and bluntly pointed. The sixth abdominal somite is about one-seventh of the total length, and has only a faintly-marked indication of a double keel on its dorsal surface. The telson narrows gradually to an acute tip. The antennular peduncle is stout, the distal end of the first segment broader than three-fourths of the greatest diameter of the eye ; the outer lobe of the first segment has its external margin strongly bent upwards, thickened and produced forwards into a strong spine which reaches the distal end of the segment. The outer edge of the antennal scale is slightly convex. Miss Rathbun states (Harriman Alaska Exp., Crustacea, p. 120) that the third maxillipeds do not reach the end of the antennal scale, but in two out of three specimens examined by me they certainly do so. The first legs reach the tip of the third maxillipeds; the palmar edge of the hand is very oblique, its terminal tooth being more than one-third of the length of the hand from the distal end. The last pair of legs reach to about the middle of the antennal scale. The first pleopod differs considerably in shape from that of C. antarcticus, the endopod being attached nearly half-way down the inner margin of the peduncle.

A difference which may possibly be of greater importance than any of those mentioned above exists, as Prof. Coutière has pointed out, in the branchial system. In addition to the five pleurobranchize possessed by C. antarcticus, C. franciscorum has on each side a well-developed arthrobranchia at the base of the third maxilliped. The statements made by various writers as to the gill-formula of the common shrimp, and of the genus of which it is the type, are curiously conflicting. Although Huxley, in 1878, Boas, in 1880, and Claus, in 1886, gave the number of gills in C. vulgaris correctly as six, more recent authors seem to have overlooked the arthrobranchia of the third maxilliped, which, although small, is not at all difficult to see. Sars, in 1890, gives among the characters distinguishing Crangon from Pontophilus, the presence of five gills in the former and six in the latter genus, and this statement is copied by Mr. Stebbing.Ortmann, in his revision of the Crangonidæ in 1895, names a number of species of Crangon which he has examined and found to have only five gills. One of the names mentioned, "typicus," does not appear elsewhere in the paper, but it may be conjectured that it refers to the typical form of the species C. vulgaris. Two other species on the list are C. affinis and C. franciscorum. In these three species, and also in C. allmanni and C. nigricauda, I find, on the contrary, that the arthrobranchia is well developed. In the absence of trustworthy data as to the occurrence of this gill in the other species from the northern hemisphere, it is not possible to estimate the importance to be attached to its absence in C. antarcticus. It may be noted, however, that it is absent in the characteristically Arctic genus (or subgenus) Sclerocrangon, which is otherwise not very sharply defined from Crangon, and to which, in its strongly sculptured carapace, the present species has some resemblance. Prof. Coutière, in his preliminary notes on the Decapoda of the ‘Belgica,’ has called attention to this resemblance to Sclerocrangon; but he suggests, with some hesitation, the establishment of a new subgenus, Notocrangon, for the Antarctic species. I have not been able to examine the structure of the male pleopods, to which he attaches some importance, but the other characters which he mentions do not seem to me to justify this step.

Larvæ.—A number of larvæ of this species were collected, all in a stage of development corresponding to the "last larval stage" of Prof. Sars. The rostrum is very long and slender, extending well beyond the eyes. There is a small median dorsal tooth on the carapace, about midway between the back of the orbit and the "cervical" groove, and a little in front of it is a rounded papilla (represented in some of Sars's figures) probably representing the problematical "dorsal organ" of some Euphausid larva. The abdomen is unarmed, except for the paired spines at the posterior end of the fifth somite, which are long and slender, almost as in Sars's figures of the larvae of Pontophilus, and, as in that genus also, the sixth somite is very long. The telson is very large, in the form of an almost equilateral triangle, with the posterior margin

concave, but not deeply notched. All the appendages are present. The first legs are subchelate. The second legs are (as in the other species of the genus Crangon) devoid of exopods. The pleopods are large but uniramous. There are only four gills on each side, corresponding to the first four legs.

Occurences. – January 13, 1902. 100 fathoms, off Coulman Island, 1 ♀.

January 22, 1902. 500 fathoms, 1 ♂(juv.) (?).

January 27, 1902. 300 fathoms, off Barrier, 1 ♀.

March 4, 1904. 254 fathoms, 1 ♀.

Larvæ of this species were taken in Winter Quarters on September 13, 1902, February 8, 1903, and March 23, 1903.

Fragments were taken from the stomachs of seals on several occasions."

(Calman, 1907)