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Comprehensive Description

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Description

Chaetonymphon australe

 

(Plate X., fig. 14.)

 

 

Nymphon australe, Hodgson (10), p. 257.

 

Chaetonymphon altioculatum, Möbius (23), p. 181.

 

 

Specific characters : --

 

Body robust, with lateral processes not widely separated but divergent; entire animal rather coarsely setose.

 

Ocular tubercle long and slender.

 

Palps five-jointed, proportions of last three as 7 : 4.25 : 4.

 

Oviger ten-jointed, denticulate spines fairly numerous, with four distinct lateral teeth on each side.

 

Legs short, tarsus a little longer than the propodus, the terminal claw with very minute auxiliaries.

 

 

Body robust, with stoutly developed lateral processes; which are distinctly though not widely separated, the interval increasing with age; the body, exclusive of the anterior part of the cephalon, forming an oval of graceful proportions.

 

 

The Cephalon is expanded, and the space between the chelifori is marked by a deep groove, wide anteriorly. The neck is well defined, and behind this is the Ocular tubercle, as structure which varies considerably in shape and size. As a rule it is rather stout, and bears four well-developed eyes, with a variable amount of pigment. It is slightly flattened antero-posteriorly, and more or less rounded at the extremity. The cephalon and the lateral processes are provided with several long setae, the latter also having a distal fringe.

 

 

The Abdomen is of moderate dimensions, pyriform, and rather thickly setose, not separable from the trunk by an articulation.

 

 

The Proboscis is cylindrical, slightly enlarged about its middle. It is directed downwards, and movably articulated to the trunk. No setae are apparent on its surface.

 

 

The length of the entire body is 8 mm.; of the body only, 6mm.; of the trunk, to the insertion of the abdomen, 4.5 mm. Its width is 3 mm.

 

 

The Chelifori are well developed; the scape is a single joint longer than the proboscis, liberally provided with long setae of irregular size, and also having a well marked distal fringe. The chelae are slender, about as long as the scape. The palm occupies half the length of the entire chela, and is covered with comparatively long setae, which are continued far on to the immovable finger. The fingers are slender and much incurved at the tips; they are provided with a large number of closely set teeth of irregular length.

 

 

The Palps are slender and five-jointed. The first joint is quite small, the second is the largest of all, and rather sparingly provided with setae, which are longest on its outer side; the third joint is a little shorter, slightly enlarged distally, the setae being more numerous and more uniform than on the preceding joint. Of the two terminal joints the distal one is a little the shorter, but together they exceed the length of the second by a trifle. These two joints are richly setose, particularly on one side.

 

 

The Oviger is ten-jointed; in the female the first three joints are very small, but progressively increase in length. The fourth and fifth are subequal and much the longer of the whole series, and slightly curved in opposite directions. The sixth joint is about three-quarters the length of the fifth. Of the four terminal joints the first three progressively shorten, the terminal one being a trifle longer than the preceding, and it bears a long slender pectinate claw with eight teeth. The first four joints bear scarcely any setae, except an inconspicuous distal fringe; on the fifth the setae are noticeable on its outer border, and those of the distal fringe are rather long and slender. In this particular the sixth joint is similar. The four terminal joints are all provided with a distal fringe and a number of long setae. The denticulate spines are arranged as usual in a single row, and, counting the joints from the base of the appendage, they bear respectively 9 : 7 : 5 : 7 of these spines (Plate X, fig. 15). These numbers are not, however, rigidly adhered to. The spines consist of flattened shafts of a slightly sinuous form, bearing four well-developed teeth on each side, with traces of a fifth in large and uninjured specimens. The third tooth from the base is usually the largest. In the fully developed male the fifth and sixth joints are remarkably swollen. The enlargement of the fifth joint affects the distal half. The eggs are large, and the spherical masses may be two in number on each limb; they are carried round the fourth joint.

 

 

The Leg extends to a length of 21 mm. These appendages do not differ appreciably in size. Of the three coxae the second is much the longest, but not so long as the other two together. The proportions of the three following joints are as 4 : 5 : 4.5. The tarsus and propodus together are as long as the femur, the former joint being the longer of the two. The terminal claw is well developed, and is accompanied by two very minute auxiliaries, not one, as stated in the ‘Southern Cross’ Collection, Crustacea, p. 258. The entire limb is setose throughout, the setae abundant, and variable in size, some of them distinctly spinous. For the most part their arrangement is irregular, but on the second tibia a linear arrangement begins to be perceptible, and this is clear on the tarsus and propodus, where the setae are much finer. The distal fringe of the first coxa is dorsal, and not so strongly developed as on the two following joints, more especially the third, where it is ventral. On the femur it is chiefly dorsal, and the setae composing it are long and stout. On the first tibia it is complete and rather spinous ventrally; this is more strongly developed on the second tibia, where there is at least one powerful spine ventrally, and generally two on each side. The male differs from the female in being more setose, the setae being longer, more irregular, but scarcely, if any, stronger. The distal fringe of the third coxa is particularly noticeable for the great length of the setae composing it.

 

 

The Genital apertures of the female are conspicuous on the second coxa of all the legs; those of the male are much smaller, and can be detected on the two posterior legs.

 

 

This species was taken in considerable numbers off Cape Adare, but inside Robertson Bay, in 20-26 fm. None were taken by the ‘Discovery.’ I have redescribed it here to remove certain defects of the original description, and on account of the capture of a form which, after considerable hesitation, I feel compelled to regard as only a variety. This species is closely allied to N. brevicaudatum Miers, with which N. horridum Böhm has been identified by subsequent investigators. N. brevicaudatum Miers, can be readily distinguished from N. australe by the following characters : --

 

 

The trunk is more setose.

 

The tarsus is shorter than the propodus.

 

The terminal claw has two distinct, if small, auxiliaries.

 

The oviger bears a very much smaller series of denticulate spines, but their lateral teeth are more numerous.

 

 

I am unable to regard the Chaetonymphon altioculatum of Möbius as a distinct species, several examples of which were taken in the vicinity of Bouvet Island during the ‘Valdivia’ expedition.” (Hodgson 1907, p.32-34)

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© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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