Boreonymphon robustum (Bell, 1855),

Phylum:Arthropoda Siebold, 1848
Class:Pycnogonida Latreille, 1810
Order:Pantopoda Gerstaecker, 1863
Family:Nymphonidae Wilson, 1878
Genus:Boreonymphon Sars, 1888
Synonyms:Nymphon robustum Bell, 1855

Netpe: male of 11 mm with young of 4 to 5 mm. Bylot Sund, 76°28.65'N, 69°21.8'W, 11 August 1968, 97 m, clay, gravel, and stones. Thule Exped. 1968, Just et Vibe St. 29. Zoological Museum, University of Copenhagen.

Body: rather slender, width of body equalling length of first two body seg¬ments together; lateral processes not quite touching. Ocular segment as long as the two following segments together. Bristles present dorsally on front and hind edges of ocular segment, on hind edge and mid-dorsally on caudal half of segments 2 and 3. Fourth segment with a low bristle-bearing node immediately in front of the abdomen. Lateral processes with scattered bristles on the dorsal surface. The entire animal is covered by minute felt-like hairs.

Abdomen : straight; proximally and distally tapering and with almost parallel sides. Surface with many short bristles.

Eye tubercle: prominent, apex rounded.

Proboscis : almost completely cylindrical, as long as ocular segment and covered by tiny hairs about 30 microns long. Width at base subequal to that of scapus base.

Chelifores : with scapi parallel and as long as ocular segment. Straight in dorsal and curved in lateral view; slightly constricted in proximal third. Space between scapi at constriction level equalling half the width of scapus at the same level. Scapi evenly covered with small bristles and a felt of tiny hairs (Fig. 1D).

Chelae robust. Movable finger slightly longer than palm and evenly curved. Fixed finger as strong as movable, but less curved. Palm inflated and covered with fine bristles and a felt of tiny hairs.

Palps : when stretched out joint 3 reaches to the tip of the proboscis. Ratio between joints 2 and 3 approximately 1:1. Joints 4 and 5 of equal length; together they are slightly longer than 3rd joint. Joints 2-5 with a sparse cover of bristles and a felt of tiny hairs (Fig. 1E).

vigers: joints 4 and 5 of equal length. Joint 6 slightly less than half as long as 5. Joints 4-6 distally widening, with evenly scattered straight or curved bristles and a dense cover of clusters of tiny hairs (Fig. 1F).

Legs: all legs have femur and both tibiae of subequal length. Femur cylindrical and as long as the corresponding three coxae together. Second tibia hardly twice as long as tarsus and propodus together. Ratio between tarsus and propodus is 3:4 (Fig. 2). Claws a little less than half as long as propodus. Auxiliary claws absent. Legs covered with small bristles, the density of which increases distally; underneath the bristles are tiny felt-like hairs.

Variations

Hairs and bristles: the cover of tiny hairs on the body may be very nearly absent. In several specimens the naked sides of tarsus and propodus of the legs appear as extremely narrow bands only.

Shape: minor variations can be found in the shape of the eye tubercle, which may appear pointed in lateral view, and in the abdomen, which is sometimes of a more or less irregular shape.

Proportions: joint 5 of the oviger of the adult male may exceed the length of joint 4 slightly. The ratio between the various joints of the legs is due to minor variations. In adult specimens the femur is often slightly longer than the three coxae together.

Others: in a few specimens auxiliary claws have been found on one or more legs.

Females: generally as males, and with the same variations; but joints 4 and 5 of the oviger are weaker and armed with scattered straight bristles.

Remarks

During his work with the present genus Knaben visited museums and collections in England and Scotland in order to locate type material. Bell’s material of Nymphon robustum could not be found, so I have considered it useful to select the neotype recorded above.

Since Sars (1877), Heller’s Nymphon hians has been considered a synonym of Boreonymphon robustum (Bell) . Knaben investigated Heller’s type material and verified the correctness of Sars’ view (Knaben, MS.) The material has not been seen by me.

Literature

  • Just J. (1972). Revision of the genus Boreonymphon G.O. Sars (Pycnogonida) with a description of two new species, B. Ossiansarsi Knaben and B. compactum Just. Sarsia, 49, 1-28.

Author: Zakharov Denis V. / 22.02.2017
Last edit: Zakharov Denis V. / 10.01.2018

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