Ascorhynchus, Sars, 1877

Child, C. Allan, 2002, Some Pycnogonida from the Eastern (Hasa) District of Saudi Arabia, Journal of Natural History 36 (15), pp. 1805-1821 : 1814-1815

Ascorhynchus parasitic species indeterminate

(gure 3)

Material examined. Saudi Arabia: Tarut Bay station Sta. 12TS-3, 13 May 1985 (one juv. badly damaged) .

Description. Much like the parasitic specimen illustrated by Hong and Kim (1987: 146–149, gures 6, 7) as Ascorhynchus stocki , except for signi cant but small appendage diVerences. Trunk and chelifore habitus similar to A. stocki except for size and height of ocular tubercle which is much lower and has a broader base. The eyes are indistinct and unpigmented. The proboscis of this specimen is missing. The abdomen is moderately long and downcurved.

Palps seven-segmented; rst segment wider than long, second 2.5 times longer than its diameter, third four times its diameter, fourth only as wide as long, fth twice its diameter, and the sixth 1.5 times its diameter. The terminal or seventh segment is unlike the same segment of any other parasitic Ascorhynchus species known. It is as long as the second segment, has ve ventral setae, the middle one originating on a raised tubercle, and the tip has a very curved stout claw longer than the segment diameter.

The ovigers are incomplete, both tips broken oV beyond the eighth segment, but each bear tiny simple spines in one or two rows. Presumably, the undamaged ovigers would have nine segments.

The legs are also incomplete with all segments missing beyond the coxae except for one with a damaged femur. They are robust with short segments. The single femur has a short broad dorsodistal spine similar to those of the ovigers.

Remarks. This specimen is judged to be a juvenile from the undeveloped spines on the oviger strigilis, the reduced size of these spines, and the lack of any sexual pores on its coxae. The very large chelifores carried widespread in this specimen, as in those of A. stocki , are not a juvenile character but are apparently normal in adults. The chelifores of Stock’s (1953: 305–307, gure 16) parasitic Ascorhynchus species A are notably smaller in relation to its trunk and there are other diVerences in the chelae and the abdomen from the above specimen.

The palp illustrated by Hong and Kim (1987: gure 7C) has a very small terminal claw and the distal segments have a normal arrangement of short setae. The terminal palp claw of another parasitic species, A. endoparasiticus Arnaud (1978: 99 –104 , gures 1, 2), is equally as long as the claw of the above specimen, although it is illustrated as straight and not sharply curved. The palp of Arnaud’s adult specimen lacks other spines but has three to four setae. Most other characters of her species disagree with both this specimen and that of Hong and Kim. The distal nozzle shape of the A. endoparasiticus proboscis, unfortunately, cannot be compared with the missing proboscis of this species.

A terminal palp claw is thus one of the many diagnostic characters of this group of parasitic Ascorhynchus , and marks a departure from ‘regular’ species of the genus which lack this claw. As far as can be determined, a palp claw is unique in this group of named species and unnamed specimens. It presumably is used to grasp the host more rmly along with the usual propodal claws and large chelae.

The Saudi Arabian specimen has a single stout spine on the only femur available, in the same dorsodistal position as the proposed cement gland tube illustrated by Hong and Kim (1987: gure 6H). This suggests that their cement gland tube is possibly a broken spine and not a glandular tube.

The extreme damage of the specimen in hand suggests that it was pulled from its attachment, leaving the proboscis and distal segments with leg claws attached to or within the host.

This small group of Ascorhynchus species apparently retains a simpli ed body plan without adornment, often has very large chelifores, a palp claw and simpleappearing legs with very few setae or spines, all of which suggest neoteny. This group of species and unnamed specimens has possibly retained some juvenile characters due to being parasitic and presumably living attached to a molluscan or other host. The retention of a palp claw and very large chelifores would assist in this attachment and permit the proboscis to be permanently inserted in the soft parts of the host until it is sucked dry. As in many other parasitic forms of normally mobile invertebrates, the legs and other possibly unused appendages atrophy or at least do not fully develop. It is probable that this group of parasitic Ascorhynchus species will be given the status of a subgenus as more examples of these neotenic specimens are discovered.