Animal Diversity Web

(sea spi­ders) Sea spi­ders ap­pear to be a sort of ma­rine "spi­der," but in fact their re­la­tion­ships are enig­matic. They may rep­re­sent a very early branch­ing of the che­licer­ate lin­eage. There are ap­prox­i­mately 1000 de­scribed species of py­c­no­go­nids, all of which are ma­rine.

Py­c­no­go­nids can be found from the in­ter­tidal re­gions to depths of around 7000 m. Most are small, but a few deep-sea forms reach up to 70 cm di­am­e­ter across the legs. They feed by suck­ing juices from soft-bod­ied in­ver­te­brates through a long pro­boscis.

Py­c­no­go­nids vaguely re­sem­ble spi­ders, with small bod­ies and rel­a­tively long, hinged legs. Unique char­ac­ter­is­tics in­clude an un­usual pro­boscis, which varies in size and shape among species, but amounts to a cham­ber with an open­ing at the dis­tal end (the true mouth lies be­tween the pro­boscis cham­ber and the esoph­a­gus). The body it­self is not di­vis­i­ble into neatly- or­ga­nized tag­mata or re­gions as it is in most other arthro­pods. An an­te­rior re­gion bears, be­sides the pro­boscis, three or four pairs of ap­pendages, in­clud­ing the first pair of walk­ing legs. Spe­cial ap­pendages called ovigers when pre­sent make up the four pair; these play a role in the brood­ing of young and are used in clean­ing. Fol­low­ing the first seg­ment is a se­ries of seg­ments mak­ing up a "trunk," each seg­ment bear­ing a pair of walk­ing legs. A ter­mi­nal seg­ment in­cludes a tu­ber­cle that pro­jects dor­sally and an anus. Some species have more than four pairs of walk­ing legs. Py­c­no­go­nids are also unique in bear­ing mul­ti­ple gono­pores, found on the sec­ond seg­ment of some or all of the walk­ing legs.

Source:

Hick­man, C.P. and L. S. Roberts. 1994. An­i­mal Di­ver­sity. Wm. C. Brown, Dubuque, IA.

Br­usca, R. C., and G. J. Br­usca. In­ver­te­brates. 1990. Sin­auer As­so­ci­ates, Sun­der­land, MA.

Pearse, V., J. Pearse, M. Buchs­baum, and R. Buchs­baum. 1987. Liv­ing In­ver­te­brates. Black­well Sci­en­tific Pub­li­ca­tions, Palo Alto, Ca.

Con­trib­u­tors

Phil Myers (au­thor), Mu­seum of Zo­ol­ogy, Uni­ver­sity of Michi­gan-Ann Arbor.