Arcturid isopods on sea spider

Here are several arcturid isopods perched on a sea spider, going along for the ride. Why would an isopod joyride on a sea spider?

Arcturid isopods cling to something, like the sea spider leg shown here, using some of their posterior pereopods (rear legs) while holding their anterior segments and pereopods up into the water for passive filter-feeding [1]. Perching on a moving object like a sea spider affords the isopod better access to its prey in the water column as the sea spider walks along the seafloor, covering a wide area. This affords the isopod far better access to food than if the isopod was attached to an immobile object and waiting for its prey to wander by one spot. The immobile bush sponge Homaxinella balfourensis almost always has arcturid isopods perched on it [4].

Arcturid valviferan isopods have a passive filtration feeding mechanism using the setal combs of their anterior pereopods, seen here [1]. More primitive genera of Arcturidae lack these setal combs on their anterior pereopods and are detritivorous [1].

Antarctic isopods have a variety of deepwater and continental shelf ecological niches including parasites of fish and other isopods and free-living predators of amphipods, polychaetes, and other invertebrates [3]. Antarctic isopods have at least 346 species and 302 of those are endemic to Antarctica (native or peculiar to Antarctica) [3]. Some Antarctic isopods occur in both Antarctica and South Africa, Australia, or South America [3]. Only one Antarctic isopod is bipolar (found in the Arctic as well as Antarctic) [3].

1: Antarctic Isopoda Valvifera. JW Wagele. Koenigstein ; Champaign, Ill. : Koeltz Scientific Books, 1991; 2: Antarctic Valviferans (Crustacea, Isopoda, Valvifera) : New Genera, New Species, and Redescriptions. A Brandt. Leiden ; New York : E.J. Brill, 1990; 3: Berichte zur Polarforschung 98: 201-240, 1991; 4: Rob Robbins, personal communication, 1999

Text ©Peter Brueggeman. Photographs ©Paul Cziko & M Dale Stokes. Photographs may not be used in any form without the express written permission of Paul Cziko & M Dale Stokes.