Icilius australis, Haswell, 1879

Watson, M. J., Lowry, J. K. & Steinberg, P. D., 2004, Revision Of The Iciliidae (Crustacea: Amphipoda), Raffles Bulletin of Zoology 52 (2), pp. 467-495 : 469-471

Icilius australis abundance and distribution on sponges. –

Icilius australis is among the most abundant amphipod species that occur on sponges in Botany Bay and in Port Jackson ( Fig. 3a ). I. australis was unevenly distributed among the twelve sponge species sampled ( Fig. 3b ): it was most abundant on Phorbas sp. , Antho (Isopenectya) chartacea , Mycale (Carmia) sp. and Callyspongia sp 1 ., a few individuals were found on Iotrochopsamma arbuscula , Callyspongia spp 2 and 3, Cymbastella concentrica , Holopsamma lamiaefavosa , Halichondria sp. and Phoriospongia cf. kirki and none were found on Raphyoxa sp. In spite of obvious differences in abundance on hosts, it does not appear that the association between I. australis and host sponges is a specialist interaction. Icilius australis was collected from every sponge species sampled, with the exception of Raphyoxa sp. The sponges on which I. australis are most abundant are not taxonomically related, being from two different orders and five different families. At a higher taxonomic level, phylogenetic tracking between the family Iciliidae and sponge families does not occur. Phylogeny is well known in dictating some specialist interactions (eg. Janz & Nylin, 1998). However, members of Icilius are associated with sponges, gorgonaceans, echinoderms, soft corals and algae. Clearly, associations between amphipods in the Iciliidae family and their hosts are not phylogenetically determined.

Feeding behaviour and gut contents analysis. – Laboratory observations of the feeding behaviour of Icilius australis revealed the following characteristic behaviour. The flattened body of I. australis is held closely to the surface of the sponge. The urosome is tucked beneath the body, and beats rapidly at all times. Beating pleopods pull water into a channel under the body and through the setose gnathopods. The gnathopods filter particulate matter from the water that is then processed by the mouthparts and either rejected or eaten. Gnathopods are held anteriorly, beneath the mouthparts. Sudden movement of the antennae is followed by rapid activity of gnathopods and mouthparts, even when the stimulus of the antennae is not readily apparent. Gnathopods occasionally comb the dorsal surface of the body. When a sand grain or other particle is encountered on the sponge surface, it is picked up by the gnathopods and worked over by the mouthparts, before being accepted or rejected.

The feeding behaviour of Icilius australis is similar to other filter feeding amphipods such as Leptocheirus pinguis . Leptocheirus pinguis pumps water through its burrow and suspended particles are filtered out with highly setose gnathopods ( Theil, 1997).

Gut contents analysis revealed several spicules in the gut of Icilius australis individuals. Some spicules were similar in morphology to spicules from the host sponge suggesting that I. australis feeds on its host. However, other shapes of spicules within the gut were unlike those of the host, suggesting that I. australis can feed on both its host, as well as other nonhost sponge tissues in the water column. Other matter also present in the gut, such as filamentous algae, indicated that I. australis also feeds on non-sponge matter. In conjunction with the behavioural observations we made, the gut contents analysis strongly suggests that I. australis is a filter feeding amphipod. Feeding methods of other amphipods within the genus are unknown, but the similar morphology of gnathopods of all members of the family indicates a comparable feeding method across the group.

Sexual dimorphism. – In amphipods where precopulatory amplexus occurs it is usually accomplished by the first or second gnathopods of the males. In Iciliids there is sexual dimorphism in pereopods 3 and 4 in all species of Icilius . It is most pronounced in I. australis where the third and fourth legs become extremely broadened in the male and the propodi become expanded distally so that a strongly setose palm develops. The dactylus closes on the palm to form a prehensile structure that may be used to grasp a female in a precopulatory position. Iciliids occur in high densities on their hosts and competition for mates may become intense. This is the first time that apparent precopulatory behavior involving pereopod 3 and 4 has been suggested. It appears to be an independently derived mechanism for mate guarding.

Figures

Fig. 3. (a) Density of Icilius australis and (b) proportion of I. australis of total amphipod community, associated with 12 sponge species at Bare Island, N.S.W., Australia. Data are means (SE) of (a) density (number per g wet weight host, n = 5 for each host) and (b) proportion (total number of I. australis divided by total number of all amphipods, n = 5 for each host). Ph, Phorbas sp.; Ac, Antho (Isopenectya) chartacea; My, Mycale (Carmia) sp.; Ca1, Callyspongia sp.1; Ia, Iotrochopsamma arbuscula; Ca3, Callyspongia sp. 3; Cc, Cymbastella concentrica; Ca2, Callyspongia sp. 2; Hl, Holopsamma lamiaefavosa; Ha, Halichondria sp.; Pk, Phoriospongia cf. kirki; Rh, Rhaphoxya sp. Density data adapted from Poore et al, 2000.