An amazing array of animals live within the feathery arms of feathers stars, including crustaceans, gastropods, and polychaete worms. One study of crinoid species in the Red Sea found 27 taxons of symbionts living on and in the bodies of feather stars, including copepods, molluscs, polychaetes, and decapods (Fishelson 1974). These commensals (and some parasites) benefit from their close association with feather stars, gaining shelter from predators and regular free meals (Fishelson 1974). 

Six specimens of A. pectinifera were collected by hand at low tide from the shallow outer reef and rubble crest zones on the northern reef of Heron Island, Queensland, Australia (23º26'27"S 151º58'49"E) on 20 September 2012. All specimens were found at depths of 0.3 - 0.9 m in crevices under boulders or large coral heads in an area consisting of both hard and soft bottom substrates. For viewing, the specimens were placed in a shallow container with enough natural seawater to just submerge the animal. Individuals were relaxed using 70% magnesium chloride administered dropwise, and examined under a stereoscopic dissecting microscope. Commensals found on 

Results

The six specimens were all approximately the same size, with central disks (the fleshy central portion bearing the oral opening) ranging in size from 0.9 - 1.2 cm and arms 7.5 - 15.2 cm long. 

5 distinct species of ectosymbionts were found between all feather stars surveyed, including 2 species of shrimp, 1 species of gastropod, and 2 species of polychaete worms. (families Myzostomidae and Polynoidae). Copepods were observed on 4 of the 6 specimens, but were not able to be collected for closer inspection. Not counting copepods, the average ectosymbiont load per feather star was 1.8. 




The majority of the ectosymbionts were less than 1.0 cm in size, but one large alpheid shrimp was just under 2.0 cm in length. This individual (shown in Fig. 4B) was a gravid female carrying numerous bright green eggs underneath the tail. All ectosymbionts found exhibited crypsis, with coloration ranging from deep red (see Fig. 4A) to black, and closely matched the coloration of their hosts.




Fig. 4. Some ectosymbionts found on Alloeocomatella pectinifera collected from the shallow outer reef and rubble reef crest of Heron Island, Qld, Australia. A) Gastropod. B) Decapod - a snapping shrimp . C) Dorsal and ventral views of a myzostomide polychaete, showing parapodia. D) Another species of decapod.  This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. 



Discussion

This preliminary survey indicates that symbionts are ubiquitous on A. pectinifera. Although identification to species of symbionts was not made, the initial results suggest that the primary taxa using A. pectinifera as a host species are the same as those found on other crinoid species, namely small polychaetes, gastropods, and decapods. 

Interestingly, many of the symbionts found on A. pectinifera were red or deep burgundy-brown in color, appearing to mimic the host coloration (Fig. 4). Similar coloration of hosts and ectoparasites is a sign of evolution having occurred over an extended period of time through the close associate of ectosymbiont and host (Bush et al. 2010). Different species of ectosymbionts appeared to favor different microhabitats. The larger, more robustly build alpheid shrimp were found exclusively on the central disk, while the smaller, slender shrimps moved freely around the margins of the central disk and along the arms. 

Additional research needs to be conducted to confirm these observations and to further characterize the density, diversity, degree of coevolution, and exact relationship of ectosymbionts living on A. pectnifera.