Commensalism and Predation

Many studies have found numerous crinoid species which have been subjected to parasitism or commensal associations by a large variety of organisms, such as gastropods, polycheates and crustaceans, covering nearly all invertebrate phyla (Alender et al 1966). The most common and longest known commensal association in crinoids is with myzostomids and is thought to have existed for over 300 million years creating a symbiosis that is now vital for myzostomids to survive (Warn 1974, Brett 1978, Meyer and Ausich 1983, Radwanska and Radwanski 2005). This relationship has lead to many myzostomids species becoming associated with only a single crinoid species (Lanterbecq et al 2006). The anatomy of a myzostomids has become so reliant on crinoids that they are unable to leave their host to move to other habitats and are unable to survive long after removal from their host, therefore limiting their dispersal capabilities (Eeckhaut and Jangoux 1993, Lanterbecq et al 2008).

Commensalism Observational Study

Three C. glebosus individuals collected from a rock wall habitat from Stradbroke Island’s Amity Point were observed for commensal organisms due to the limited information on C. glebosus and their relationships with other organisms.  All three individuals were of similar size and were housed within the same aquarium tank after collection. For the observational study each individual was removed from the tank and transported to the laboratory where they were placed under a dissecting microscope in a shallow container of aquarium seawater that allowed the animal to be fully submerged. The organisms found on the individuals were recorded by phyla and class along with its position on the crinoid.

A variety of organisms were found on C. glebosus but not in large quantities. This could be due to the fact that the C. glebosus specimens were collected almost a month prior to the observational study and therefore commensal organisms would have had to survive the transport from Stradbroke Island and life in the aquarium, or the organisms were already present within the aquarium. Myzostomid polycheates were found on all C. glebosus individuals but were unfortunately displaced during the movement between aquarium and laboratory so the position on the body could not be recorded. Each of the myzostomids were ofa cryptic nature, with a colouration closely resembling that of C. glebosus; the myzostomids were a deep red/maroon colour compared to the dark brown/black of C. glebosus. Decapod crustaceans were found on two of the C. glebosus individuals, a crab and shrimp both found between the cirri and the centrodorsal. Both of these crustaceans had cryptic colouration and were a deep red/brown colour. The third individual had a non-cryptic gastropod mollusc on one of its arm. The size of the polycheates, crustaceans and gastropods found on the individuals were 1-2 cm in size. The largest individual had not only a myzostomid and a decapod but also a semi cryptic ophiuroidea, brittle star, which lay over the oral side of the central disc with its arms extending over the calyx cup (Figure 4). The arms of the brittle star were approximately 5 cm in length and was light purple in colour and was able to hide in between the pinnules and arms enough to blend in.

The cryptic colouration of the myzostomids and crustaceans in regards to C.glebosus is a sign of a possible evolutionary symbiosis that has occurred over a long period of time, like myzostomids on other studied crinoid species (Bush et al 2010). Research of the species within a field environment where it hasn’t been subjected to aquarium condition will provide a greater understanding of the organisms found within C. glebosus and its evolutionary relationships. The similar but not quite cryptic colouration of the brittle star could be a natural relationship or one that arose from aquarium conditions and having to settle for the closest coloured species available. As C. glebosus hasn't be studied in detail, future studies should include commensalism and evolutionary symbiotic relationships.



Figure 4 - Ophioroidea, Brittle Star, found on a C. glebosus individual during an observational study. Photo taken by Sophie Horsfall

Predation

Many studies have shown that crinoids, especially comatulids, are ignored by fish even when they are richly coloured and fully exposed on coral surfaces, or in an environment limited in food (Alender et al 1966). The stomach contents of various reef fish have shown no sign of crinoidal fragments which has led to the conclusion that crinoids are generally not subjected to predation by fish (Randall, 1967). The release of toxic chemicals via secretion from the mucous glands at the bases of the tube feet are thought to be the reason behind the low predation rates making their main enemies their own kind during larval stages (Alender et al 1966).