Behaviour

Comatulid crinoids are suspension feeders that rely on the movement of water through their outstretched arms and pinnules to deliver plankton and other food particles to the erect tube feet (Ruppert et al 2004). Their arms are placed in a parabola or plane shape and they orient themselves perpendicular to the water flow during feeding, with the adjacent pinnules and tube feet creating a tight lattice to catch suspended food (Ruppert et al 2004). The trios of tube feet are used like tentacles using mucus secreted from papillae situated along their length; the primary tube foot of each trio is long and outstretched during feeding to make a food catching mesh, where food particles that stick to the tube foot are moved to the mucus-lined ambulacral grooves (Alender et al 1966, Ruppert et al 2004). Cilia then transport the food particles along the ambulacral grooves to the mouth to be ingested (Ruppert et al 2004). Toxic substances secreted along with the mucus from the tube feet are thought to produce a paralytic effect on crustaceans and other planktonic organisms to utilize food capture (Alender et al 1966).

The placement and size of a crinoid species’ tube feet, has been found to have a strong relationship with the living and feeding positions found within the certain species (Meyer 1979). Species that have longer and more widely spaced tube feet correlate to species that live within the infrastructure of the reef, where they hold their arms in a multidirectional posture (Meyer 1979). Species with shorter and more closely spaced tube feet tend to to be found on the exposed parts of the reef (top of the reef) and form filtration fans (Meyer 1979).  These differing styles of tube feet correlate to taxonomic groupings with non-comasterids usually found in exposed positions while comasterids are found within the infrastructure of the reef (Meyer 1979). The differing feeding positions and tube feet characteristics relate to the water movement through the reef. Since the reef has many structures, water flow in one direction is scattered and velocity is reduced, while above the reef the water flow is at its highest velocity and is not hampered by other structures (Meyer 1979). Taking into account these differing water flow environments, crinoid species have adapted different feeding postures that increase their efficiency in catching food (Meyer 1973). This has in turn adapted the spacing and length of the tube feet to creating a greater filtering efficiency in the different water velocities (Meyer 1979).

When observing the three C. glebosus individuals within the aquaria, they were positioned on top and within the coral architecture present with their arms in held in a multidirectional posture (Figure 5). C. glebosus seems to follow other comasterid species in regards to its feeding position, and with the tank having little to no water movement, the position of the individuals on top of the coral infrastructure would not imply it was a high water velocity species. Along with the robust and short cirri and its preference of rocky substrates, C.glebosus seems to follow other behavioural traits seen within the comasterids family.

Figure 5- Photo of C. glebosus in their feeding pose in the shape of a parabola. Photo taken by Sophie Horsfall


Locomotion

Comatulids, such as C. glebosus, are the free-living group of crinoids and are capable of swimming using their arms or crawling with their cirri (Ruppert et al 2004). Feather stars always move with their oral surface facing upwards alternately raising and lowering their arms when swimming or lifting its body and moving around on its downward-flexed arms when crawling (Alenderet al 1966, Ruppert et al 2004). During crawling there is a regular use of the arms and pinnules, which have small hooks, to pull it over substrates and surfaces (Ruppert et al 2004).

Members of Comasteridae have been found to be creepers (Alender et al 1966). They regularly use their cirri and arms to move over substrates but do not use their arms to swim, even when mechanically stimulated (Alender et al 1966). Swimming is usually used as an escape route but in the case of comasteridaes they remain still clinging onto the substrate unless they use their complex creeping movements that involve pushing and pulling their cirri and arms across the substrate (Alender et al 1966).

When collecting the three C. glebosus individuals from the tank they clung to the rock and didn’t swim away when mechanically stimulated like other comasteridaes. During the substrate tests the individuals used the creeping method of pushing and pulling cirri and arms over the substrates to pull itself up onto the rocky substrate. Although there have been no studies on C. glebosus an inference on their locomotion can made from the substrate test where they were seen to use the creeping methods used by other comasteridaes.

Community Formation

Unlike other echinoderms, crinoids are rarely a dominant or subdominant member of benthic communities but can be found in considerable numbers on reefs and seafloors during favorable conditions (Alender et al 1966)