M.S. Marine Biology
Nudibranchs are soft-bodied marine heterobranch gastropod molluscs which lack a shell and mantle cavity. The basic body plan is bilaterally symmetrical with an expanded notum, but in regards to other physical characteristics they exhibit a wide range of forms. Compared to other molluscs, evolutionarily the head and body became flattened and streamlined and tentacles have been lost or shifted to different areas of the body. Nudibranchs are found in many variations of size and color; despite the fact that these animals in general are noted for flamboyant colors and prominent external anatomical structures, many species rely upon a more cryptic appearance in order to remain inconspicuous as a defense mechanism.
Nudibranch means “naked gill” since they possess dorsal external gills and branchial plumes. Consisting of over 3000 species, they are the largest clade of heterobranchs (Bouchet & Rocroi 2005) and are found in a wide variety of biogeographic regions. The almost exclusively carnivorous nudibranchs are one of the top predators of filter feeding organisms such as corals, hydroids, and sponges (Garcia 1990). With the loss of the shell came increasing development of chemical and biological defenses, and different dietary specializations emerged. As a result, these mostly benthic, soft-bodied animals became virtually immune to attacks by predators (Gosliner 1987).
This paper provides a comprehensive review of the nudibranchs that feed on octocorals (Cnidaria, Anthozoa) emphasizing their feeding physiology and strategies, including prey location and selection. In particular, the question of whether feeding mechanics and morphology are similar among nudibranch corallivores that prey on related octocorals is addressed. One way this paper will identify worldwide patterns in the nudibranch/octocoral relationships is through the investigation of co-evolution. For example, in the North Pacific, neighboring colonies of the encrusting soft coral Discophyton rudyi retract their polyps in response to chemical cues as they are preyed upon by Tritonia festiva, which attempts to neutralize this strategy by launching brief, rapid surprise attacks on its target (Goddard 2006). If correlations can be found between feeding structures and type of prey, it may be possible to predict the diet of nudibranchs with unknown prey.
The relationship between nudibranchs and their food is extremely important. Nudibranchs have a varied diet of mostly toxic and chemically well-defended prey including corals, hydroids, sponges, and other nudibranchs (McDonald 1999). Having lost the protective shells possessed by closely related species, nudibranchs have replaced them with a variety of alternate defenses (Slattery 1998), including toxic chemical deterrents sequestered from their cnidarian prey, and cryptic coloration designed to mimic the substrate on which they reside. For example, Phyllodesmium jakosenae resembles its Xenia prey, and Phyllodesmium briareum has external organs (cerata) that resemble its Briareum prey (Wagle 2005). Alternatively, many species advertise their toxicity with vivid colors, making them some of the most striking creatures in the ocean.
Eric Brown. 2011. Nudibranch Predators of Octocorallia. Capstone. Nova Southeastern University. Retrieved from NSUWorks, . (23)