Last week our lab studied and explored organisms of the Phylum Mollusca. The mollusk that I found most interesting, as well as most colorful, was Flabellina iodinea. Flabellina iodinea is a brightly colored species of nudibranch belonging to the Aeolid family. Aeolid nudibranchs are different from other nudibranchs in that some members of the group have evolved to feed specifically on hydroids and other small anemones. Interestingly, these animals have evolved and adapted into a lifestyle that has allowed them to prey on potentially harmful cnidarians without reducing their fitness. Furthermore, these nudibranchs have taken advantage of this unique feeding ecology by incorporating unfired nematocysts into their own tissues, allowing them to have an efficient defense mechanism without the presence of a shell. A lingering question that I felt would be interesting to research is how Flabellina iodinea are able to accumulate the nematocysts of their Cnidarian prey onto projections on their dorsal backside.
Flabellina nudibranchs can be found in the marine intertidal areas of Baja California all the way up to British Columbia, Canada. Within this habitat distribution, Flabellina preys selectively on the orange polyps of an athecate hydroid, Eudendrium ramosum. Eudendrium hydroids are likely the prey item of choice for Flabellina because they lack thecae: polyps that can be retracted into protective cups. This leaves the hydroid particularly prone to the rather slow and unsuspicious Flabellina nudibranch. Hydroid eating nudibranchs such as Flabellina avoid being stung by their prey’s nematocysts by secreting a mucous that inhibits discharge. Research (Greenwood 2004) has shown that the type of mucous secreted is specific to the type of prey the nudibranch consumes In addition, studies (McBeth 1972) have shown that the distinct orange and scarlet hues of the cerata and rhinophores of Flabellina iodinea are all derived from astaxanthin, a single carotenoid pigment obtained from its Eudendrium prey.
Once a Flabellina has consumed Eudendrium hydroids, how does it secrete and incorporate un-discharged nematocysts onto its body? To answer this, we must understand the anatomy of nudibranch cerata. Morphologically, cerata are anatomical structures that resemble lateral outgrowths projecting from the dorsal side of the organism. Functionally, these cerata are responsible for creating surface area for gas exchanges, and in the case of Flabellina and other aeolid nudibranchs, they also serve as defensive structures. Each cerata is connected to the organism as an extension of its digestive system. When a hydroid is consumed, its nematocysts are stored in special sacs located on the tips of cerata known as cnidosacs. These cnidosacs are composed primarily of a single layer of cndiophages, or phagocytizing cells. Consumed nematocysts are processed in the stomach and later diverted to a hollow structure called the diverticulum, which is connected to the dorsal cerata. Once inside the lumen of the cnidosacs, the nematocysts are phagocytized and stored within the cnidophages (Kälker & Schmekel 1976).
When irritated or threatened, the Flabellina will curl its body and when in contact with a predator, specialized muscles will squeeze the nematocysts out of the cnidosacs. When in contact with the external saltwater environment, the nematocysts forcefully discharge as if they were still located on a polyp of a hydroid. In lab, this curling was observed, and thankfully, when touched, I felt no excruciating pain from the discharged nematocysts.
Research conducted on how nematocyst-bearing aeolid nudibranchs fared in the presence of a predatory sea slug, Navanax inermis, showed that of the eight tested species of nudibranchs, only Flabellina iodinea was capable of deterring the slug from predation. The results of these studies may indicate, that because only one out of the eight species of aeolid nudibranch was not predated upon, the secretion of un- discharged nematocysts by most aeolids may be a no longer useful trait, which may not serve an inherent function. Perhaps some non-toxic aeolid nudibranchs display aposematic signaling by Batesian mimicry.
The answer is still unclear, as the defense mechanism described above has never been studied in a controlled experiment. Nonetheless, some aeolid nudibranchs do posses a powerful sting, which is likely correlated with how powerful the nematocysts were in the prey item. Reports of swimmers being stung by Glaucus sp. (a species of aeolid nudibranchs), which preys on the Portuguese-man-of War, have been reported in regions of Australia where both species persist (Greenwood 2014).
Kälker H, Schmekel L (1976) Bau und Funktion des Cnidosacks der Aeolidoidea (Gastropoda: Nudibranchia). Zoomorphologie 86: 41–60.
McBeth, J. W. (1972). Carotenoids from nudibranchs. Comparative Biochemistry and Physiology, 41(B): 55-68.