Nudi-warfare

Chemical warfare, firing missiles, hidden weapons of mass destruction! Don’t stress it’s not the apocalypse, just Nudi-warfare!

Marine organisms possess an extensively wide array of different defenses to predation. With their expressive, flamboyant coloration and soft, artistic body forms, nudibranchs would appear to be an easy target. They possess no sharp spines, hard carapaces or shells to defend themselves from predators, but don’t be deceived these organisms are well equipped to protect themselves from would be attackers.

Nudibranchs are not only great fighters, but excellent thieves. Many species are able to utilize components from their food sources for their own defense.  Some species utilize spicules from the sponges they feeds on, whilst some nudibranchs are even able to incorporate un-fired stinging cells from their cnidarian diet as their own defense. Cnidarians are a phylum of organisms that posses stinging cells called cnidae used to capture prey, this includes corals, anemones, hydroids, jellyfish and many others.

These firing missiles intended for the defense of the cnidarian pass through the digestive sacs of the nudibranch and are maintained in a functional state in cavities called cnidosacs at the tips of the cerata (projections of the digestive tract) of the nudibranch, ready to fire!

Diagram of internal structure of a nudibranch

The selection of these specific cnidae for incorporation within the tissue of the nudibranch is a function of diet and food choice, of which the dynamics of selection not fully understood. There are over 25 types of cnidae present within the many different taxa of cnidarians, believed to have different functions. These cnidae have been found in varying combinations and proportions among nudibranchs. This has led to research interest in the factors that influence cnidae uptake.

Different types of cnidae

Changes in abundance of certain cnidae types within the cerata can be affected by preferential selection of specific cnidae from food available. However ratios of cnidae present can vary between organisms with the same diet, suggesting that factors other than pure availability of food may affect cnidae incorporation. Frick, 2003 investigated the  effect of a biotic factor, in the form of predation pressure, had as an influence on cnidae uptake.

Nudibranch: Flabellina verrucosa

Frick, 2003, investigated the links between population level variation in cnidae sequestering and incorporation, and predation pressure in the nudibranch Flabellina verrucosa. This was achieved by identifying changes in cnidae uptake by nudbranchs in response to chemical cues from the predators; Crossaster papposus (sunflower sea star), Tautogolabus adspersus (cunner) and Carcinus meanas (green crab) compared to those not exposed to the presence of predators.

F.verrucosa were fed two different hydroids (cnidae containing cnidarian organims); Turbelaria spp. and Obelia geniculata, and exposed to one of the predators for two weeks, or kept unexposed (as a control). Cerata (see diagram above) were plucked from each nudibranch before and after this two week treatment and the cnidae inside observed to determine the change in uptake caused by the predation pressure.

Hydroid; Tubularia sp.

Hydroid; Obelia geniculata

Results of this study indicated that in the presence of C.papposus and T.adspersus, F.verrucosa showed similar changes in cnidae abundance. An increase in the incorporation of one type of cnidae (microbasic mastigophores; see cnidae diagram above) and decreases in one of two other kinds of cnidae. There was no significant change in cnidae incorporation observed in nudibranchs exposed to C.meanas.

This result is significant in suggesting that F.verrucosa modulates cnida incorporation in response to the presence of a specific predator. The increased incorporation of one specific type of cnidae may suggest its high level of effectiveness at defending organisms against predators.

This study paves the way for future research into the effect of predation pressure and other biotic factors on the uptake of cnidae in other species of nudibranchs. The study raises questions regarding the effectiveness of specific forms of cnidae as defense from predators, the ability of different species of nudibranch to regulate cnidae uptake, and evolutionary development of this defensive adaptation.

There is still much to know about nudi-weaponry!

REFERENCE;

Frick, K., 2003, Response in nematocyst uptake by the nudibranch Flabellina verrucosa to the presence of various predators in the Southern Gulf of Maine, Biological Bulletin, 205, 367-376

Frick, K., 2005, Nematocyst complements of nudibranchs in the genus Flabellina in the Gulf of Maine and the effect of diet manipulations on the cnidom of Flabellina verrucosa, Marine Biology, 147, 1313-1321

Greenwood, P., 2009, Acquisition and use of nematocysts by cnidarian predators, Toxicon, 54, 1065-1070

Hay, M., 1996, Marine chemical ecology; what’s known and what’s next, Journal of Experimental Marine Biology and Ecology, 200, 103-134

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