“You are what you eat” and other defensive strategies of marine invertebrates, Part II

It has been a few months since I posted part one of this blog. It has been a busy spring. A number of events have come and gone. The Biology Club’s Annual “Save Nemo” event was again sold out, netting a respectable contribution to the maintenance of the reef tank. Additional pay cheque withdrawal donations have helped us maintain a healthy balance in the UNBC Reef Tank fund. A big thanks to everyone who kindly supports the tank – the animals appreciate it!

The 8th Prince George Reef Tank Tour gave reef tank enthusiasts the opportunity to view some excellent tanks and to discuss issues relevant to the hobby. Photos from the different tanks can be viewed in this series of posts on CanReef http://www.canreef.com/vbulletin/showthread.php?t=103966 (scroll down and you will get to them).

The UNBC Reef Tank has experienced some minor issues relating to water quality and I am continuing to address those by frequent water changes. The manifestation is that some corals are not expanding their polyps properly, and some algae growth indicates that there is a nutrient excess. It is a never-ending task to keep things on an even keel, so to speak.

“You are what you eat” and other defensive strategies of marine invertebrates, Part II
I have mentioned that corals and their relatives have unique cells, the cnidocytes (stinging cells) which contain a capsule, a nematocyst, which when touched releases a stinging thread, which sometimes can deliver a more-or-less potent venom. No other animals have these cells, hence corals, sea jellies, hydras and their relatives are classified together in the Phylum Cnidaria, the stinging animals. If you touch a tide pool anemone on the coast, you will notice that it feels like the tentacles are sticky. This is due to the stinging threads being fired into your skin. These anemones don’t deliver potent venom, however, so it doesn’t hurt. At the other end of the scale are some box jellyfish, which deliver such a potent venom that it may be lethal, and at best even minor stings can cause agonizing pain and muscle cramps that last for days or even weeks. You can get an idea of the potential problem in this video https://www.youtube.com/watch?v=9CHshkF8GDU. Heeding warnings of stingers is definitely a good idea!

Interestingly, some species of one other Phylum of animals, the molluscs, have figured out ways to a) get around the defense of the stinging animals, and b) to use it to their own advantage. Some version of this is found in several Classes, e.g., anemone fish and some crustaceans live among the tentacles of anemones without triggering the nematocysts (Mebs 2009), and males and immature females of the blanket octopus carries tentacles of its siphonophore (Portuguese Man-o’-War or blue bottle, Physalia sp.) prey for defense. Some Gastropods (snails and slugs), and more specifically in the marine aeolid nudibranchs, an Order of often unimaginably colourful sea slugs have taken this a step further, however. They will incorporate the unfired nematocysts, called kleptocnidae, into its own tissues. A few marine flatworms and at least one species of comb jelly also do this, but very little is known about them (Greenwood 2009).

Berghia coerulescens (Laurillard, 1830). Photo by Parent Géry. Licensed under Public domain via Wikimedia Commons – http://commons.wikimedia.org

Because these nudibranchs tend to be predators of Cnidaria (stinging animals) and other organisms, many are likely to feed on corals, and because they also tend to be a bit difficult to ship and keep, they are rarely seen in the reef tank trade. Some related organisms are available from time to time, however. and the UNBC tank has had two types, both unsuccessfully. One is the blue velvet nudibranch (Chelidonura varians Eliot) or head shield sea slug. In spite of its common name, this is not a nudibranch, but a member of the head shield slugs (Order Cephalaspidea). It is a predator of marine flatworms (Phylum Platyhelminthes), and is used in the reef tank hobby to bring these organisms under control. The specimen brought in for the UNBC tank was in poor condition when it arrived, and simply disappeared very quickly after being introduced into the tank. The second species brought in was a so-called lettuce nudibranch (Elysia crispata (Mörch)). This is not a nudibranch either, but a sacoglossan. There is a story related to the theme of this story with respect to this group, however. Sacoglossans are vegetarians, feeding on various algae. Some species will retain intact, live chloroplasts, a phenomenon called kleptoplasty (Cruz et al. 2013). For that reason, these some species of sacoglossans are able to use the chloroplasts to photosynthesize, and as such they would be unique in the animal kingdom. A recent study has cast some doubt on this, however (Christa et al. 2014), arguing that the kleptoplasts may represent a form of food storage because the genes necessary for the chloroplasts to function is not present. Wägele and Martin (2014) summarize knowledge on how photosynthesis is possible in the absence of a horizontal gene transfer from the algae into the slug genome.

But back to the kleptocnidae. Greenwood (2009) reviewed research on acquisition and functional significance of this stolen weaponry. As is always the case in biological systems, there is no simple and straightforward explanation. Acquisition is aided by physical protection in the nudibranch gut, whereby those nematocysts that do fire do not damage cells. Furthermore, mucus, either acquired from the prey or produced by the nudibranch may inhibit nematocyst firing. Interestingly, this is prey specific, so a nudibranch feeding on one species does not have protection from a different prey species. Some species of nudibranchs can switch prey, whereby the gain protection against the new prey species, but lose protection against the “old” prey species. Yet other nudibranchs acquire immature nematocysts which cannot fire, but mature in the cerrata (fleshy extensions lining the aeolid nudibranch body) where the kleptocnidae are housed (see below). Size also matters! Large nematocysts may prevent predation. Some nudibranchs prefer to feed on tunicates rather than cnidarians, but appear to feed on the latter solely to acquire nematocysts.

In the cerrata, the nematocysts are engulfed by cnidophage cells, and over time they are actually digested (Greenwood 2009). The cnidophages are found in the tips of the cerrata in a cnidosac, which is an extension of the basal digestive diverticulum (See Figure 2 in Greenwood 2009). The kleptocnidae appear to be important for defense in most species, but in some their value may be marginal. Nevertheless, fish and crustaceans generally would eat nudibranchs when the cerrata were removed, whereas most would not eat intact specimens. Nudibranchs do have chemical defenses as well, however, so it is unclear what the relative role of the kleptocnidae may be (Greenwood 2009). In any case, the acquisition and use of nematocysts by aeolid nudibranch sea slugs is unique among metazoans, and exemplifies well that for every evolutionary invention to defend against predation, there will likely be an invention to get around that defense.

References
Christa, G., V. Zimorski, C. Woehle, A. G. M. Tielens, H. Wägele, W. F. Martin and S. B. Gould. 2014. Plastid-bearing sea slugs fix CO2 in the light but do not require photosynthesis to survive. Proc. R. Soc. B 281: 20132493

Cruz, S. R. Calado, J. Serôdio, and P. Cartaxana. 2013. Crawling leaves: photosynthesis in sacoglossan sea slugs. J. Exp. Bot. 64: 3999-4009.

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

Jones, E.C. 1963.Tremoctopus violaceus uses Physalia tentacles as weapons. Science 139: 764-766.

Mebs, D. 2009. Chemical biology of the mutualistic relationships of sea anemones with fish and crustaceans. Toxicon 54: 1071-1074.

Wägele, H., and W.F. Martin. 2014. Endosymbioses in sacoglossan seaslugs: plastid-bearing animals that keep photosynthetic organelles without borrowing genes. Pp. 291-324 in Endosymbiosis (W. Löffelhardt, ed.). Springer-Verlag, Vienna.

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