In July 2020, an interesting article (Camps-Castellà et al., 2020) was published in the prestigious journal Marine Ecology Progress Series (MEPS), which, under an apparently trivial title, announces a paradigm shift: “Not all nudibranchs are carnivores“.
Nudibranchs (Mollusca, Nudibranchia) have traditionally been considered one of the most highly specialized groups of predators (Megina & Cervera 2003) and, in fact, they are classified into four groups based on their diet: sponge eaters, bryozoan eaters, hydrarian eaters, and another “miscellaneous” group whose diet includes other animals such as tunicates and even other nudibranchs or their egg spawns. Thus, nudibranchs, as a group, were considered carnivores (Clark, 1975; Wagner et al. 2009).
The article we are commenting here studies the case of a tiny nudibranch called Polycerella emertoni that was found in waters of the Ebro Delta (Tarragona, Spain) associated with the bryozoan Amathia verticillata (Camps & Prado, 2018) on which it was supposed to feed.
It may seem very easy to determine what a nudibranch feeds on, but it is not, especially when the animal measures 3-4 mm and lives perfectly camouflaged on the substrate. We know that many nudibranchs that live on sponges feed on them, that many of those that live on hydrarians feed on them, that many of those that live on bryozoans feed on them, and that many of those that live on sand feed on a multitude of organisms that live in this environment. So why would one more nudibranch living on a bryozoan not feed on it?
The first suspicions appeared after an small video recorded with a microscope in which the nudibranch was feeding. It apparently nibbled on the bryozoan, but the tissue of the bryozoan appeared unscathed. But, if the supposed food was not nibbled, could it be because the nudibranch was not actually feeding on it but rather feeding on something that is not seen? What could it be?
Polycerella emertoni on Amathia verticillata, vídeo by Judith Camps © 2018
A combination of methods was used to determine this point, including stable isotope analysis along with Bayesian mixing models, examination of stomach contents and video recording of the animal’s feeding behaviour. A radular analysis was also carried out to determine possible relationships of the radular morphology with the feeding behavior of the animal and, finally, for more than a year, a study of the seasonal abundance of both the bryozoan and the nudibranch was carried out to determine the nature and stability of their association.
Contrary to what was previously believed, the results of the work show that the nudibranch Polycerella emertoni is a microherbivore that feeds on the periphyton that covers the bryozoan Amathia verticillata (up to 99% of the diet according to the results). This periphyton is composed of various species of microscopic algae called diatoms that were found in the stomach content of the nudibranch.
Diatoms are a class of unicellular algae characterized by having a silica coating called a frustule. These frustules have a great variety of shapes and designs, some very beautiful. Most diatoms are pelagic (live in open water) but some species are benthic (live on the sea floor). Diatoms are especially important to the oceans, providing about 45% of total primary ocean production, the basis of marine life, and are found in all seas, from the poles (where there are few species but in large quantities) to the tropics (where there are more species but in less quantity).
Are there other herbivorous nudibranchs?
There is very little information on adaptations of the radula morphology to nudibranch feeding strategies (Nybakken & McDonald, 1981), therefore, general perceptions of trophic ecology and this group biology need to be re-evaluated (Todd et al. 2001).
The results of the study of the nudibranch Polycerella emertoni show a radular formula and a characteristic morphology that, compared to other similar nudibranch species, suggest the presence of multiple feeding strategies: not all eat the same type of food.
Existing literature (see McDonald & Nybakken, 1997 compilation) often confuses substrate with food (although in many cases it is the same). It is a difficult subject to study due to the inherent problems (small animals, seasonal or occasional appearance, difficult and expensive collection by scuba divers) and, therefore, there are very few ecological works on the subject. We highlight a publication on the heterobranch Tylodina perversa (see Becerro et al., 2003) that pointed in the same direction as the present work.
Although the study shows that this nudibranch does not feed on the bryozoan, the association between the nudibranch and the bryozoan suggests a certain symbiosis between them: the nudibranch camouflages itself over the bryozoan, also obtaining benefits from improved dispersion since the bryozoan breaks easily, and each piece (with its associated nudibranchs) retains the ability to continue growing where environmental conditions allow (Amathia verticillata is well known for its ability to grow rapidly on the hull of boats –fouling– and other marine structures). On the other hand, the elimination of the periphyton that covers the bryozoan, in which the nudibranch is an active part, can have positive effects on the reproductive cycle of the bryozoan (Dahms et al, 2004), thus improving its survival rate and facilitating its dispersion.
The original article can be requested directly from the authors, by clicking here.
- Becerro, M. A., Turón, X., Uriz, M. J., & Templado, J. (2003) Can a sponge feeder be an herbivore? Tylodina perversa (Gastropoda) feeding on Aplysina aerophoba (Demospongiae). Biological Journal of the Linnean Society, 78(4): 429-438. DOI: https://doi.org/10.1046/j.0024-4066.2002.00165.x
- Camps, J. & Prado, P. (2018) Polycerella emertoni associated to Amathia verticillata in the Ebro Delta, NE Spain (Western Mediterranean). In: Yokes, Andreou, Bakiu, Bonanomi and others (2018) New Mediterranean Biodiversity Records (November 2018). Mediterranean Marine Science 19: 673−689, p 675. DOI: https://doi.org/10.12681/mms.19386
- Camps-Castellà, J., Ballesteros, M., Trobajo, R., Pontes, M. & Prado, P. (2020) Not all nudibranchs are carnivorous: trophic ecology of Polycerella emertoni in the Ebro Delta. Marine Ecology Progress Series 645: 67-82. DOI: https://doi.org/10.3354/meps13379
- Clark, K.B. (1975) Nudibranch life cycles in the Northwest Atlantic and their relationship to the ecology of fouling communities. Helgoländer wissenschaftliche Meeresuntersuchungen 27: 28−69. DOI: https://doi.org/10.1007/BF01611686
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- McDonald, G. R, & Nybakken, J. W. (1997) List of the Worldwide Food Habits of Nudibranchs. Veliger, 40(2). Retrieved from https://escholarship.org/uc/item/0g75h1q3
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- Nybakken, J., & McDonald, G. (1981) Feeding mechanisms of west American nudibranchs feeding on Bryozoa, Cnidaria and Ascidiacea, with special respect to the radula. Malacologia 20(2): 439-449
- Todd, C.D., Walter, J. & Daviee, J. (2001) Some perspectives on the biology and ecology of nudibranch molluscs: generalisations and variations on the theme that prove the rule. Bolletino Malacologico 37 (5-8): 105−120
- Wagner, D., Kahng, S.E. & Toonen, R.J. (2009) Observations of the life history and feeding ecology of a specialized nudibranch predator (Phyllodesmium poindimiei), with implications for biocontrol of an invasive octocoral (Carijoa riisei) in Hawaii. Journal of Experimental Marine Biology and Ecology 372: 64−74. DOI: https://doi.org/10.1016/j.jembe.2009.02.007
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