University of São Paulo: At the bottom of the sea, a game of life and death in exuberant colors

Scientists at USP’s Marine Biology Center (Cebimar), in São Sebastião, SP, managed to record, for the first time, in photo and video, the food interaction process between prey and predator of two marine species, a mollusk (nudibranch Spurilla braziliana ) and a sea spider (Picnogonid Pigrogromitus timsanus ). The study generates basic knowledge about animals that can be interesting models in future toxicology studies.

Highlighted in their habitat for their exuberance and vibrant colors, nudibranchs, molluscs popularly known as sea slugs, are rarely seen as prey in marine waters. Although they look fragile because they lack a shell that would protect them as adults, the tiny molluscs (this species was 4 cm) have excellent defense strategies: from a chemical arsenal that includes the incorporation of toxins from their prey, production of acidic substances , even a camouflage system and the typical warning coloring. However, even with all this ammunition, predators do exist, and it was this exact moment that USP researchers managed to register for the first time, in observations carried out in the Cebimar laboratory.

“The discovery of a new predator of the nudibranch was almost by chance, or driven by the intrinsic curiosity of the researcher’s activity”, tells the biologist Licia Sales, who holds a doctorate from the Institute of Biosciences (IB) at USP, to Jornal da USP . In 2017, she was on an expedition in the Araçá Bay, in São Sebastião, SP, collecting animals for her doctoral research, whose theme was marine slug reproductive strategies, when she found a kind of nudibranch different from the focus of his doctoral work, and he decided to bring some specimens to better observe them in the laboratory.

In an aquarium and with the help of a magnifying glass, she verified that there was a tiny sea spider (the pycnogonid), only a few millimeters long, attached to the back of the nudibranch, feeding on it through the base of the cerata (cerata, plural and waxes, singular – filamentous structure that the nudibranch has throughout the body and where it concentrates anti-predator chemical substances in the tips).

Main photo: Sea spider (Pychnogonid Pigrogromitus timsanus ) attached to the back of the nudibranch, feeding on it. It was camouflaged among the cerata that are distributed on the animal’s back and sides – Credit: Álvaro Esteves Migotto/Cebimar-USP

Biologist Lícia Sales, author of the research and PhD at the Institute of Biosciences (IB) of USP – Photo: Personal archive
Micropredation recorded in photo and video
Licia and Professor Álvaro Esteves Migotto, from Cebimar, her supervisor for the master’s degree and who was at the site, realized that they were facing a rare event and began recording in photo and video the entire process of food interaction between these two species. In all, ten days of takes of filming and photographic clicks until, in the end, the nudibranch was found dead in the tank bottom. However, it was not known whether the cause of death was micropredation, as the sea spider did not feed on a substantial part of the nudibranch, explains the biologist.

According to the researcher, throughout the observation period, the sea spider remained attached to the mollusc’s back. His tiny trunk was often seen sucking the base of the cerata from the nudibranch. In one of the moments, the researchers tried to remove the sea spider from the back of the mollusc, when one of the cerata ended up being removed along with the pycnogonide, which continued to feed on the wax for at least another 16 minutes, when there was an interruption of the footage. After returning to the aquarium, the pyknogonid climbed again on the mollusk’s back and continued micropredation. And finally, after ten days, although it looked healthy, the “slug” was found dead for unknown reasons.

“Until then, there were no reports that anyone would have seen this species of nudibranch being preyed upon by sea spiders. The defenses of nudibranchs seem to be in fact very efficient, as few are the known organisms capable of feeding on nudibranchs”, explains the biologist and first author of the article An unprecedented observation of the nudibranch Spurilla braziliana as a food resource of the pycnogonid , published March 2021 in The Nautilus . Professor Migotto also signs the article. The question that came up after following the experiment was how the sea spider managed to bypass the mollusc’s defense system to feed on it.

Strategy to bypass the defense system
The biologist explains that this species of nudibranch stores urticating microscopic structures (nematocysts, which produce a sensation of burning or painful irritations) that it captures from its prey at the tip of the cerata, to use them in its own defense. Thus, “because the pyknogonid kept feeding on the base of the cerata, the hypothesis was proposed that the sea spider was trying to stay away from the nematocysts, and ‘bypassing’ the defense mechanism of the nudibranch”, he says.

According to the biologist, the experience could also have been interpreted as parasitism due to the fact that the sea spider was feeding for ten days in a row. However, she explains that parasitism happens when a species feeds on a single host for an entire stage of its life (the predator). And, in this case, the pyknogonid had already been seen eating other organisms as an adult, so the food interaction observed was qualified as micropredation, that is, when an organism feeds on more than one prey during a life stage, without necessarily killing it or harm it, such as mosquitoes and leafhoppers.

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