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Cryptic Cannibals: Sea Slugs in the Bahamas

The following is a blog post by Carolyn Belak, sea slug enthusiast and scientific aid at the California Department of Fish and Wildlife, and Andy Kough, postdoctoral research associate at Shedd Aquarium.

A speckled varietal of Chelidonura berolina. © Carolyn Belak
A speckled varietal of Chelidonura berolina. © Carolyn Belak

When one thinks of the Bahamas, they probably picture miles of white sandy beaches, dolphins dancing through crystal blue waters and colorful fish flickering amongst coral. However, the 700-island archipelago is comprised of many different marine ecosystems each with a variety of creatures, including many that are hidden and unfamiliar yet important nonetheless. We recently visited the Exuma Cays Land and Sea Park, the oldest Marine Protected Area in the wider Caribbean, to map and assess different habitats, and survey Queen Conch (Lobatus gigas) population abundance and health. While our research focus was on Queen Conch, along the way we were able to observe incredible, yet diminutive wildlife. Of particular note while snorkeling in a small, otherwise barren, sandy bay, we observed one thing and one thing only: sea slugs! Where no large predators, no fish, little algae and no corals permanently resided, slugs dominated.

Marine ecosystems consist of many trophic levels, or hierarchical levels: primary producers, primary consumers, secondary consumers, detritivores, etc. Food webs are complex, with interactions between groups that depend on which animals are present and the environment that they are in. So what’s happening when these trophic levels remain unseen? Bacteria and microorganisms often create their own complicated communities, but in the realm of cryptic macrofauna (creatures visible to the naked eye), what else is in play? After seeing an almost exclusively slug-filled environment, presumably with a single trophic level of macrofauna, we had many unanswered questions about this paradox: What are they eating? Is anything eating them? When/where/how are they reproducing?

The dark color morph of Chelidonura berolina, next to a millimeter-marked ruler, for scale. © Carolyn Belak
The dark color morph of Chelidonura berolina next to a millimeter-marked ruler for scale. © Carolyn Belak

The specific family of slugs that we came across was the Aglajidae, a type of headshield slug. While the nudibranchs may be the most familiar opisthobranch, their cousins, the headshield slugs, are commonly less colorful, but are beautiful and sleek and charismatic all the same. And we’re not talking about slugs the size of golf balls that you can spot from meters away. These guys typically range anywhere from 2 to 6 mm, with the rare exception of an 18 to 20 mm individual. Think of small, slimy and gorgeous grains of rice sliding along the sand. To see them, grab a mask and dive down to take a close look at the bottom. All of the little dark specs in contrast with the legendarily pristine, white Bahamian sand suddenly become much more interesting. We searched for slugs along three haphazardly placed 50 m transects of the bay, and we found 81 individuals! These were identified as Aglaja felis, Chelidonura berolina, Philinopsis pusa, and Costasiella ocellifera species, but 93 percent of the slugs we observed were members of Aglajidae. We saw many slugs, but what were they doing in what appeared to be sandy area, mostly devoid of vegetation or structure?

Headshield slugs follow prey using chemical cues and mucus trails. © Carolyn Belak
Headshield slugs follow prey using chemical cues and mucus trails.
© Carolyn Belak
At a whopping 19mm, this Philinopsis pusa individual stood out as our largest slug found. © Carolyn Belak
At a whopping 19mm, this Philinopsis pusa individual stood out as our largest slug found.
© Carolyn Belak
Other than aglajids, the sacoglossan Costasiella ocellifera was found on a small piece of algae. © Carolyn Belak
Other than aglajids, the sacoglossan Costasiella ocellifera was found on a small piece of algae.
© Carolyn Belak

A little research told us that these slugs are voracious predators and can even become cannibals, often feeding on their own family and species. They differ from other slugs by lacking a radular tooth structure. Instead, they evert a proboscis-like buccal bulb, and swallow prey whole. Slugs being slugs, they also lack the hard protective shell of a snail, and must make up for this exposure with other defenses. As with most slugs, they harbor a noxious chemical toxin in their tissues to ward off prey. These toxins teach potential predators they are unpalatable, and the only remaining predators capable of consuming slugs are the slugs themselves. So in a shallow, sand-filled bay, where few other predators thrive due to lack of prey, these slugs seem to reign supreme with their cannibalistic capabilities.

Our 50 m long surveys in search of slugs were within a sandy bay, on a cay, on one of the Bahamian banks. © Shedd Aquarium
Our 50 m long surveys in search of slugs were within a sandy bay, on a cay, on one of the Bahamian banks.  © Shedd Aquarium

Now, one might think that this small, 30,000 square meter bay is insignificant as an ecosystem on its own, so why should we care about these slugs and their unique niche? Well, geologically speaking, the Bahamas platform consists of a few very large, calcareous banks with ridges sticking out as islands (cays) and warm, shallow sand in between. If we scale-up our small bay (right) to consider similar habitat throughout the cay, then from the cay up to the banks, then up to the entire platform, similar shallow sandy habitats actually makeup the majority of the Bahamian benthic marine environment! While we were surveying conch, we also used cameras attached to snorkel tow boards to digitally map over 300 km of habitat adjacent to the islands. As it turns out, after travelling about 5 km away from the islands, the lush seagrass beds, macroalgal flats and patch reefs began to disappear. Instead, we typically documented sandy, sparsely-vegetated habitat, just like what we found in our slug-filled bay. So if slugs make up the primary biomass of these bays, and similar barren sandy habitat covers much of the Bahamas, how many slugs are there and how important are they? The answer just may be that the Bahamas is dominated by slugs.

While not found within this particular survey, Chelidonura hirundinina, another member of the Aglajidae family, was also seen nearby. © Carolyn Belak
While not found within this particular survey, Chelidonura hirundinina, another member of the Aglajidae family, was also seen nearby.
© Carolyn Belak

As with any marine destination, there is always more to the ecosystem than meets the eye. Predatory attacks and fights for survival come at many scales. Although often unseen and under-considered, struggles for survival between slugs may be an important force shaping the underwater environment. However, our knowledge on the biology and ecology of these fascinating creatures lags behind the charismatic fish and invertebrates which monopolize marine research. As we continue our research on Queen Conch in the Bahamas, we look forward to taking time for explorations on various species of marine sea slugs and hope to find out more about these hidden treasures creeping across the sand.

Though we wish every day could be spent diving to the Bahamian sea floor to seek out unappreciated sea slugs, we encourage you to adventure outside your home, flip rocks and keep a keen eye peeled for other small but important and interesting denizens of your local environment.

For information about sea slugs, visit www.seaslugforum.net. You can also learn more about our research on queen conch in the Bahamas on Shedd Aquarium’s website.