Hard corals are generally thought of as autotrophs, or organisms which produce most of their own metabolic energy and contribute to the base of the food chain in the ecosystem. Although most reef-building corals do feed on plankton, they meet 75-95% of their energy demands by harvesting symbiotic micro-algae within their tissues, and are thus getting their sugars and carbohydrates indirectly through photosynthesis. However, this view of corals is currently in debate, and it looks like a more diversified diet could be utilized by some corals to meet their daily energy demands. This has been the study of Conservation Diver Instructor and Board Member, Rahul Mehrotra.
Investigating what corals can eat
Initially, Rahul showed back in 2015 that Heteropsammia corals used their large mouth size to opportunistically consume salps during seasonal blooms. This was observed In Situ on several occasions in the ‘muck’ areas around Koh Tao, and documented in the journal Marine Biodiversity. By chance, a student of the New Heaven Reef Conservation Program, Joel Rohrer observed a mushroom coral ingesting a sea slug during a coral reef survey, and alerted it to Rahul’s attention. From that single observation, investigations began into the question, “Are reef building corals actively feeding on sea slugs for additional energy?” A question which would take 4 years to investigate.
Experiments were devised as a series of both in situ and ex-situ trials to test his hypothesis, which was that corals take advantage of their proximity with Sacoglossan sea slugs and their large gape to facilitate heterotrophy. The experiment looked at several factors, the size of the coral, the size and species of the sea slug, and the likelihood of ingestion. He used aquarium feeding trials to test whether or not it was even possible for the mushroom corals to capture, transport, and ingest the slugs, and looked at the rate of rejection based on slug size and species. Following those trials, he also performed field tests to ensure that this adaptation was actually going on in the wild populations.
The study’s conclusions
Based on 240 trials, it was concluded that prey species and prey size are the greatest factors in successful predation of sea slugs by corals, rather than the type or size of the corals themselves. Of the six species of prey slugs tested, consumption rates were highest for species of the genera Costasiella and Elysia, with significantly less success when attempting to ingest those of the Plakobranchus genus. Additionally, when these sea slugs were rejected after consumption by the corals, the slugs were covered with a mucus that is thought to be a defensive mechanism by the corals to prevent damage caused by the slug’s defensive measures.
What this means
Rahul’s work has helped to expand the way we think about coral feeding behaviors, as well as their place in the trophic structure of reefs. We now know that they feed on large plankton (salps and small jellyfish) in addition to the nearly microscopic prey they have always been known to. Furthermore, we now see that they are actively feeding on benthic organisms, not only those floating in the water column. This may be an adaptation to increase the amount of energy they receive from heterotrophy to allow them less reliance on their symbiotic algae, or as a way to gain beneficial micro- and macro-nutrients that could be essential for their growth and development. Additionally, the data adds to the incredibly sparse understanding on predators of sea slugs (which are all toxic) and overall ecology in tropical reef habitats. As these new observations are investigated further, we are sure they will yield some more surprising and exciting information which will continue to modify our views of this very important class of reef organisms.