Heteropsammia are a unique group of stony corals which do not attach to the substrate. These free living corals have a flat base, and live in calm sandy areas, usually in the depths outside of the coral reefs. With many of the species of Heteropsammia, the larvae settle down onto the shell of very small snails, which are then engulfed as the coral grows. They also have an obligate commensal relationship with a small worm, known as a sipunculid, which protrudes from a small hole in the bottom of the coral and helps the coral right itself, move around, and prevents it from being buried in the sand. Because this worm can also move the coral around, the corals are known in the aquarium trade as “Walking Dendros.”
Almost the entire upper portion of the 2.5 cm wide coral is a mouth, meaning that it is able to consume much larger prey than most other hard coral species. Being a coral which lives deep, this is an advantage as there is less light available at depth to power photosynthesis for the coral’s symbiotic algae. These corals thus rely more on predation than the functionally autotrophic corals of the shallow oceans. But what exactly do they eat is a question that is still being answered. Recently, on the island of Koh Tao in the Gulf of Thailand, the corals have been observed eating salps, an observation that was recently published in the Journal of Marine Biodiversity.
Salps are a type of pelagic tunicate that resemble jellyfish, but are actually more closely related to us, as they are in the phylum Chordata along with humans. These tunicates bloom in the Gulf of Thailand every year just before the new moon in April-August, washing up onto the beach in the millions. In some cases they can cover the beach in 2-3 cm of rotting slime, a problem for beach-goers and local businesses, as well as the shallow reef environments. The decomposition of these salps causes a high amount of nutrients and bacteria in the water, which can contribute to coral disease and the overgrowth of hard corals by macro-algae and cyanobacterial mats.
Each year, the coastal seas around the word are becoming more loaded with nutrients, and combined the removal of important fish species, means that more and more of the larval salps are growing to adulthood. In order to combat this problem for the reefs and beaches, the salps need to be consumed before rotting. On surveys around the island of Koh Tao during the salp blooms, no fish were seen feeding on the living or dead adult salps. Parrotfish and Damsel fishes occasionally tried, but always spit them back out. Interestingly, some of the animals most threatened by the salps blooms are also the ones feeding on them, namely the mushroom corals and Heterosammia corals.
Almost all species of mushroom corals feed on the salps, using their large mouths to ingest the 3-4 cm transparent tunicates. It was observed that mushroom corals living deeper (below 8m) were more likely to feed on the salps than those in the shallow reefs. During these times, almost all of the Heteropsammia corals were feeding on the salps.
As the effects of human activities on land, combined with those of climate change, are affecting the nutrient and chemical cycling of our oceans problems such as these are becoming more pronounced and frequent. Everyday humans are removing the important parts of the ecosystem (in this case planktivores fishes) and putting back only bad things (such as pollution, sedimentation, and excess nutrients). The situation with the salps is analogous to the creation of ocean dead zones, which occur following die-offs of jellyfish populations blooms. The dead zones are areas where all of the oxygen has been removed from the water by aerobic bacteria feeding on decomposing organic matter, and can be as large as some European countries and persist for years. At least in this battle we and the reefs have a humble ally, the abundant mushroom corals and the often overlooked Heteropsammia corals.