What can coral identification reveal about coral reefs?
By Deepika Sharma and Adyasha Nayak
Coral reefs are the celebrities of the oceans: exceptionally photogenic, colourful and always the talk of the town when it comes to marine biodiversity. They also happen to be infamous models to show the negative impacts of climate change on the oceans. These are some of the aspects of coral reefs which are in the limelight. But there is so much more to the coral reef universe that we recently discovered through coral identification. Put simply, it involves identifying the taxonomic groups of corals. Initially the first question that crossed our minds was — what value does coral identification hold?
We started off by going back to the basics.
A web search on coral reefs showed us a variety of inanimate structures. These are the colonies formed by corals which are invertebrate animals i.e. they have no spine (no puns intended). Originating 500 million years ago, corals have been on the planet long before humans arrived. In all these years, corals have evolved into various taxonomic groups with a variety of characteristic traits. These traits are important determinants of the overall health and resilience of reefs. For instance, colonies with massive dome structures tend to be slow-growing and ancient. They can sustain strong waves and temperature variability. In contrast, branching colonies are comparatively fast-growing and therefore are also more susceptible to external stressors.
Despite being confined to our homes due to the lockdown, we were able to learn and identify corals from the pictures of the underwater survey conducted by the WCS-India Marine Program in December 2019 at Angria Bank, a submerged plateau located off the west coast of Maharashtra. As a part of the survey, researchers took close-up pictures of individual coral genus at Angria Bank. These pictures were then catalogued systematically into different folders for analysis and identification.
Vardhan Patankar, Head – Marine Program, WCS-India, who has been identifying corals for years then took a workshop with the team members on how to go about with coral identification. It turned out to be quite the exercise though as we referred to three massive volumes of Corals of the World, which was like lifting weights! Additionally, we also used the Coral Finder, a slim step-wise visual guide of coral identification as well as a navigation tool for the three volumes of the book. It was overwhelming to learn about dozens of traits in these guides, but then with a bit of guidance we got familiar with common terminologies used in coral identification.
The main feature for identifying corals are corallites. These are small units of calcium carbonate which make up the coral colonies. Wrapped within corallites are coral polyps, which are sac-like- structures, composed of very soft tissue, responsible for all the vital life processes from food procurement to reproduction. Each corallite has tiny embellishments on it namely- septa and costae. Septa is present on the inner side of the corallite, while costae on the outer side. A wall separates septa and costae in certain corals. Whereas in the absence of a wall septa and costae merge together.
Corallites were the most difficult features to deal with. They are starkly different in some corals while extremely puzzling in others. For instance, we could easily identify Galaxea colonies by looking at their spikey corallites. Pachyseris colonies are also easy to identify because of their characteristically parallel ridges with thick surface texture. It is also why Pachyseris have earned the moniker ‘elephant skin coral’.
On the other hand, colonies of Favites and Goniastrea are like identical twins. Corallites in both the colonies are of the same sizes and also have joint walls. The only difference is that Goinastrea has tiny skeletal plates, called paliform lobes, circling inside the corallite wall. Even though this is absent in Favites, yet it was difficult to make the distinction with Goinastrea. It took us multiple iterations to train our eyes to spot these features and distinguish corals.
Coral identification becomes most challenging when corals are feeding. Corals predominantly indulge in nocturnal binge eating by extending their tentacles out from the polyps to catch free-floating zooplanktons from the surroundings. Extended tentacles obscure the corallites making it difficult to identify corals. But Goniopora colonies are an exception, since their tentacles are permanently extended and therefore, are its characteristic features.
While identifying corals, we also checked for the colour of coral colonies, which indirectly reflects their health. Corals get their colours from algae nestled within the coral polyps. Algae are like the chefs of the coral colonies, which carry out photosynthesis to produce essential nutrients. Healthy coral colonies, therefore, display bright and vivid colours. However, if the ocean temperature rises, even by 10 C, the algae disassociate from coral colonies leaving them bleached. As opposed to a healthy coral colony, a bleached colony is white because only the calcium carbonate skeleton structure remains without live coral polyps.
For all the corals that we examined, the majority of corals showed no signs of bleaching. Moreover, half of the coral population here belongs to the resilient taxonomic genera including Favites and Porites. Both these corals form dome structures which makes them resilient to temperature variability. The taxonomic genera also, therefore, have important cues in determining the health of the reefs.
However, identification of corals is just one part of the jigsaw puzzle. Coral reefs also have a constant entourage of fishes and diversity of invertebrates, which find food and home here. In return, coral colonies get the benefit of regular housekeeping, especially from fishes, to get rid of any excessive algal growth that might take over the corals and cause a phase shift. That meant many more virtual tours in the ocean, and weight lifting of massive field guides to identify reef-associated fishes and invertebrates, which will complete the jigsaw puzzle of marine biodiversity at Angria Bank.
This entry was posted on Friday, July 31st, 2020 at 3:44 PM
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