Reef Check Spotlight: Mystery in the Red Sea – Circular feeding scars observed on fire coral

Feeding scars on Millepora dichotoma at Marsa Shagra, Egypt, Red Sea	Leopard blenny Exallias brevis guards his nest, Hawaii, Central Pacific
E. brevis with nest inside M. dichotoma, Red Sea	E. brevis deep inside the Millepora thicket at Marsa Shagra, Egypt, Red Sea
Feeding scars on Porites caused by singular bannerfish Heniochus singularis	Feeding scar on Porites caused by E. brevis
Unidentified feeding scars on Millepora complanata at Conch Reef, Florida Keys	Unidentified feeding scars on M. complanata at Puerto Morelos reef, Mexico, Caribbean Sea
Unidentified feeding scars on M. complanata at Puerto Morelos reef, Mexico, Caribbean Sea

By Reef Check EcoDiver Course Director Stephan Moldzio

During our Reef Check (RC) surveys in February 2012 at Marsa Shagra, Red Sea, Egypt, we observed some strange circular blotches on a fire coral Millepora dichotoma. We sent these pictures to RC and made an inquiry to several experts with no conclusive outcome.

Initially we took the following explanations into consideration:
– Feeding scars by Coralliophila snails
– Feeding scars by juvenile Crown of Thorns seastars (COTS)
– Some kind of coral disease
– Anomaly of Millepora

Coralliophila is frequently observed on Porites corals, where it produces slight feeding scars, whereas the tissue remains mainly alive and intact. But Coralliophila feeds exclusively on Porites and is most unlikely to feed on any other corals, including Millepora. Another point against Coralliophila is that they were obviously absent on Millepora, whereas they are usually observed in close vicinity due to their small feeding territory.

Juvenile COTS are ruled out, because COTS tend to create paths instead of single blotches. We actually have observed a young COTS feeding scar on a Favia coral during our surveys; the tissue was completely removed – in contrast to those circular blotches on Millepora that appeared a bit bleached but not grazed down to the bare skeleton. Later, we found out from Prof. Rupert Ormond that COTS actively avoid Millepora because they are stung by it!

Some kind of coral disease might also have been the cause, but an infection with ciliates or bacteria tends to spread over the whole colony and does not form such equal sized and sharp edged blotches.

At that time, we preferred the explanation of an anomaly of Millepora. I observed in my aquarium that Millepora often forms “tissue bubbles” at branches as well as “bleached” small areas; but not such perfectly round patches.

So we uploaded the pictures to RC Europe´s homepage and posted an inquiry through the “NOAA Coral List Server,” from which we received more than 20 emails with possible explanations. These ranged from feeding scars by COTS, Coralliophila, Drupella, parrotfish, damselfish, butterflyfish, filefish, blennies (Exallias brevis), corallivorous flatworms, White Pox disease, a secondary infection of a feeding scar and a special form of bleaching.

Zvuloni et al. (2011) described exactly the same phenomenon in Millepora as “Multifocal Bleaching”, but they didn´t determine the reason for these patches. They suggested a form of bleaching, a “new syndrome in Millepora,” possibly caused by a microbial infection.

Finally, Dr. Bruce Carlson in Hawaii solved this mystery by uploading a video showing the feeding behaviour of the leopard blenny, Exallias brevis, and describing the exact mechanism of how it forms these remarkable feeding scars: Exallias brevis is an obligate corallivore that scrapes off the tissue with its upper jaw while anchoring the mouth with its lower jaw, producing sharp edged, circular feeding scars! Indeed, all of those blotches, even those published in Zvuloni´s paper, showed a faint white line under the circular feeding bite! “That line represents the lower jaw that anchors the mouth while the upper jaw sweeps over the coral and removes the tissue,” Carlson stated. He also mentioned that on the picture “some of the older scars have regenerated a bit and a newer scar overlaps the older scar. That can only be produced by feeding, i.e., it rules out bleaching or disease.” Carlson is submitting a paper to Marine Ecology Progress Series with a complete description of the feeding behavior of Exalllias brevis.

Carlson (1992) found out that only the superficial coenosarc tissue was removed while the polyps remained mostly intact within calyces and that these marks regenerated within 50 days. He observed E. brevis feeding exclusively on living corals, at rates of 13.9 and 28.4 bites per hour for males and females, respectively. He sized the circular feeding bites on Porites lobata to 2.04 ± 0.42 cm².

Dr. Carlson has observed E. brevis feeding on Millepora as well at Enewetak Atoll in the Pacific. Dr. Jürgen Herler stated that E. brevis removes coral tissue, at least for breeding, and he sent a photo with the fish and its egg patches on a Millepora in the Red Sea. Finally, Christian von Mach confirmed that he observed E. brevis feeding on Millepora in the Gulf of Aqaba/northern Red Sea.

In the following days we had a lively discussion upon this matter….
Why have these circular patches not been reported until now?
Why has it been such a problem (for many experts) to determine these patches as Exallias brevis feeding scars?

We have conducted RC surveys in that area for four years, but this was the first year we observed these circular patches. If Exallias brevis is not very abundant, divers would have seen just a minor density of feeding scars. Thus, this phenomenon may not have been obvious, being perceived only by a watchful observer, who knows about coral diseases, coral bleaching and feeding scars. Additionally, E. brevis hides deep within Millepora thickets, so any observer would have to come really close and rest for a while to watch E. brevis feeding. Such an observer would have to be quite persistent to get an answer about whatever is responsible for these blotches. So we think that this phenomenon has simply not been recognized and/or been acknowledged to be published so far.

Furthermore, most divers and snorkelers may avoid coming too close to Millepora, because it´s also called “fire coral”. Also, the preferred habitat of Millepora and E. brevis is around the reef crest at shallow depth, whereas most divers are going to 10-30m depth.

E. brevis may show some ecological differences within its range from the Red Sea, Madagascar and India, to Australia and Hawaii. In the Egyptian Red Sea we´ve observed E. brevis always within the Millepora thickets at shallow depth around the reef crest. We didn´t observe it on any other corals, e.g. Porites, where it was mainly observed by Dr. Carlson in Hawaii. 

It´s quite possible that in some cases, disease may in fact simply be some kind of feeding scar.

Stories like this one happen when thousands of Reef Checkers put their eyes on the reef, with a focus on all kinds of human impacts, coral damage, recently killed corals, bleaching, coral diseases, and COTS feeding scars.

But one part of this mystery remains: similar scars on Millepora complanata have been observed by scientists from Bermuda, the Florida Keys, the Mexican Caribbean and Pernambuco, Brazil. So far, no one has identified the fish that creates these spots. Exallias brevis does not occur in the Caribbean but it has close relatives there. Charles Delbeek mentioned that Ophioblennius atlanticus has a very similar mouth structure to E. brevis and lives in close proximity to stands of Caribbean fire coral.

There are many secrets on the reef waiting to be uncovered and we are still searching for a corallivore equivalent of E. brevis in the Caribbean.