Who can say that they have ever observed stony corals getting in a sort of "hunting fever?" The role of corals as an important predator in the reef is mostly unknown. Food acquisition at night remains mostly obscured to divers, and the naked eye of the beholder, due to the microscopic size of the coral’s prey and the light-sensitive (or photophobic) response of the polyps to disturbance from the divers’ torch.
Quite commonly known is the mutual symbiosis between reef-building corals and single-cell algae (zooxanthellae) thriving in the tissues of the polyps. These algae are responsible for the colouration of the coral; and supply carbohydrates (organic carbon compounds) to the corals in exchange for nitrogen and phosphorus. Also, the zooxanthellae facilitate limestone precipitation of the coral by removing CO2 from the polyps’ tissues. Most corals are virtually inactive during daytime displaying retracted polyps. However they have an army of unicellular algae working for them all sunlit daytime long and performing photosynthesis, although it would seem inconspicuously.
This is the general appearance of stony corals that divers and snorkellers are well familiar with. Some observant divers may also have studied corals during their active feeding phase at night: A lawn of polyps moving their heavily armed tentacles with the water, each of them furnished with myriads of stinging cells. Thus, at night the corals have little in common with their appearance during daytime and pose a deadly threat for plankton organisms such as crustacean, mollusc or fish larvae. Indeed all these potential prey organisms run the danger of being pierced and narcotized, or stuck at a touch of the hostile tentacles. The tentacles transfer the captured food directly into the “stomach” (gastric cavity), where it is decomposed except for the indigestible parts (skeletons, shells) and where nutrients are taken up (reabsorbed) by the “stomach wall”. After the meal, the polyps egest the indigestible remains through the mouth opening. In this fashion, innumerable populations of corals are filtering tons of plankton out of the water night by night, while we are basically unaware of it. On the next morning, usually nothing is left over to provide evidence of the precedent nocturnal feast.
In one instance, however, a different situation was encountered: In a particular morning in spring we saw a lot of coral colonies seemingly covered with “needles”, resembling a hedgehog. What had happened? An unusual nocturnal banquet of the coral community appeared to be responsible for the traces still visible on the next day.
The event had obviously been triggered by a mass occurrence of tiny wing snails from the plankton; pteropods, in scientific terminology. This group of snails comprises species with various forms of shells, or without any shell, and with a foot reshaped to some sort of wing.
What we observed was that the majority of polyps still had the elongated shells stuck in their mouths with the thicker end of the shells inside. Perhaps the polyps had not completed their meals yet or they encountered some problems in finally ridding themselves of the bulky snail remainders over 1cm in length.
Most of the observed coral colonies riddled with “needles” were representatives of the honeycomb and brain corals (family Faviidae). These corals mostly exhibit larger polyps with larger mouth apertures. So just in case you happen to observe a coral during your dive that looks a bit like a hedgehog, you will know now what it is all about!
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