Sea fans, gorgonians

Scientific name : Octocorallia, Alcyonacea

Sea fans are animal characterizing the deep reef environment, a dominating organism in North Sulawesi seas like in many other world's seas. The large, colourful, fan-shaped branches, bent by the current, are always a good sight for divers and under water photographers. With the term sea fan or gorgonian we indicate all the octocorals with a horny skeleton. The phylum is Cnidaria, including animals with two very basic possible growth stages, polyp or jellyfish. Sea fans are typical colonial organisms, formed by many polyps, all equal, originating from a founder polyp by asexual division. Sea fans belong to the class Anthozoa, subclass Octocorallia, order Alcyonacea (previously they where considered in an order Gorgonacea). Also the majority of the so-called soft corals belong to the same order (Alcyonacea). We prefer to keep the distinction among sea fans and soft corals, even if science has united the two orders, because this is a natural difference for divers. Sea fans are colonial animals, made up by polyps. The typical octocoral polyp has a cylindrical columnar body ending in a mouth, surrounded by 8 pinnate tentacles (with lateral expansions). The internal anatomy is very primitive, practically reduced to a large stomach, with only one opening (the mouth). The undigested food is rejected. The polyps of a colony communicate, exchanging food but also information (through a nervous network). When the tip of a branch is disturbed, the whole colony reacts with polyp contraction. The colonial skeleton is normally branching, seldom unbranched (whip gorgonians). The skeleton has a horny axis, made up mainly with a protein called gorgonin. In the living colony, the skeleton is covered with a colonial tissue (coenenchyme), in which polyps can withdraw. The coenenchyme is more or less fibrous, due to calcareous sclerites. The whole skeleton can be calcareous in few species, for example the mediterranean red coral (Corallium rubrum). The typical colony shape is a fan, with all the branches on a single plan, and the colony diameter can be 3 m and more. There are gorgonians in all environments, but they are dominating, with the maximum diversity, along the deep reef. Mostly they like shaded environments, with the exception of few species with symbiotic zooxanthellae, taking advantage from the light in shallow environments. They are passive filter feeders, the polyps' tentacles collect particles from the water. The nematocysts can paralize the motile preys. The colony has normally a fan shape, with branches on a plane perpendicular to the dominating current. The branches' network intercepts the current, break them into curls bringing back the food particles to the polyps. Most gorgonians have separate male and female colonies. In few cases eggs and sperms are released simultaneously for an external fecundation, but in most species the female catches the sperms, and breed the eggs. In all cases, a swimming larva develops from the fecunded egg, that, after a metamorphosys, generates a new founder polyp. Asexual propagation, for colony fragmentation, is uncommon among gorgonians. Junceella fragilis drops its uppermost branch tip, which attaches on the substratum and grows up as an independent colony. Confusion is possible with some other organisms. Soft corals, that are placed by the modern taxonomy in the same order (Alcyonacea), have a skeleton without proteic axis, kept in shape by the sclerites and by the internal water pressure. Some hydrozoan colonies recall a gorgonian, especially when covered with Parazoanthus sp. 1. Other Parazoanthus species (P. sp. 2, P. sp. 4) can grow on dead gorgonians skeleton. The difference is in the number of tentacles: the Parazoanthus polyp has many tentacles, the gorgonian polyp 8 pinnate tentacles. Whip gorgonians can be confused with wire corals (black corals). Again the tentacles are different (see Junceella).

	Description	Author	Place	Size
	Octocorallia, Alcyonacea	DANIELE IOP	Indonesia, Bunaken	1369 kB