of
Tubipora musica. Octocorals resemble the
stony corals in general appearance and the size of their polyps, but lack the distinctive stony skeleton. Also unlike the stony corals, each polyp has only eight tentacles, each of which is feather-like in shape, with numerous side branches, or
pinnules. Octocorals are colonial organisms, with numerous tiny polyps embedded in a soft matrix that forms the visible structure of the colony. The matrix is composed of
mesogleal tissue, lined by a continuous
epidermis and perforated by numerous tiny channels. The channels interconnect the gastrovascular cavities of the polyps, allowing water and nutrients to flow freely between all the members of the colony. The skeletal material, called
coenenchyme, is composed of living tissue secreted by numerous wandering
amoebocytes. Although it is generally soft, in many species it is reinforced with calcareous or horny material. The polyp is largely embedded within the colonial skeleton, with only the uppermost surface, including the tentacles and mouth, projecting about the surface. The mouth is slit-like, with a single
ciliated groove, or
siphonoglyph, at one side to help control water flow. It opens into a tubular
pharynx that projects down into a gastrovascular cavity that occupies the hollow interior. The pharynx is surrounded by eight radial partitions, or
mesenteries, that divide the upper part of the gastrovascular cavity into chambers, one of which connects to the hollow space inside each tentacle. The
gonads are located near the base of each mesentery. and have shown high recruitment rates post-die-off events caused by
El Niño events.
Bioluminescence is found in 32 genera, a trait estimated to have evolved 540 million years ago, the earliest timing of emergence of bioluminescence in the marine environment. ==Phylogeny==