Credit: NERC CHESSO project
Relicanthus daphnaea: Incerti ordinis A recent study of sea anemone genetics revealed a surprising finding—a species formerly included amongst the “swimming anemones” was in fact a completely different order of hexacoral. This large colorful species is known only from hydrothermal vents in the ocean depths. Calling this species “large” is a bit of an understatement, as the body can reach up to a meter across, with tentacles up to two meter long. This is in fact the largest polyp known to science! Its former placement amongst the “swimming anemones” is indicative of a high degree of morphological convergence, particularly amongst the nematocysts. But the incongruities in body size, habitat and genetics strongly indicate this species occupies its own unique branch of the hexacoral tree of life. More work remains before the issue is settled and a complete classification is in place. Button Polyps: Order Zoantharia
The unusual dimorphic mesenteries of Zoantharia, & the “yellow polyp” Terrazoanthus sp.. Credit: 1911 Encyclopaedia Britannica & BeakerBob
Sister to the bizarre Relicanthus is a familiar group of small, colonial polyps known variously as button polyps or zoas. Several important characters help define this group: 1) the incorporation of sand/sediment into the tissue to add support (note that this is absent from the common aquarium genus Zoanthus) 2) two rows of tentacles encircling the oral disc 3) a unique arrangement of dimorphic mesentery pairs. The unusual morphology and growth of the mesenteries has been suggested as a possible derivation from the serial septal growth seen in the extinct Rugosa.
The solitary Sphenopus, opened and closed. Credit: Reimer et al 2012
While the vast majority of species are colonial, remaining interconnected either with stolons or embedded within a common mat, there is one group whose species remain solitary throughout their lives. The three species of Sphenopus are found in sandy areas throughout the warm waters of the Indo-Pacific. These large polyps (up to 3.5 cm across) are azooxanthellate and presumably micropredaceous suspension feeders. Molecular study indicates this bizarre group is actually derived from within the colonial genus Palythoa. Sadly, I have yet to hear of these strange polyps being exported for aquarists. While Sphenopus may not be as chromatically brilliant as your typical zoa, this is a fascinating organism—one which deserves a place in the aquarium hobby! Honeycomb Corals: Order Tabulata †
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