Leptoseris corals love the gloom. They grow in shaded nooks and crevices and on the sea floor at depths where many other corals do not thrive. They are particularly common in mesophotic reefs, where the light is muted and all but a few wavelengths are filtered out. Several species live at more than 100 m and one, Leptoseris hawaiiensis, has been recorded at 160 m at Johnston Atoll in the Pacific Ocean.
This low-light lifestyle makes it awkward for the endosymbiotic zooxanthellae that live within the coral’s tissue. Zooxanthellae are photosynthetic, requiring a hit of light to drive the chemical reactions that build carbohydrates. Down in the shadows and the perpetual twilight of the mesophotic zone there’s not a lot of illumination to go around. But they make do. Elements of the Leptoseris skeleton — especially the ledge-like menianes that fit in close with the polyp’s tissue — are thought to play a part in snaffling light and making it available to the endosymbiotic zooxanthellae. Survival is a team effort.
Until recently, all species were known to be zooxanthellate, but a newly described Leptoseris from the western Pacific breaks that pattern.
Leptoseris troglodyta Hoeksema, 2012, lives in marine caves in water between 5 and 35 m deep and has been recorded from sites in eastern Indonesia, the central Philippine Islands, Palau, Guam and the Louisiade Archipelago of Papua New Guinea.
Although this species lacks zooxanthellae, it still possesses the menianes that help to harvest light in other Leptoseris species. This combination raises the question of which came first in Leptoseris — zooxanthellae or no zooxanthellae?
It might be tempting to assume that L. troglodyta had zooxanthellate antecedents and has ditched the endosymbionts as part of its adaptation to cave-dwelling, leaving the menianes as evolutionary hangers on. But research on Dactylotrochus cervicornis, another azooxanthellate and menianes-possessing genus in the same family (Agariciidae) suggests that things might not necessarily have gone that way. So the question remains: Did the zooxanthellae precede the menianes or did the menianes precede the zooxanthellae? This, as Dr Hoeksema says, is ‘a “chicken or the egg” causality dilemma’. Further studies will shed light (ahem) on the conundrum.
Hoeksema, BW. (2012) Forever in the dark: the cave-dwelling azooxanthellate reef coral Leptoseris troglodyta sp. n. (Scleractinia, Agariciidae). ZooKeys 228: 21–37, doi: 10.3897/zookeys.228.3798
Kitahara, MV., Stolarski, J., Miller, DJ., Benzoni, F., Stake, J. & Cairns, SD. (2012) The first modern solitary Agariciidae (Anthozoa, Scleractinia) revealed by molecular and microstructural analysis. Invertebrate Systematics 26: 303–315. doi: 10.1071/IS11053