Twilight’s home: A new cave-dwelling coral from the western Pacific

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.

Living specimens of Leptoseris troglodyta sp. n. a) Philippines, Cebu Strait, W of Bohol, NW of Cabilao Island, 10–30 m depth (7 November 1999); b) Indonesia, NE Kalimantan, Berau Islands, S of Derawan Island, 7–10 m depth (4 October 2003). (Image from Hoeksema, 2012)

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.

 

References

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

A trick of the tail: how snails escape snail-eating snakes

There’s not much chance of a land snail outrunning a predator, so a slow-moving gastropod must rely on other measures to stay out of trouble. The shell is an effective defence, but it’s not perfect. Some predators can crush them. Others, such as the song thrush and pitta, smash them open on rocks. Carabid beetles and firefly larvae are small enough to breach the defences by slipping through the only gap in the armour — the shell aperture. Even the best protection has a weak spot.

Not only do snails in some parts of Asia have to survive these usual suspects — brute force crushers and smashers and those sneaky nibblers — but they also have to deal with specialist predators: snail-eating snakes. Iwasakii’s snail-eater, Pareas iwasakii, has asymmetrical jaws that are adapted to extract snails from their shells. (Click here to to watch a video of the snake in action.)

Researcher Masaki Hoso of the Naturalis Biodiversity Centre in the Netherlands has discovered that Satsuma caliginosa, a camaenid land snail from Japan, has evolved counter-measures to escape its serpentine hunter.

The range of Satsuma caliginosa partly overlaps that of Iwasaki’s snail-eater. Where the two are separate, the interior of the Satsuma shell aperture is smooth, but where predator and prey occur together, the shell aperture is lined with several low barriers that narrow the opening and prevent the snake grabbing hold of the snail and dragging it out. Unfortunately for Satsuma, these barriers only develop in adults.

Masaki Hoso found that Satsuma employs an equally effective but more drastic method of staying alive until the shell barriers develop. When snagged by a snake, the snail sheds its tail. (Really, the hind part of the foot.) Result: predator gets a bite-sized morsel of prey and prey gets to live another day.

(a–c) Foot regeneration of S. caliginosa in the wild. (a) S. caliginosa caliginosa with an intact, (b) a regenerating and (c) a regenerated foot. (d) Proportion of S. caliginosa with a regenerating or regenerated foot in the wild. (Image from Hoso, 2012)

But this process of autotomy is costly. Regrowing the shed tail requires a lot of energy and while that is taking place, resources are diverted from shell growth. For immature snails, this involves a trade off between defence mechanisms. Dropping the tail is a life-saver, but delays the development of apertural barriers. Still, dropping the tail means that the snail can survive long enough to grow those apertural barriers.

Reference

Hoso, M. (2012) Cost of autotomy drives ontogenetic switching in antipredator mechanisms under developmental constraints in a land snail. Proceedings of the Royal Society B. doi: 10.1098/rspb.2012.1943