Thursday, October 11, 2012

[Coral • 2012] Leptoseris troglodyta • Forever in the dark: the cave-dwelling azooxanthellate reef coral (Scleractinia, Agariciidae) from Indo-Pacific

The coral species Leptoseris troglodyta sp. n. (Scleractinia, Agariciidae) is described as new to science. It is the first known azooxanthellate shallow-water agariciid and is recorded from the ceilings of caves at 5-35 m depth in West Pacific coral reefs. The corals have monocentric cup-shaped calices. They may become colonial through extramural budding from the basal coenosteum, which may cause adjacent calices to fuse. The size, shape and habitat of Leptoseris troglodyta are unique compared to other Leptoseris species, many of which have been recorded from mesophotic depths. The absence of zooxanthellae indicates that it may survive well in darkness, but endolithic algae in some corals indicate that they may be able to get some light. The presence of menianes on the septal sides, which may help to absorb light at greater depths in zooxanthellate corals, have no obvious adaptive relevance in the new species and could have been inherited from ancestral species that perhaps were zooxanthellate. The new species may be azooxanthellate as derived through the loss of zooxanthellae, which would be a reversal in Leptoseris phylogeny.

Keywords: Cavernicolous, colonial, dwarfism, extramural budding, monocentric, skiophilous, solitary, troglobiotic

Diagnosis. Corals cave-dwelling, azooxanthellate. Calices small, cup-shaped, monocentric or fused, forming buds at basal coenosteum.

Etymology. The epithet troglodyta (noun) means cave dweller in Latin, derived from ancient Greek for “one who dwells in holes”.

Distribution. Records are from coral reefs, usually in areas with limestone outcrops: Indonesia (East Kalimantan, North Sulawesi, Southeast Sulawesi), the central Philippines (Cebu, Bohol), Palau, eastern Papua New Guinea, and the Marianas (Guam) (Figure 9).

Figure 1. Living specimens of Leptoseris troglodyta sp. n. 
a Philippines, Cebu Strait, W of Bohol, NW of Cabilao Island, 10–30 m depth (1999) 
b Indonesia, NE Kalimantan, Berau Islands, S of Derawan Island, 7–10 m depth (2003).

A new cave-dwelling reef coral discovered in the Indo-Pacific
Coral named Leptoseris troglodyta sheds light on coral-algal symbiosis

Coral specialist Dr. Bert W. Hoeksema of Naturalis Biodiversity Center in Leiden, The Netherlands, recently published the description of a new coral species that lives on the ceilings of caves in Indo-Pacific coral reefs. It differs from its closest relatives by its small polyp size and by the absence of symbiotic algae, so-called zooxanthellae. Its distribution range overlaps with the Coral Triangle, an area that is famous for its high marine species richness. Marine zoologists of Naturalis visit this area frequently to explore its marine biodiversity.

Reef corals in shallow tropical seas normally need the symbiotic algae for their survival and growth. Without these algae, many coral reefs would not exist. During periods of elevated seawater temperature, most reef corals lose their algae, which is visible as a dramatic whitening of the reefs, a coral disease known as bleaching.

Most reef corals generally do not occur over 40 m depth, a twilight zone where sunlight is not bright anymore, but some species of the genus Leptoseris are exceptional and may even occur much deeper. At greater depths, seawater is generally colder and corals here may be less susceptible to bleaching than those at shallower depths. Despite the lack of zooxanthellae and its small size, the skeleton structures of the new species indicate that it is closely related to these Leptoseris corals, although it has not been found deeper than 35 m so far.

The species is named Leptoseris troglodyta. The word troglodyta is derived from ancient Greek and means "one who dwells in holes", a cave dweller. The discovery sheds new light on the relation of reef corals with symbiotic algae. The new species has adapted to a life without them. Consequently, it may not grow fast, which would be convenient because space is limited on cave ceilings. The species description is published in the open access journal ZooKeys.

Figure 2. Living specimens of Leptoseris troglodyta sp. n.
a Philippines, Cebu Strait, W of Bohol, NW of Cabilao Island, 10–30 m depth (1999)
b Indonesia, Tukang Besi Islands (Wakatobi), Binongko, 20 m depth (2003)
c Indonesia, North Sulawesi, S of Bunaken Island, 17 m depth (2008; photo B.T. Reijnen)
d Palau, W of Ulong Island (Rattakadokoru Island), W off barrier reef, 32 m depth (2002)
e Papua New Guinea, Misima Island, 6–10 m depth (1998; photo G. Paulay)
f Guam, Blue Hole, 35 m depth (2000; photo G. Paulay).

Figure 9. Distribution map of Leptoseris troglodyta sp. n. showing records at
(1) Palau, (2) East Kalimantan, (3) North Sulawesi, (4) Wakatobi, (5) Bohol, (6) Guam, (7), eastern Papua New Guinea.

Hoeksema, B.W. 2012. Forever in the dark: the cave-dwelling azooxanthellate reef coral Leptoseris troglodyta sp. n. (Scleractinia, Agariciidae). ZooKeys. 228: 21. doi: 10.3897/zookeys.228.3798