Styx: the murky, dismal and dark

Pogoneleotris heterolepis (Günther, 1869)

Possessing a head which appears to be eyeless, this species conjures visions of the underworld, which is not far from the truth. In the Melanau language of Sarawak, this fish is known as “Ilong mapak” or blind fish. This name alludes to the eyes which are greatly reduced and barely visible in photographs of this species. This appears to be the result of its habitat.

This species inhabits soft muddy-silty bottoms of rivers close enough to the sea to be affected by tides. The waters can be extremely turbid and fast flowing. The eyes of this species have become reduced as they would putatively not confer any advantage in this environment.

The natural history of this species is also rather murky and until recently it was only known from a handful of long-preserved specimen. In a paper published in August 2018, researchers from Malaysia and Singapore made observations on a series of specimens and were able to feature photographs of this species soon after they had been caught. This species is not found outside of the ASEAN region and is only known from Sarawak in Malaysia.

Fangtooth
Anoplogaster cornuta (Valenciennes, 1833)

This species exemplifies the dark and gloomy deep-sea world, as well as our knowledge of this realm (or rather, lack thereof). Because so little was known about this species and partly because they look so different, the adults (in 1883) and the juveniles (in 1833) were thought to be distinct species. Adults have a fearsome appearance with huge teeth giving rise to its common name. The juvenile looks like a fat version of ‘Nemo’ painted light brown.

It was not until 1955 that scientists realised that these two ‘species’ were just different life stages of the same fish! This species is found in deep water worldwide from the tropical to temperate latitudes. In the ASEAN region, it is found in deep water off Cambodia, Indonesia, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam.

Christmaplax mirabilis Naruse & Ng, 2014

This species of crab inhabits the submarine caves of Christmas Island in the Indian Ocean. Though politically not part of ASEAN today, this island shares biological affinities with Southeast Asia. It is also geographically very close to the ASEAN region. As with other cave-dwellers, the eyes of this crab have become reduced and are not even visible in the dorsal view!

The body also appears to be very lightly pigmented. It was discovered and named by scientists from Japan and Singapore who have been conducting expeditions to Christmas Island. The species name “mirabilis” means miraculous in Latin. Because of its distinctness, a new genus and new family were created for it. This species is only known from Christmas Island, Indian Ocean.

Sloan’s Viperfish
Chauliodus sloani Bloch & Schneider, 1801

With its large teeth, big eyes and dark body, this species is positively stygian. When its jaws are closed, the teeth form a cage that trap smaller prey. For larger prey, its jaws can open to a 90-degree angle to swallow prey up to two-thirds its own size! It uses a dorsal fin ray tipped with bioluminescence to lure prey close to its mouth. This species is found in deep water between 200 and 4,700 metres worldwide. Within the ASEAN region, it is found in Indonesia, Malaysia, Myanmar, the Philippines, Thailand, and Vietnam.

Tioman Cave Loach
Speonectes tiomanensis (Kottelat, 1990)

Although it is troglodyte that spends its life in the murky world of caves, this species is not without eyes or body pigment. The eyes are only somewhat reduced and the body and fins are uniformly pink. It is likely to be the only exclusively cave-dwelling fish known in Peninsular Malaysia. It lives on the bottom of streams in caves and scientists think that it may feed on the droppings of swiftlets that roost and nest in the cave.