Ophiocoma wendtii
The brittle star Ophiocoma wendtii inhabits coral reefs from Bermuda to Brazil. It is known for its advanced compound eyes.
Brittle stars have long, thin arms emanating from a small, disk-shaped body and are about the size of an outstretched human hand. They belong to the phylum of echinoderms, which includes sea urchins, sea cucumbers, and sea stars.
Its arms are covered with calcite crystals. In addition to functioning as an armor and giving structural support, the crystals make up its unique visual systems. They minimize spherical aberration and can detect the direction of incoming light. The lenses work by filtering and focusing light on an underlying photoreceptor system. Nerve bundles under each lens, presumed to be light-sensitive, transmit the optical information to the rest of the nervous system.
The only known animals to employ a similar visual system were the now-extinct trilobites. Phototropic chromatophores can change O. wendtii's color and regulate how much light will reach the photoreceptors.
Mekong Giant Catfish
The Mekong giant catfish, Pangasianodon gigas, is a species of catfish (order Siluriformes) in the shark catfish family (family Pangasiidae), native to the Mekong basin in Southeast Asia.
The Mekong giant catfish is perhaps the most interesting and most threatened species in the Mekong river. For this reason conservationists have chosen it as a sort of “flagship” species to promote conservation on the Mekong (Hogan et al. 2004, MGCCG, 2005). With recorded sizes of up to 10.5 ft (3.2 m) and 660 lb (300 kg), the Mekong’s giant catfish currently holds the Guinness Book of World Record’s position for the world’s largest freshwater fish (Mydans et al. 2005, Hogan et al. 2004, Hogan et al. 2007). Although research projects are currently on going, relatively little is known about this species.
Historically the fish has a natural range that reaches from the lower Mekong in Vietnam (above the tidally influenced brackish water of the river’s delta) all the way to the northern reaches of the river in the Yunnan province of China, spanning almost the entire 4,800 km length of the river (Lopez et al. 2007, Hogan et al. 2007). Due to threats which will be discussed below, this species no longer inhabits the majority of its original habitat, now believed to only exist are small, isolated populations in the middle Mekong region (Hogan et al. 2004). Fish congregate during the beginning of the rainy season and migrate up-stream to spawn (Hogan et al. 2004). They live primarily in the main channel of the river where the water depth is over 10 m (Mattson et al. 2002) while researchers, fishermen and officials have found this species in the Tonle Sap river and lake in Cambodia (also a UNESCO Biosphere reserve). In the past fishermen have reported the fish in a number of the Mekong’s tributaries, however today essentially no sightings are reported outside of the main Mekong river channel and the Tonle Sap region.
In infancy, this species feeds on zooplankton in the river and is known to be cannibalistic (Pholprasith, 1983 as cited in Mattson et al. 2002). After approximately one year, the fish becomes herbivorous, feeding on filamentous algae probably ingesting larvae and periphyton accidentally (Pookaswan, 1989 and Jensen, 1997 as cited in Mattson et al. 2002). The fish likely obtain their food from algae growing on submerged rocky surfaces, as they do not have any sort of dentition (Pholprasith, 1983 as cited in Mattson et al. 2002).
