Spider Crabs, Majidae, are a distinctive group of Crabs with carapaces that are longer than they are wide and come to a point at the front. They typically have extremely elongate legs for Crabs, and tend to be covered in bristles, which are often covered in Algae, providing a form of camouflage. However, members of the Subfamily Planoterginae differ from other Spider Crabs in having extremely reduced legs. These are short in members of the American genus Hemus, and almost absent in Planotergum mirabile, the species that gives the group its name. Planotergum mirabile is found living on the north and west coasts of Australia, as well as on the coasts of Sumatra, Java, and Vietnam. Its lifestyle is poorly understood, but from its flattened morphology and, short, hooked legs it is thought to live attached to rocks.
In a paper published in the journal Zoological Studies on 31 July 2019, Ivan Marin of the A.N. Severtzov Institute of Ecology and Evolution of the Russian Academy of Sciences, and the Biological Department at Altai State University, Vassily Spiridonov of the Shirshov Institute of Oceanology of the Russian Academy of Sciences, and Peter Ng of the Lee Kong Chian Natural History Museum at the National University of Singapore, describe a new species of Planotergum from the southern Red Sea.
The Red Sea is considered to be a biodiversity hotspot, having a unique fauna with many endemic species (species not found elsewhere). This has been able to develop due to the Red Sea's relative isolation, being connected to the Gulf of Aden and Indian Ocean by the shallow Strait of Bab al-Mandab, which is prone to variations in temperature and salinity that present a barrier to many marine organisms. The Red Sea has a unique deep-sea environment, where the waters remain constantly at about 22°C and have a salinity of about 40‰ (compared to about 35‰ for most seawater). This has led to the development of a vast reef system with many organisms unlike anything found anywhere else. Unfortunately, high shipping levels since the opening of the Suez Canal have led to many problems for this ecosystem, both from pollution and alien organisms carried on the sides and in the ballast tanks of ships, and decades of political instability and conflict in the nations around the Red Sea have meant that much of the area is poorly studied or protected.
The new species is described from a single specimen collected in 1963 by marine biologist Galena Murina aboard the Research Vessel Akademik Alexander Kowalevski, during by beam-trawl dredging in the Strait of Bab al Mandab. The specimen was examined by Crab-specialist Nikolai Zarenkov, then placed in the Zoological Museum of the M.V. Lomonosov Moscow State University, with a label giving information on the sampling vessel, station number and date, where the specimen was collected. The geographical coordinates given with the specimen are clearly erroneous, but Marin et al. were able to reconstruct the correct collection location from Galena Murina's original notes and maps.
The new species is named Planotergum kowalevski, in honour of the Research Vessel Akademik Alexander Kowalevski. The specimen from which it is described is a female measuring 8.6 by 11.7 mm. It clearly differs from Planotergum mirabile in the shape of its carapace, and the structure of the antennae, the third maxilliped (part of the feeding aparatus) and the ambulatory legs.
Planotergum kowalevski female (8.6 × 11.7 mm), Red Sea: (a) and (b) general dorsal view; (c) ventral view; (d) lateral view. Scale bars are 5.0 mm. Marin et al. (2019).
The discovery of a new species of Planotergum in the Red Sea is not in itself surprising, given the high rate of endemicity seen in the fauna there, and the distance from the known range of Planotergum mirabile. However, the sampling location of the only known specimen of Planotergum kowalevski, in the Strait of Bab al Mandab, leaves it unclear as to whether Planotergum kowalevski is a true Red Sea endemic, or whether it has a broader distribution within the western Indian Ocean.
Map of localities where representatives of the genus Planotergum have been reported. The species 'Anomalopisa incongruens' was created by Desmond Johnson in 1965 to describe specimens from off the coast of Pulau Sikijang Pelepah (Lazarus Island), which were later shown to belong to Planotergum mirabile. Marin et al. (2019).
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