A new species of Eagle Ray from the Northwest Pacific.

Eagle Rays (Myliobatidae) are large Batoid Fish (bilaterally symmetrical fish with cartilaginous skeletons descended from Sharks), that predominantly live as free swimming animals in the water column rather than on the sea bottom. They are strong swimmers, and some species have been observer to leap several meters from the water. The plankton-feeding Manta Rays and Devilfish are highly specialized Eagle Rays, though most members of the group feed on Molluscs and other shellfish, which they crush with strong, flat teeth, in a similar way to other Batoids.

In a paper published in the journal PLoS One on 31 December 2013, William White of the Wealth from Oceans Flagship of CSIRO Marine & Atmospheric Research and Keisuke Furumitsu and Atsuko Yamaguchi of the Faculty of Fisheries at Nagasaki University, describe a new species of Eagle Ray from the northwest Pacific.

The new species is placed in the genus Aetobatus and given the specific name narutobiei, meaning 'Naru Eagle Ray', a reference to the island of Naru, one of the Goto Islands, which are part of Nagasaki
Prefecture, where the new species was first encountered. Aetobatus narutobiei is a cryptic species; the populations referred to the new species were previously known, and referred to the wide ranging Aetobatus flagellum, which is recorded from across the Indian Ocean as well as the northwest Pacific. The new species is very hard to distinguish from the old visually, it is slightly larger and has slightly different colouring, but this is based upon a small sample size for both species. The species can only be reliably differentiated by genetic testing.

Aetobatus narutobiei, male specimen in dorsal view. White et al. (2013).

Eagle Rays assigned to the species Aetobatus flagellum have been subjected to a predator control program since 2001, due their impact on commercial shellfish stocks; this is despite the face that Aetobatus flagellum, like many Shark and Ray species, is listed as Endangered on the IUCN’s Red List of Threatened Animals. White et al. suggest that this policy needs to be reviewed as a matter of priority as it appears that these Rays are part of a smaller overall population.

See also A Hybodont Shark from the Middle Permian of Northern Arizona, Fossil Tapeworm eggs from the Permian, Satellite tracking Manta Rays off the coast of Mexico and New species of Catshark from the Galapaagos.

Follow Sciency Thoughts on Facebook.

The effect of parasitic Nematodes on European Eels.

European Eels, Anguilla anguilla, have a complex life-cycle; they hatch from eggs in the Sargasso Sea in the southwest North Atlantic, then migrate to European inland waterways where they grow for up to 25 years before returning to the Sargasso to breed. This life-cycle exposes leaves them exposed to a number of human-related threats, including overfishing, habitat loss and pollutants, and the species has suffered a dramatic decline in numbers since the 1970s, to the alarm of conservationists. Since the 1980s the species is also known to be widely suffering from infection from an invasive parasitic Nematode worm, Anguillicoloides crassus, which probably entered Europe with imported Eels from Southeast Asia, and against which the European Eel apparently has no natural defenses. 

The European Eel, Anguilla anguilla. Animal Base/Staats-und Universitätbibliothek Göttingen.

Anguillicoloides crassus infects the swim-bladders of Eels, and from where it drains blood. The Nematode has a much shorter life-cycle than the Eel, and an infection will purge itself since eggs are laid outside the host, but multiple infections can damage or even destroy the swim-bladder, removing the Eels ability to control its own buoyancy and killing it. Anguillicoloides crassus is not reliant on the Eels to survive, it can use a variety of hosts including many types of Fish and Crustaceans, so a reserve of Nematodes is likely to remain in the environment even if the Eels become locally extinct, so that new Eels recolonizing an area will be infected immediately.

The parasitic Nematode Worm, Anguillicoloides crassus. AquaNIS.

Eels raised in the laboratory and infected with Anguillicoloides crassus, tend to be underweight compared to other Eels of the same age. Such infected Eels are approximately the same length as uninfected Eels, but are notably thinner. Based upon this studies of Eels in the field have tended to use weight-to-length ratio as a proxy for Nematode infection, though this has never actually been tested in wild Eels.

In a paper published in the Proceedings of the Royal Society Series B Biological Sciences on 16 January 2013, a team of scientists led by François Lefebvre of Station Biologique de la Tour du Vala, describe the results of a study of Eels in a complex of brackish lagoons in the Rhône River delta, in which Eels were actively checked for Nematode infection and signs of past infection (scarring of the swim bladder), and the rate of infection was compared to the weigh-to-length ratio of the Eels.

Surprisingly this study found that the relationship between infection rate and age appeared to be reversed in wild Eels compared to laboratory-raised Eels; fatter Eels showed signs of bearing a heavier parasite load that thinner Eels, suggesting that the model used in many previous studies is completely wrong and must be reassessed. Lefebvre et al. suggest that while infection in the laboratory is deliberate and not influenced by diet, this is not the case with wild Eels, which become infected by consuming Nematode eggs, thus making more voracious Eels more likely to become infected, while giving Eels that consume less, either through more selective feeding or for other reasons, a better chance of avoiding the Nematodes.

See also An invasive Serpulid Worm in the La Encrucijada Biosphere Reserve, Mexico, Blue Flatworms invade Menorca and A living fossil eel discovered in Palau.

Follow Sciency Thoughts on Facebook.