Sunday 25 May 2014

A new species of Ribbon Worm from the North Pacific.

Ribbon Worms (Nemertea) are a small group of worms related to Platyhelminth Flatworms. They have a simple anatomy, essentially a long, ribbon-like body with a dermal layer (skin), a through gut and a three layers of muscle. They have an extendable proboscis or stylet, which forms a cavity when retracted, and which is turned inside out when extended. This is used to subdue prey, often by penetrating their bodies, and in some species produces toxic secretions. Most Ribbon Worms are small, under 20 cm and often only a few mm, but there have been reports of Ribbon Worms over 50 m long, which if true would make them the longest known animals.

In a paper published in the journal ZooKeys on 14 January 2013, Hiroshi Kajihara of the Faculty of Science at Hokkaido University and Armand Kuris of the Marine Science Institute & Department of Ecology, Evolution and Marine Biology at the University of California, Santa Barbara, describe a new species of Nemertean Worm found in egg masses of the Red King Crab, Paralithodes camtschaticus, from the Sea of Okhotsk, waters off the island of Hokkaido and the coast of Alaska.

The new species is placed in the genus Ovicides, which contains four previously described species, all predators of Crustacean Eggs, and given the specific name paralithodis, which derives from the host species (Paralithodes camtschaticus). Ovicides paralithodis is an eyeless, sexually dimorphic Nemertean Worm, the females reaching about 1 cm long, the males 5 mm. 

Female specimen of Ovicides paralithodis in an egg mass of the Red King Crab, Paralithodes camthaticus. Kajihara & Kuris (2013).

Ovicides paralithodis feeds on the eggs of the Crab by piercing the egg membrane with its stylet, then consuming the spilt content of the egg. The female King Crabs carry and nurse their eggs; juvenile Worms were found on male Crabs and non-egg carrying females, suggesting that the immature worms can transfer from Crab-to-Crab till a food source becomes available, and may be able to survive moulting events (in which the Crab sheds its exoskeleton). 

More than 50% of Red King Crabs were infected with Ovicides paralithodis at thirteen locations in Alaska, with the infection rate reaching 100% at five locations. At six of these locations the infestation levels reached over 1000 worms per pleopod (female Red King Crabs have six egg-bearing pleopods), and at one location, Terror Bay on Kodiak Island, infected Crabs were found to have over 24 000 Worms per pleopod. 

Ovicides paralithodis. (A) Egg strand, (B) magnification of (A). Kajihara & Kuris (2013).

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