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Cooper of the sea, Phronima sedentaria

Ananta F. Benvenuto Tuesday, August 3, 2010
Phronima sedentaria sets up house by attacking and then hollowing out a transparent jellyfish-like animal called a salpa (as pictured).

The tiny crustacean then lays its eggs inside the "barrel," according to the Woods Hole Oceanographic Institution. When the babies hatch inside, the female, now outside the barrel, "watches over" her babies by pulling the makeshift nursery around via hooked claws.

The cooper of the sea, Phronima sedentaria, is a species of amphipod crustacean found in oceans at a depth of up to 1 km (1 mi).

Characteristics

Females are 14 mm in length, males are 15 mm in length. Males have an elongated basis of the first antenna. The eyes make up nearly a quarter of the body, giving rise to the infraorder name Physocephalata, meaning "large head". Each of the two eyes is divided into a medial and lateral eye.

Ecology

These crustaceans are usually found in midwater pelagic habitats, but can be found migrating all the way to the surface. Females live in the barrel-like bodies of salps, and use their strong pleopods to propel their homes through the water. The amphipod can somersault quickly in the barrel and thus change directions. Phronima sedentaria is carnivorous on zooplankton, krill, and arrowworms.

Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Amphipoda
Suborder: Hyperiidea
Family: Hyperiidae
Genus: Phronima
Species: P. sedentaria

Description
Phronima sedentaria is one of the most well-known and common hyperiid amphipods in the plankton, and its relationship with gelatinous zooplankton such as siphonophores, pyrosomes, and salps has been central to their invasion of the pelagic zone. These amphipods have adapted to the pelagic realm through various morphological, physiological, and behavioral specializations. Behavioral specializations include the formation of transparent barrels from salps, which they pilot through the water and on which they deposit their young (demarsupiation). Feeding behavior appears to vary with the consistency of the gelatinous organism being ingested. Pereiopods 3 and 4 are used to pick and pull soft-bodied prey toward the mouth, where the mouthparts may remove smaller pieces and push them into the esophagus. If the tissue is somewhat tough, pereiopods 1-3 hold the tissue across the mouthcone area where the mandible can bite and tear off small pieces. On the other hand, if the material is fairly fluid (e.g., salp stomach contents), the mouthparts flatten against the wall of the mouthcone and the contents are sucked into the foregut using the muscles of the esophagus and gut. The internal anatomy is roughly similar to other described amphipods but differs in certain respects. The digestive system differs from the generalized amphipod as depicted by McLaughlin (1980) in that the foregut is reduced and completely enclosed by a midgut chamber. The midgut caeca are reduced in size. The brain is circumesophageal and has four pairs of major nerves, which lead to the dorsal and ventral compound eyes, ventral nerve cord, and antennules. The reproductive and circulatory systems are described. 

Photograph courtesy H. Bahena, Felder, D. L. and Camp, D. K. (eds.) 2009. Gulf of Mexico­Origins, Waters, and Biota. Vol. 1. Biodiversity. Texas A&M Press, College Station, Texas.

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