1. The unusual animal, called Phronima,
an Amphipod, is one of the many strange species recently found on an expedition
to a deep-sea mountain range in the North Atlantic.
2. Amphipoda is an order of
malacostracan crustaceans with no carapace and generally with laterally
compressed bodies.
3. The name amphipoda refers to the
different forms of appendages, unlike isopods, where all the thoracic legs are
alike.
4. Of the 7,000 species, 5,500 are
classified into one suborder, Gammaridea.
5. The remainder are divided into two or
three further suborders.
6. Amphipods range in size from 1 to 340
millimetres (0.039 to 13 in) and are mostly detritivores or scavengers.
7. They live in almost all aquatic
environments; 750 species live in caves and the order also includes terrestrial
animals and sandhoppers such as Talitrus saltator.
8. The body of an amphipod is divided
into 13 segments, which can be grouped into a head, a thorax and an abdomen.
9. The head is fused to the thorax, and
bears two pairs of antennae and one pair of sessile compound eyes.It also
carries the mouthparts, but these are mostly concealed.T
10. The thorax and abdomen are usually
quite distinct and bear different kinds of legs; they are typically laterally
compressed, and there is no carapace.
11. The thorax bears eight pairs of
uniramous appendages, the first of which are used as accessory mouthparts; the
next four pairs are directed forwards, and the last three pairs are directed
backwards.
12. Gills are present on the thoracic
segments, and there is an open circulatory system with a heart, using
haemocyanin to carry oxygen in the haemolymph to the tissues.
13. The uptake and excretion of salts is
controlled by special glands on the antennae.
14. The abdomen is divided into two
parts: the pleosome which bears swimming legs; and the urosome, which comprises
a telson and three pairs of uropods which do not form a tail fan as they do in
animals such as true shrimp.
15. Amphipods are typically less than 10
millimetres (0.39 in) long, but the largest recorded living amphipods were 28
centimetres (11 in) long, and were photographed at a depth of 5,300 metres (17,400
ft) in the Pacific Ocean.
16. Samples from the Atlantic Ocean with
a reconstructed length of 34 centimetres (13 in) have been assigned to the same
species, Alicella gigantea.
17. The smallest known amphipods are less
than 1 millimetre (0.04 in) long.
18. The size of amphipods is limited by
the availability of dissolved oxygen, such that the amphipods in Lake Titicaca
at an altitude of 3,800 metres (12,500 ft) can only grow up to 22 millimetres
(0.87 in), compared to lengths of 90 millimetres (3.5 in) in Lake Baikal at 455
metres (1,500 ft).
19. Mature females bear a marsupium, or
brood pouch, which holds her eggs while they are fertilised,and until the young
are ready to hatch.
20. As a female ages, she produces more
eggs in each brood.
21. Mortality is around 25%–50% for the
eggs. There are no larval stages; the eggs hatch directly into a juvenile form,
and sexual maturity is generally reached after 6 moults.
22. Some species have been known to eat
their own exuviae after moulting.
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