The Open Sea by Alister Hardy

Author:Alister Hardy [Alister C. Hardy]
Language: eng
Format: epub
ISBN: 9780007509768
Publisher: HarperCollins Publishers

The Amphipoda are close relatives of the isopods, but whereas the latter tend to be flattened from above downwards, these instead appear as if squeezed flat by pinching from the sides. The most typical amphipods are like our common freshwater shrimp, Gammarus, and belong to the sub-order Gammaridea; but few of these are to be found in a plankton-sample except when the net is used very near the bottom. Apherusa (Plate 14), however, may be regarded as truly planktonic and A. cleevei is quite often to be taken high up in the water in the southern North Sea. The more characteristic amphipods of the plankton, however, belong to another sub-order, the Hyperidea.

In addition to the lateral flattening, the following amphipod characters are noteworthy: no distinct carapace, eyes sessile, and the first thoracic segment fused with the head (or in a few forms the second also). As for their limbs, those of the thorax are uniramous—the first pair being maxillipedes, and the second and third usually ending in little prehensile claws; those of the abdomen are biramous and in two sets—the first three pairs being multi-articulate and the other three pairs usually simple and flat (all three in fact like uropods); the telson is very small. The typically planktonic amphipods, the Hyperidea, have the characters just described, but their most noteworthy feature is their enormous eyes. One of the commonest is Themisto gracilipes; in some years it occurs in immense numbers off the coasts of Scotland and Northumberland and may be quite an important item in the diet of the herring caught in the Shields summer fishery. It grows to three-quarters of an inch in length and is heavily pigmented with deep red stellate chromatophores (Fig. 53). Closely related to this form is Hyperia galba, which has already been mentioned (see here) for its association with the larger jelly-fish; it has a much rounder body and is shown in Plate 14.

A very rare visitor to our seas—one of the exotic Lusitanian forms found by Dr. Fraser to the north of Scotland (see here)—is the remarkable Phronima which is so common in the Mediterranean and subtropical Atlantic. It is a hyperid amphipod with huge compound eyes, but the whole animal is perfectly transparent. It attacks salps and other similar animals,8 and from them, by eating away parts, fashions a gelatinous barrel-like house in which it shelters and rears its young. One may sometimes be seen swimming and pushing a barrel full of young ones in front of it. A specimen taken by Dr. Fraser in north Scottish waters is shown in Fig. 53.

The third and last main sub-order of the amphipods is the Caprellidea which contains those remarkably modified forms such as Caprella which feed upon hydroids. We should hardly expect to find the caprellids in the plankton; but in fact, in the shallower regions of the southern North Sea, one of these little forms, Phtisica acaudata, is very commonly met with. It is particularly common after rough weather when the bottom

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.