Arthropods Marine

Arthropods are the most numerous animals on earth in terms of described species, with approximately 1.25 million taxa. However, that probably represents only a portion of the total number of arthropod species in the world, with estimates of diversity (including unde-scribed species) starting at 2 million. People are perhaps most familiar with members of the Insecta, and indeed they constitute the bulk of arthropod species whose successful colonization of land led to their rapid radiation in form and function and their habitation of nearly all terrestrial regions, with many species occurring in freshwater as well. Insects, however, represent only a portion of the morphological diversity present in the arthropods and are not the dominant arthropods of the seas.

The organisms that are grouped together under Arthropoda are those that possess metamerism (that is, segmentation) and a pair of appendages on each segment (although these may be lost on some segments in the more derived taxa). Arthropods also possess such synapomorphies (shared characters) as a chitinous exoskeleton that is molted at intervals (controlled by the hormone ecdysone) and an absence of cilia on or in the body. Many scientists consider the velvet worms (Ony-chophora) and water bears (Tardigrada) to be the two phyla most closely related to the arthropods.

Of all described arthropods on the planet, only a small percentage, perhaps 3 to 5 percent, are adapted to an exclusively marine life. This includes the majority of Crustacea (approximately 50,000 species); all of the Pycnogonida, or sea spiders (with more than 1,000 species), and Xiphosura, or horseshoe crabs (4 species); and a few Arachnida (approximately 1,000 species). The Insecta, with more than 1.1 million described species, are extremely poorly represented in the marine environment, with only a few hundred species in the intertidal zones and near-shore environments, and a mere five species of water-striders (Halobates) and perhaps a few midges (Diptera) being found on the open ocean. The myriapods (millipedes, centipedes, and so forth) have no marine representatives. In keeping with the fact that the oceans constitute a semi-continuous worldwide habitat, almost all the marine arthropod higher taxa (except the horseshoe crabs) today have an essentially worldwide distribution, at least within a broad latitudinal range.

The small percentage of arthropods found in the marine environment stands in sharp contrast to the fact that the seas were home to the earliest arthropods. Early representatives, such as the chelicerate horseshoe crabs (Xiphosura) and sea scorpions (Eurypterida), as well as primitive crustaceans such as those found in the Burgess Shale deposits in Canada, shared the seas with the now-extinct trilobites. Although the trilobites were a moderately

A) Copepod (Copepoda) Aphotopontius forcipatus; B) Horseshoe crab (Xiphosura) Limulus polyphemus; C) Water-strider (Insecta) Halobates micans; D) Amphipod (Amphipoda) Stenothoe marina; E) Sea spider (Pycnogonida) Pycnogonum stearnsi; F) Parasitic isopod (Isopoda) lone cornuta, a: female, b: male successful group of wholly marine animals (approximately 4,000 species), they were, for reasons unknown, unable to survive the climatic changes at the end of the Permian and left no descendants. Although their basic body plan did not vary as widely as in some other arthropod groups, which may have contributed to their extinction, they did exhibit a remarkable array of variation in form and armature.

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