Distribution

Occurs worldwide from the equator to polar regions at all ocean depths. Latitudinal variation in taxonomic composition is pronounced, even at the level of orders. The shallow-water tropics to warm temperate regions are the most diverse and are dominated by members of the aspidochirote families Holothuriidae and Stichopodidae. The diversity of these families peaks on coral reefs, where 20 species per 2.5 acres (1 hectare) is not uncommon. Dendrochirotes live here as well but become a dominant part of the holothuroid fauna only in shallow cool-temperate to polar seas. Whereas the apodan Synaptidae, subfamily Synaptinae, are primarily found in the tropics, the diversity of Apodida generally increases away from the equator. Molpadiidae also are primarily found at higher latitudes or in deeper water. In the tropics, the Caribbean fauna is distinct from that of the Indo-Pacific region. The waters around southern Africa and New Zealand harbor numerous unusual endemic forms.

Marked taxonomic variation in depth also occurs. The as-pidochirote Holothuriidae, in addition to being an essentially tropical family, is primarily a shallow-water group. Most holothuroid habitat, however, is in the deep sea. Many families of holothuroids have at least some deep-sea members. Most Dactylochirotida live at depth. All species of the aspi-dochirote Synallactidae and Gephyrothuriidae, apodan Myri-otrochidae, and the order Elasipodida are found in the deep sea. Among the Elasipodida, species in Laetmogonidae live primarily at bathyl depths (3,000-6,000 ft [915-1,830 m]), whereas those in Psychropotidae and the Peniagone species in Elpidiidae characterize abyssal depths (6,000-18,000 ft [1,830-5,490 m]). The region below approximately 18,000 ft (5,490 m) comprises only approximately 1% of the area of the ocean floor, and a noticeable decrease in species diversity occurs. These depths consist of geologically less stable and inclined substrata in oceanic trenches that extend to 36,000

ft (10,970 m). Nevertheless, in these regions holothuroids dominate the benthic fauna in terms of weight of living organisms. Although nearly all holothuroids are restricted to particular depth ranges, a few species are remarkably indiscriminate. For example, Elpidia glacialis live in waters as shallow as 230 ft (70 m) in northern Europe to as deep as 33,000 ft (10,058 m).

Essentials of Human Physiology

Essentials of Human Physiology

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