Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 94

Georges Bank is a shallow (45 m depth) hummock in the Gulf of Maine, USA, made famous by its former bountiful harvests of cod (Gadus morhua), sadly now depleted. The area was the subject of an intensive multidisciplinary 1990–2000 oceanographic study as part of the international GLOBEC (GLOBal Ocean ECosystem Dynamics) program, funded by the USA’s National Science Foundation. The mission of GLOBEC was to describe the interaction of physical and biological processes in the life history of important species. In the case of George’s Bank, the target species was cod.

A GLOBEC sampling cruise in May of 1994 revealed large numbers of suspended hydroid colonies in the zooplankton over the shallows of Georges Bank (Madin et al. 1996). The colonies were typically fragments of 2–5 polyps each, were widely distributed over the bank, and were the overwhelming dominant component of the net‐caught zooplankton, reaching densities of 10 000 m⁻³ in the water column and 25 000 m−³ nearer the bottom. Colonies were found to be the polypoid life stage of the hydromedusan genus Clytia, predominantly Clytia gracilis, which normally grows attached to rocks, seaweed, or other available benthic substrate. The polyps suspended in the water column instead of being attached to substrate were not only alive, they were actively hunting. Examination of gut contents and shipboard experiments revealed that the hydroids were catching and digesting larval cod as well as copepod eggs and larvae. Madin et al. (1996) estimated that the hydroids were capable of ingesting half the daily production of copepod eggs and a quarter of the standing stock of copepod larvae per day: an impressive figure for a benthic life stage.