382 Invertebrates, Marine, Overview Figure The invertebrate animals found in a 0.25 mÀ2 layer of sea sediment from the central North Pacific by Hessler and Jumars A worm shape is the most common body form, nematodes being particularly abundant Note that the size of the animals is exaggerated so that they may be seen (Reproduced with permission from Gage JD and Tyler PA (1991) Deep-Sea Biology Cambridge: Cambridge University Press.) polychaete worms (Annelida), while meiofaunal nematodes (Nematoda) dominate the benthos, at least in terms of abundance and diversity of meiofauna Arthropods are also important in both the macro and meiofaunal size classes The average marine benthic infaunal invertebrate is a soft-bodied worm (Figure 2), but vermiform animals are less important components of the megafauna Littoral benthos from sediments is sampled by inserting a corer into the deposits and washing the removed sediment over an appropriately sized sieve Sampling offshore is more problematical Megafauna are collected by equipment such as Agassiz trawls, anchor dredges, or epibenthic sledges towed along the bottom Such equipment has problems collecting consistent samples and is suitable only for producing qualitative or semiquantitative data Initially, the smaller infaunal animals were collected with grabs, and these are still used where circumstances dictate but they are inefficient The development of the box corer by Hessler and Jumars was a critical improvement in the accurate quantitative sampling of macrofauna for biodiversity studies Development of quantitative meiofauna samplers, notably the Scottish Marine Biological Association’s multiple corer, similarly revolutionized investigation into offshore meiofauna biodiversity Marine samples taken with modern corers provide data that are more quantitatively accurate than most terrestrial studies, and this has influenced the type of biodiversity questions asked by marine scientists Marine research has focused on alpha, or ecological, diversity, which is commonly analyzed using diversity indices that incorporate some measure of evenness as well as species richness The two most common indices employed are the Shannon-Wiener information function (H0 ) and, in the deep sea, the Sanders rarefaction method expressed as a curve or index that predicts an expected number of species per number of individuals in a sample Terrestrial biodiversity analysis tends to focus on larger scale measures of diversity based on species richness per area so it can be difficult to compare published marine diversity patterns with those obtained for terrestrial environments Pelagos Functional classification of pelagic organisms is based on locomotion, size, and trophic level or ecosystem function Plankton are generally passively drifting organisms carried by water movements; examples include bacteria, algae, and small animals Nekton are actively swimming organisms and so can to a certain extent migrate in the horizontal plane; examples include squid, fish, and some crustaceans Phytoplankton, marine plants, are primarily unicellular and responsible for primary production They are the main food source for all life in the open ocean Zooplankton are animals with a planktonic life style Division of pelagic organisms into size classes is related to retention by the different mesh sizes of nets and filters used for sampling (Table 3) Pica and nanoplankton are usually collected with remotely operated opening and closing bottle devices, allowing control of the depth zone sampled To collect organisms sized from microplankton upward, various types of net sampling gear are used ranging from simple ring nets, which are hauled vertically, to complicated remotely operated opening and closing net systems that can sample horizontal depth layers (e.g., RMT net, Omori net) Nekton are sampled with large, pelagic fish trawls Often pelagic samplers are used in combination with other equipment that measures physical factors such as depth, temperature, salinity, and light Due to the vastness of the pelagic realm relatively little has been studied, although there has been reasonable geographic coverage It is estimated that less than 5% of its huge volume