Offshore Plankton Research Articles

Ecological and evolutionary significance of pelagic spermatophores of vermetid gastropods

Sperm are disseminated in vermetid gastropods by spermatophores released freely into the ocean. Spermatophores from 7 species of vermetids, including 3 genera, were obtained from reproductively active males or offshore plankton tows or both. Each vermetid spermatophore consists of a sperm mass containing highly ordered eupyrene and apyrene sperm enveloped concentrically by 3 transparent capsules. Generic and specific differences occur in the size, shape and composition of the sperm mass and capsule. The complexly layered spermatophore is assembled in the male reproductive tract, which includes and elaborate set of pallial reproductive glands for capsule production. Spermatophores are liberated by males into the seawater, and there dispersed by water movements. Some of these drifting spermatophores become entangled in the mucous feeding nets of female vermetids and are then activated by feeding movements. Activation of the complex ejaculatory apparatus appears to result from an interplay of osmotic and mechanical mechanisms. Spermatophores remain viable in the laboratory for 12 to 20 h after release from the male; after 20 h, the sperm mass breaks down and the sperm become immotile. Plankton tows capture some spermatophores with motile sperm and others in which sperm are no longer active. Evolutionary implications of the relationship between production of pelagic spermatophores, sessile mode of life and other aspects of vermetid biology are discussed.

The environmental and climatic distribution of dinoflagellate cysts in modern marine sediments from regions in the North and South Atlantic Oceans and adjacent seas

Quantitative treatments of 168 dinoflagellate cyst assemblages from modern marine sediments were used to decipher the salient environmental and climatic features of the distribution of common living cyst-based taxa. Surface sediment samples were studied by routine palynological methods from estuarine, continental shelf, slope and rise zones, and abyssal plains between latitudes 62°N and 27°S within fourteen geographic regions of the North and South Atlantic Oceans, the Caribbean and Mediterranean Seas and from one region in the southeastern Pacific Ocean near Peru, to meet this general objective. The results were expressed as percentages of species present and statistically analyzed by multivariate Q-mode factor analysis, cluster analysis and by calculation of a diversity index, using pre-existing computerized routines. Inshore to offshore and latitudinal (climatic) variations in distribution were identified and they involved individual taxa, associations of species, species diversity and specimen densities (cysts per gram of sediment). They were related empirically to changing surface water environments above and this suggested that biologic-ecologic phenomena, which involve species-water type relationships for different taxa, combined with hydrodynamic (current) systems, are the most important factors that control cyst distribution in the bottom thanatocoenosis. However, geologic factors such as recycling of older specimens and outcroppings of relict Pleistocene-Early Holocene sediments exert their influence too. An ecologic classification for extant cyst-based species is proposed in the format of classical “plankton elements”. This format is dictated by the existence of dual trends (environmental and climatic) in cyst distribution. It is suggested that genesis of ecologic species-groups of this nature during evolution can be interpreted by or “predicted” from stability-predictability concepts as developed in contemporary ecologic work, if surface water masses are considered to represent unique hydrographic climates with innate stability and predictability characteristics. This concept of “hydroclimatic stability” is used to identify fossilizable living dinoflagellates as being environmentally adapted to unstable conditions around continental margins and comparable shallow-water environments around oceanic islands. In addition there is a tendency for the more stenotopic extant cyst-based taxa to be adaptively specialized towards more stable sectors of nearshore and offshore plankton environments which develop at the terminal and subterminal ends of temperature and salinity regimes or profiles. One taxonomic change is introduced: Planinosphaeridium choanum (Reid 1974) nov. comb. is proposed from the basionym Ataxiodinium choanum Reid 1974.

A diver-controlled plankton net

Many types of plankton net have evolved for various sampling needs, but in the main they are designed for sampling in mid-water, or over fine particulate substrates where they can operate within a sledge frame. The conventional plankton nets are mostly towed behind a boat and can only be used in areas where there are no obstacles to impede their course. Many nets are described in the literature and the reviews by Tranter et al. (1966) and the UNESCO monograph on zoological sampling methods (Tranter, 1968) give further details. Nets for taking benthic plankton are generally mounted on a sledge frame and either towed behind a boat (Russell, 1928; Wickstead, 1953), towed in shallow water by persons wading and pulling the net behind them (Colman & Segrove, 1955) or pushed by SCUBA divers (Fager et al. 1966). However, all these methods still leave the need to sample rocky areas and irregular substrates where conventional methods are not possible. Such areas are often extremely rich in planktonic organisms which appear to accumulate within the shelter offered by gulleys and depressions in the bottom topography (personal observation). Often species associated with inshore algal belts are poorly represented in conventional offshore plankton samples (Russell, 1973), and it may be that such species spend much of their larval existence close in shore or in association with shallow water reefs. The present paper describes a diver-controlled plankton net which has been designed to sample rocky areas. By having direct diver control this plankton net can be used to sample other areas where conventional methods are not appropriate.

Seasonal Changes in Population Density and Vertical Distribution of prosobranch veligers in offshore Plankton at Plymouth

Vertical and horizontal hauls were taken at approximately fortnightly intervals from April 1969 to April 1970 at L 3 (lat. 40° 17.7′ N, long. 4° 11.2′ W) and L 4 (lat. 50° 15′ N, long. 4° 12.5 W). Thirty-two species of prosobranch veliger were present at L 35 these larvae were not as numerous at L 4 and only 26 species were recorded. The number of species was highest in the summer: the number of veligers was highest in February. After mid October both the number of species and the abundance of veligers decreased rapidly and remained low until early February. Veligers of some species occurred later at L 3 and L 4 than in more inshore waters and these were probably individuals carried beyond tidal influences which would normally lead to settlement on the shore. They includedLacuna vincta, Littorina littoreaandL. neritoides.Veligers of all ages were found at all depths. At their time of greatest abundance veligers of many species occurred maximally at 5 or 10 m, with a variable decrease towards greater depths and a sudden one towards the surface. This distribution later changed giving a proportionally greater number at greater depth. When numbers were low the larvae scattered through the water column with little or no indication of a preferred depth. An examination of the age composition of veligers of rissoids,Natica alderi, Nassarius reticulatusandPhilbertia linearisfrom certain catches showed that the surface accumulation at the time of abundance was composed of a high percentage of young veligers: in an ageing population there was a higher percentage of larvae, especially the older ones, at greater depths, except forNassarius reticulatuswhich consistently showed maximal numbers above 10 m until the larvae became scarce.

PRELIMINARY OBSERVATIONS ON CORAL REEF PLANKTON1

Plankton collections near coral reefs were made by hand‐towing nets while swimming and by using a suction device for sampling caves. Plankton in sheltered areas was considerably different from that in nonsheltered areas; some plankton forms maintained position near coral reefs, indicating that the terms planktonic and epibenthic may represent extremes of a behavior continuum. Copepods were observed to swarm and mysids to school on the reef. Mysids and nesting pomacentrids exhibit a loosely developed commensalism. Offshore plankton probably represents a net energy gain to the reef.

Relative Iron Requirement of Some Coastal and Offshore Plankton Algae

Relative Iron Requirement of Some Coastal and Offshore Plankton Algae