Tom Fenchel

The Ubiquity of Small Species: Patterns of Local and Global Diversity

Abstract Small organisms (less than 1 millimeter in length) tend to have a cosmopolitan distribution. This is a consequence of huge absolute population sizes rather than any inherent properties of particular taxonomic groups. At the local scale, the diversity of small species exceeds that of larger organisms, but at the global scale this relation is reversed, because endemism is largely responsible for the species richness of large organisms. For small organisms, the relationship between species and area is flat, and a latitudinal diversity gradient is absent or weak. These patterns are explained by some of the assumptions underlying the unified neutral community model.

Local versus Global Diversity of Microorganisms: Cryptic Diversity of Ciliated Protozoa

Microbial species diversity. both global and local, is still poorly understood. In this study all species of ciliated protozoa were recorded microscopically from ∼1 cm 2 sediment collected from a small lake and from a marine shallow-water bay. Additional adjacent sediment samples (together representing <50 cm 2 ) then incubated under a variety of culture conditions to reveal cryptic species that are present as resting cysts or are too rare to be found microscopically. About 85 and of the total number of observed species from the limnic and marine sediment, respectively, were such cryptic species. In both cases the number of species found in < 50 cm 2 of sediment represented about 75% of all ciliate species ever recorded from these two previously well-studied habitats, and about 8% of all named free-living ciliates. These observations support the assumption that in the case of microorganisms everything is everywhere and that their global species diversity is relatively limited.

THE ECOLOGY OF MARINE MICROBENTHOS II. THE FOOD OF MARINE BENTHIC CILIATES

Abstract The paper brings together knowledge on the food of marine benthic ciliates with the exception of sessile forms. References are given to 260 species of which 90 have been studied by the author. The classification of ciliates according to their natural food and the specificity in choice of food is discussed and the ecological significance of discrimination of food according to size is emphasized.

Microbial diversity and activity in a Danish Fjord with anoxic deep water

Abstract Microbial diversity and activity were studied in a stratified basin of Mariager Fjord, Denmark in August 1994. The basin is about 30 m deep and the lower half of the water column is anoxic and sulphidic. The hydrographical and biological features of the system are described. Based on chemical gradient profiles and measurements of process rates, we found that the relative importance of sulphate reduction, denitrification and methanogenesis in terms of anaerobic terminal mineralisation was about 5:1:0.4. It is possible, however, that methanogenesis is underestimated because an unknown fraction of the methane production escaped by ebullition. It was estimated that 10–15 % of the net primary production is mineralised anaerobically. The mean residence time of methane, sulphide and ammonia beneath the chemocline is within the range 1.6–2.3 yrs. Chemolithotrophic production in the chemocline (sulphide oxidation and nitrification) accounted for about 3% of the net primary production of the system. Methan...

Vertical distribution of photosynthetic pigments and the penetration of light in marine sediments

Methods for studying the vertical distribution of some photosynthetic pigments in marine sediments (chlorophyll a, bacteriochlorophyll a, phycocyanin and a watersoluble cryptomonad pigment) are described and examples of their vertical distribution and of absolute counts of microalgae from various types of shallow water sediments are given. In all types of sediments many pigmented cells are found below the photic zone. Light penetration in different kinds of sediment was measured for white and colored light. In pigment-free sand long waved light penetrates furthest; this is of significance for the photosynthetic bacteria which live deeper in the sediment than algae and utilize near-infrared light for photosynthesis. The photic zone of very fine sand is of half the thickness of that found in coarse sand. The percentage of light absorbed by the photosynthetic pigments was estimated to be less than 10-20% of the total incident light in rich localities, a low value compared to other plant communities. The amount of chlorophyll a per unit area of the photic zone of different sediments was estimated and found to be lower than estimates from other plant communities.

The ecology of marine microbenthos I. The quantitative importance of ciliates as compared with metazoans in various types of sediments

Abstract An investigation of the quantitative importance of protozoans — especially ciliates — has been undertaken in Scandinavian waters. Ciliates were found in numbers from 106 to 4 × 107 individuals per m2 representing 0.03–2.3 grammes wet weight per m2. Highest numbers were found in fine sand and in localities with a rich growth of sulphur-bacteria. In these sediments ciliates are 10 to 100 times more numerous than the total number of metazoans, and their biomass is of the same order or sometimes larger than the biomass of the micro-metazoans (nematodes, turbellarians, gastrotrichs etc.). In coarser sand ciliates are fewer, and micro-metazoans play a larger quantitative role. This distribution is explained by the small size and other morphological adaptations making ciliates able to inhabit the small interstitia of fine sand, and also their ability to endure reducing and anaerobic conditions. In the detritus layers which cover clayey and muddy sediments ciliates are few in number and play a small role...

Patterns of reproduction in different populations of five species of the amphipod genus Gammarus

Different features of reproduction (breeding periods, number of generations per year, clutches per generation, clutch sizes, egg sizes) were studied in five species of Gammarus in the Baltic Sea and in the Limfiord, Denmark and in two species at localities along the west coast of the European continent. Breeding periods, numbers of annual generations (one or two), and number of clutches per generation show some geographical variation within each species; this is explained by interspecific relations (reproductive character displacement) in conjunction with local water temperature. Winter breeding forms have fewer and larger eggs than summer breeding forms; this is explained as an evolutionary adaptation to a larger juvenile mortality resulting from a low growth rate at low temperatures. Reproductive effort (total egg volume per clutch) is nearly identical for four of the species within each area, but differs between areas. No explanation for this effect is attempted; it is pointed out that considerations on rand K-selection are futile for understanding such data. This will require much more detailed data on age specific mortality in nature under different environmental conditions than is normally available for invertebrate populations.

The diversity of microbes: resurgence of the phenotype

The introduction of molecular genetic methods has caused confusion about the nature of microbial species. Environmental DNA extraction has indicated the existence of a vast diversity of genotypes, but how this relates to functional and phenotypic diversity has not been sufficiently explored. It has been implied that genetic distance per se correlates with phenotypic differentiation and thus reflects subtle (but undiscovered) adaptive fine-tuning to the environment, and that microbes may show biogeographic patterns at the genetic level. Here, we argue that no theoretically based species concept exists; species represent only the basic unit in the taxonomic hierarchy. The significance of naming species is that it organizes biological information. The reason why microbial species collectively represent large genetic differences is owing to huge absolute population sizes, absence of allopatric speciation and low extinction rates. Microbial phenotypes are, therefore, ancient in terms of the geological time-scale and have been maintained through stabilizing selection. These problems are discussed with special reference to eukaryotic micro-organisms.

Habitat Selection and Distribution Patterns of Five Species of the Amphipod Genus Gammarus

The distribution patterns of five brackish water species of the amphipod genus Gammarus from the inner Danish waters and from the central Baltic Sea are compared. In the former area, the species show zonation patterns which correlate with salinity. In the Baltic, with a low, stable salinity, such patterns are absent, and the species show a much higher degree of coexistence, although habitat selection correlated with the degree of exposure, substrate and depth is indicated. Comparisons with the literature show, that while regionally coexisting Gammarus spp. always show some habitat compression, the actual relationships to environmental gradients of the single species have evolved in different directions in different areas. The findings are discussed in terms of the selection forces which have resulted in habitat selection. It is suggested that selection against interspecific precopula-formation leading to sterile matings is an important factor and that displacements of reproductive periods are important

Ciliates from Scandinavian molluscs

Abstract The present paper describes the ciliate fauna living on the ciliated epithelia of aquatic invertebrates, especially bivalves, from a systematical and an ecological point of view. In 64 Scandinavian invertebrate species, of which 57 were lamellibranchs, 47 morphologically distinct ciliate species were found of which 14 are new to science. The majority of these ciliates belongs to the order Thigmotrichida, the remainder being representatives of the families Urceolariidae, Licnophoridae and Euplotidae. A new family, Peniculistomatidae is created to contain Peniculistoma mytili (De Morgan), which possesses many characters in common with the pleuronematine hymenostomes and therefore must be considered as the most primitive thigmotrich. Conchophtheridae and Thigrnophryidae are considered to be specialized families. Three types of morphological adaptations were found within the ciliates of lamellibranchs. In all types, however, the organelles of attachment and a body shape giving minimum resistance to w...