Maintenance of two genetic entities by habitat selection

Summary In the laboratory, the two species of copepodsLepeophtheirus thompsoni andLepeophtheirus europaensis, ectoparasites of flatfishes, can meet and mate on at least one host species. In the wild however, these two species are found isolated on their sympatric hosts. Habitat selection theoretically represents a powerful enough mechanism to explain the maintenance of genetic heterogeneity in the wide sense. In this paper, the host colonization process is studied for both parasite species. It is shown that each parasite can develop and reach adult age on each host species. However,L. thompsoni is highly selective; it almost totally refuses to colonize hosts other than its natural one.Lepeophtheirus europaensis, on the contrary, readily infests turbot and brill in single-host experiments, but strongly prefers the brill when it has a choice. It appears that these two genetic entities are sympatrically maintained due to strong habitat selection. Such a pattern could theoretically only occur in a soft-selection context (density dependence). This point is discussed with respect to the different patterns in host use found in the geographical distribution of these parasites.     

Some notes on the ecology of a GermanBranchipus schaefferi population (Crustacea: Anostraca)

Habitat preference, seasonal occurrence, starvation resistance, hatching eggs ofBranchipus schaefferi, and effects of predation onB. schaefferi were studied.Branchipus was only present in turbid, unvegetated ponds and absent in ponds which contain higher aquatic vegetation and theSpirogyra sp. The first individuals ofB. schaefferi appeared in April when water temperature was 10 °C and the last adults in November at a water temperature of 3.5 °C. Up to 6 reproducing generations were observed during this period. Abundance ofB. schaefferi was higher in temporary ponds than in permanent ponds. Sex ratio was close to unity for most of the year. Body size ofB. schaefferi males and females was significantly positively correlated with pond volume. Without foodB. schaefferi could survive for 1.5 to 2 days at 20 °C and 4 to 5 days at 10 °C. Hatching success of eggs decreased when eggs were dried for 7 months. Freezing of eggs had no effect on hatching success. From] the predators tested,Chaoborus sp. larvae clearly selected smallB. schaefferi; one consumed approximately 6Branchipus d–1 at a density of 6 to 12 prey 1–1. The other predators, dragonfly larvae, and larvae and adults ofTriturus alpestris selected alternative prey types, for exampleTubifex sp. and ostracods.     

Habitat-specific predation susceptibilities of a littoral rotifer to two invertebrate predators

The rotifer Euchlanis dilatata lives associated with submerged vegetation in the littoral zone of freshwater lakes and ponds. I assessed habitat-specific predation susceptibilities for this rotifer in the presence of three aquatic macrophytes (Myriophyllum exalbescens, Elodea canadensis, and Ceratophyllum demersum) and two predators (damselfly nymphs — Enallagma carunculata; and cnidarians — Hydra). Rotifer survival was greatest on Myriophyllum in the presence of both predators. Conversely, the presence of the other macrophyte species actually increase rotifer suspectibility to predation by damselfly nymphs. I also manipulated plant structural complexity. As predicted, decreasing the relative complexity of each plant resulted in lower rotifer survival.     

Habitat selection by grayling—I. Spawning habitats

Grayling spawning sites were investigated in two French rivers, the Pollon (1 year) and the Suran (2 years) and described by current velocity, water depth, and substrate composition, completed by an assessment of bottom shear stress with FST-hemispheres. A comparison was made between used and available habitats, the latter being characterized by random sampling of 300 m long (Pollon) and 510 m long (Suran) river sections, both including three riffle/pool sequences. Mean velocities observed on spawning sites did not differ significantly between rivers or years (overall mean 48–9 cm s^-1 S.D. = 11.9, range 25.8–91.7 cm s^-1, n =150). Most water depths ranged from 10 to 40 cm in both rivers, but mean depths were significantly different ( P <0.05). Substratum of spawning grounds was dominated by gravel and pebbles (2–64 mm) in both rivers. Most spawning sites (99%) were characterized by a narrow range of hemispheres (nos 9–13), i.e. a range of shear stress of about 5–16 dyn m^-2. In the Pollon, spawners between spawning acts were found in a resting pool located immediately downstream from the spawning area and characterized by slow-flowing water (mainly <20 cm s^-1) and great depth (mainly >60 cm), with cover provided by overhanging branches and tree roots.     

Relation to habitat in rotifers

Rotifera should be especially suited for an analysis of habitat relations because this group contains such a high number of species, inhabiting diverse environments. Furthermore, rotifers are to a large extent cosmopolitan, implying that ecological barriers, rather than geographical, are decisive of their distribution. In this review a short characterization of the rotifer fauna in different habitats is given, whereby macroenvironments and microenvironments are reported separately. The macroenvironments are classified as follows: harmonious lakes and ponds, arctic and antarctic waters, hot springs, hypertrophic-saprobic environments, mires, strongly acidic waters, saline waters, temporary water bodies, subterranean waters, running waters, oceans, terrestrial environments. The following microenvironments are distinguished: macrophytes (housing periphytic rotifers), open water (with planktic forms), minerogenous sediments (with psammon and hyporheos), organogenous sediments, other organisms (i.e. parasites and epizoans).Many rotifers are more or less euryecious, while relatively few are strongly restricted in their choice of habitat. In extreme environments a low number of species is found, but often a high number of individuals within these species. These rotifers are usually primary consumers, and for natural reasons extreme environments are characterized by a low number of trophic levels.In environments with a high species number the separate species differ very much in their morphology, making it difficult to find common traits which may be interpreted as adaptations to the respective habitats. The most apparent adaptations ought to be found among the planktic rotifers, and these adaptations seem to constitute largely a protection against predators. Rotifers in extreme environments are usually not very apart in a morphological or taxonomical respect, with their most close relatives living in normal habitats and sometimes euryecious (an apparent exception from this rule is formed by the class Seisonidea). Adaptations to deviating chemical and physical environments may develop relatively rapidly (seen from a geological perspective), while the more fundamental changes (occurring during a longer period of time) seem to be a response to biotic factors (e.g., the development of different types of trophi for facilitating food collection).     

Seasonal migration promoting assortative mating in Littorina brevicula on a boulder shore in Japan

Littorina brevicula Philippi is one of the most common snails found in the upper intertidal zone of Japan. In Amakusa, some of the population of L. brevicula migrate to the lower zone in the winter, while the rest stay in the upper zone. Thus, during the winter, which is its reproductive season, the population of L. brevicula divides into two sub-populations. This leads to a hypothesis that the migration pattern in winter is genetically controlled and this behavioural dimorphism is maintained by reproductive isolation between the two sub-populations. In order to test this hypothesis, the following three points were investigated: (1) whether the same snails migrate in a similar way every winter, (2) whether there is a significant tidal level preference in snails, and (3) whether reproductive isolation occurs between the two sub-populations. The results showed (1) the migration behaviour of each snail was consistent over two successive winters, i.e. the same group of snails migrated downward every winter and the same group of snails stayed in the upper zone every winter, (2) transplanted snails moved toward the original zones where they were caught, suggesting that the snails actively selected their tidal zone in winter, and (3) most of the snails copulated within each sub-population. Therefore, reproductive isolation between the two sub-populations was considered to be established to some extent by the dimorphic migration behaviour. In conclusion, the migratory behaviour of L. brevicula is determined separately for each individual and might be genetically controlled, and the behavioural dimorphism may be maintained by partial reproductive isolation between the two sub-populations.     

Pulling the threads together: habitat diversity

The dependence of fungi on host and habitat is emphasized and the reticulate pattern of the interactions explored. Habitat diversity, often defined by the vascular plant element, vertebrate and invertebrate associations, and non-phanerogamic mutualisms are considered. The role of man especially in changes in habitat availability is stressed. Infraspecific variability is mentioned. The diversity of habitats is as much a part of the Rio Convention as molecular, cellular and organismal variation.     

Mangrove zooplankton of North Queensland,Australia

Food consumption, growth, fish length distributions,population sizes and habitat use of the salmonids intwo lakes in the Høylandet area were studied in1986–89. The allopatric brown trout (Salmotrutta L.) in the tarn Røyrtjønna (27 ha) fed mainlyon organisms at the lake surface , crustaceanplankton, Trichoptera and Chironomidae. Only 5% ofthe trout reached an age of 6 years and a length of25 cm. Sexual maturation started at age 3 and a lengthof 14 cm. Through mark – recapture technique thenumber of trout >10 cm was estimated to 115 ha^-1.Growth, fish length frequencies and sexualmaturation of the sympatric brown trout and Arcticcharr (Salvelinus alpinus (L.)) in LakeStorgrønningen (530 ha) were not much different. TheStorgrønningen charr fed chiefly on zooplankton whichby volume represented 33% for the trout. The foodconsumption of Storgrønningen trout was at maximum inJuly with 2.06 mg food (d.w.) per g live fish and forcharr in September with 1.26 mg food. The maximumsize-independent growth rate of trout was 5.2%day^-1 in late June, and for charr 4.1%day^-1 in late July. Seventy percent of theirseasonal growth took place before 15 August. The charrstayed mainly deeper than 3-4 m, at water temperatures<15 °C. Brown trout stayed mainly the littoralzone and in near surface water of the pelagic. Thenumber of pelagic charr was estimated hydroacusticallyto 50 ind. ha^-1. The charr spawn in thelake. Mean numbers of juvenile trout in the twolargest tributaries were 26 and 48 per 100 m².Their annual length increment was 2.8–3.4 cm. Noindication of acidification or other human inducedimpacts were found. The lakes and their tributariesrepresent complex aquatic systems, representative forpristine oligotrophic Norwegian lowland lakes.John W. Jensen died shortly after easter in 1996     

Larval habitat preference of the endemic Hawaiian midge,Telmatogeton torrenticola Terry (Telmatogetoninae)

Telmatogeton torrenticola Terry is a large endemic chironomid (lastinstar >20 mm) commonly found in high gradient Hawaiian streams on smoothrock surfaces with torrential, shallow flow and in the splash zones ofwaterfalls. We have quantified benthic water flow in larval habitat in a 50m segment of Kinihapai Stream, Maui using a thermistor-based microcurrentmeter. Under base flow conditions at sites suitable for larval attachment,depth was measured and bottom water velocity measurements were made 2 mmabove populations. Larval densities ranged from 386.9–1178m^–2, habitat bottom water velocities from 13.4–64.2 cms^–1, and water depths from 1.5–50 cm. Bottom velocitiesof sites with zero larvae ranged from 20.8–21.8 cm s^–1with depths from 50 to >160 cm. Larval densities were greatest inareas with high bottom water velocities and shallow depths. Stepwisemultiple regression analyses showed that density could be confidentlypredicted best by Froude number (r=0.81; p=0.008). In the absence of Froudenumber as a regression term, the best variable to predict larval density wasbottom velocity ratio: relative depth ratio (r=0.75; p=0.019). In addition,the torrential habitat of the larvae was always characterized by aperiphyton community that appeared to be the primary food resource for thelarvae. These data suggest that torrential flows over appropriate substratesare important factors regulating habitat availability for T. torrenticolaand that reduced discharge (e.g. affected by water diversions) couldsignificantly reduce the amount of available habitat for this organism andother flow sensitive stream fauna.     

Cryptic coloration and choice of escape microhabitats by grasshoppers (Orthoptera: Acrididae)

Cryptic coloration is found in many Orthoptera, especially in Acrididae, showing a great variety of forms. In a grasshopper assemblage in southeastern Brazil, preferences for escape places were detected; grasshoppers tended to escape to backgrounds in which they seem to be more cryptic. Coloration was measured using the Simpson diversity index, to quantify 'aspect diversity' (diversity of colours and shapes of patches along the insect's body). A weak positive correlation was found between grasshoppers' aspect diversity and diversification in use of escape places (use of many backgrounds to escape). Grasshoppers with similar colour patterns tended to use the same structures (leaves, sandy soil, stones) to escape.