Annelids in Temporary Waters: Ecology and Adaptations

Annelids. Annelid worms known from temporary waters primarily comprise species from the classes Oligo- chaeta and Hirudinea, although freshwater Poly- chaeta (Sabellidae: Manayunkia) have occasionally been found (Yozzo and Diaz 1999). Species belonging to the Branchiopdellida might also be expected to occur in these habitats by virtue of their ectosymbiotic relationship with freshwater crustaceans, in particular astacid crayfishes.

Leeches tend to live in lakes, ponds, and the slower-flowing reaches of streams and rivers, and are often an important component of the benthic community; in small ponds they often represent the top predators (Davies 1991). Several species of leech are known to tolerate degraded water conditions, including anoxia (for up to 60 days). Their distribution is related, at least in part, to the total dissolved solids (TDS) content of the water. For example, the mortality of Erpobdella punctata, a common wetland species, has been shown to be strongly influenced by an interaction among water temperature, ionic content, and TDS, whereas Nephelopsis obscura was relatively unaffected. For both species, cocoon production was most influenced by temperature. Interestingly, cocoon production and viability in E. punctata increased in waters with high TDS (Linton et al. 1983), the latter often signalling the end of the hydroperiod. Some leech species (such as those belonging to the Erpobdellidae) seem preadapted to living in temporary waters as their thick-walled cocoons have a spongy outer layer thought to reduce water loss. Further, some species actually deposit their cocoons out of water, in damp places.

Although it is likely that all leeches are genetically iteroparous (i.e. they reproduce several times in their life), Davies (1991) believes that they tend to exhibit semelparity (single reproductive event in their life) under most conditions. He suggested that such flexibility would promote the long-term persistence of a variety of genotypes in aquatic environments that are highly variable. Helobdella stagnalis, a species commonly found in temporary ponds, for example, can produce either one or two generations in a year, depending on water temperature (Davies and Reynoldson 1976). Growth of another common wetland leech, N. obscura, was found to be more sensitive to prey variation between years than to variations in temperature (Davies 1991). In turn, this leech has been shown to strongly influence the spatial distributions of the benthic chironomid larvae upon which it feeds (Rasmussen and Downing 1988).

Species of leech that occur in temporary waters seem to have cosmopolitan distributions, probably related to the ease with which some are transported, attached to waterfowl and/or macrophytes, and also to their reproductive flexibility. Some are also capable of surviving droughts as adults in mucus-lined cysts.

Many of the oligochaete species that live in temporary waters are also cosmopolitan, and frequently occur in permanent waters. Also, there appears to be less separation of species between running- and standing-water habitats than is seen in many other animal groups. Further, most species seem tolerant of a range of sediment sizes (e.g. sand to mud), and niche separation may depend more on the organic and microbial contents of sediments, together with the intensity of interspecific competition (Brinkhurst and Gelder 1991). Temporary waters support species chiefly from the families Lumbriculidae, Enchytraeidae, Naididae, and Tubificidae.

Lumbriculids typically live among vegetation at pond and stream margins, and commonly appear in temporary water faunal lists. Adult Lumbriculus variegatus have the ability to form cysts, but, under experimental conditions, survival without water has not lasted longer than 4 days—although cysts protected by mineral particles may fare better. A different type of cyst is developed in response to freezing (Olsson 1981), and some lumbriculids have been observed to undergo fission while encysted. Tenagodrilus musculus, a new genus and species of lumbriculid has recently been described from a temporary pond in Alabama (Eckroth and Brinkhurst 1996), which perhaps underscores their lack of study in such habitats.

The Enchytraeidae is generally regarded to be a terrestrial family, however there are numerous records of these worms occurring in both saturated and moist sediments, including pond basins, stream beds, and marshes (e.g. Williams 1993; Yozzo and Diaz 1999).

Naidids occur in a wide variety of aquatic habitats but reach their greatest abundance in running waters with fairly coarse substrates. In standing waters they are more common in the littoral zone, especially on macrophytes with finely divided leaves; populations tend to be reduced on mud and silt substrates. Reproduction is primarily asexual, via budding and fragmentation, and may be controlled by food availability and water temperature. Among those populations that produce mature individuals, there appears to be only one sexual generation per year, with adults dying after cocoon production (Whitley 1982). There is some evidence to suggest that adults are more resistant to environmental stress and hence sexual reproduction is not used to survive adverse conditions (Brinkhurst and Gelder 1991). Genera that commonly appear in the wetland literature include Chaetogaster, Dero, Nais, Stylaria, and Pristina (e.g. Gathman et al. 1999; Taylor et al. 1999).

Tubificid worms represent a large heterogeneous family whose species often possess respiratory pigments that allow them to survive in oxygen-poor conditions such as the terminal stages of many temporary waters. Species often have interesting physiological and behavioural properties such that species mixtures may exhibit reduced respiration rates together with increased growth rates and assimilation efficiencies, compared with monocultures (Brinkhurst and Austin 1978). Because they often occur at very high densities, and through feeding on basin deposits, tubificids can exert a significant effect on sediment properties. Reproduction is typically by sexual means, but parthenogenesis is known in some species. Development can be rapid, depending on the food supply and water temperature both of which may promote considerable local variation in life cycles. Once mature, species frequently breed several times each year. Cosmopolitan tubificid species that commonly appear in the wetland literature include L. hoffmeisteri, Tubifex tubifex, and Branchiura sowerbyi (e.g. Euliss et al. 1999; Hall et al. 1999). Adults are capable of surviving drought by forming cysts. As such, T. tubifex, for example, has been shown to be able to survive for 14 days in dry sediment, but for up to 70 days if occasionally moistened. Once moistened, individuals can resume normal activities within 20 h (Kaster and Bushnell 1981). Presumably, the cocoons have some desiccation resistance, too.