Taxon Attribute Profiles

Cybister tripunctatus

Family Dytiscidae

Predaceous Water Beetles

Introduction

Dytiscidae (predaceous water beetles) is one of the largest and most commonly encountered groups of aquatic beetles. Both adults and larvae are predaceous, and will attack a wide variety of small aquatic organisms. Although most species are small to medium sized, some adults can attain a length of 35 mm.

Taxonomy and Ecology

Synopsis of included taxa

Australia has a rich fauna of Dytiscidae, with 226 species in 42 genera (Gooderham & Tsyrlin, 2002). Watts (1978) supplied a revision of Australian Dytiscidae; and Pederzani (1995) provided keys to all dytiscid genera and subgenera for the world. Watts (2002) provided a series of easy to use keys to the genera of both adult and larval dytiscids. A checklist of Australian Dytiscidae is available on the Australian Biodiversity Information Facility website (Lawrence et al., 2002).

General overviews of Australian Dytiscidae, including biological and ecological information, are provided by Lawrence et al. (1991) and Gooderham & Tsyrlin (2002).

Life Form

Dytiscids have a characteristic appearance, and can generally be recognized by having a hard, smooth, oval body, without any ventral spine, having the hind legs flattened and with a fringe of hairs so that it can act like a paddle, and having long, thin antennae. In the water, dytiscids swim by moving their hind legs simultaneously, like oars, while the similar appearing Hydrophilidae alternate the movement of their hind legs.

Eretes australis	Lancetes lanciolatus
Megaporus howitti	Sternopriscus hansardii

Click to enlarge map

Distribution

Dytiscidae are all aquatic, and are common throughout the continent, with the greatest number of species found in the south-east. Adults are capable of flying to isolated habitats, which has allowed their spread to aquatic habitats throughout Australia.

Habitat

Dytiscids generally prefer slow moving or stagnant water, such as ponds, lakes, billabongs, dams, and pools at the edges of streams. They require atmospheric air, and the adult beetles go to the surface to gather air which they store in a chamber underneath their elytra (wing covers) to enable them to increase the time they can be submerged. Larvae lack this ability, but many species use a siphon in the form of long filaments at the end of the abdomen.

Status in Community

Both adults and larvae are predaceous. The adults are capable of eating through a normal mouth opening. Most larvae do not have a mouth opening, but have long, sickle like jaws which enable them to suck fluids out of their prey items. The larvae will attack animals much larger than themselves, and have been known to feed on other insects, crustaceans, worms, leeches, mollusks, tadpoles and small fish.

Reproduction and Establishment

Reproduction

A variety of mating behaviours occur in the Dytiscidae, and many of them reflect the fact that females are more selective in choosing a mate than males (Miller, 2003). Males have a variety of methods to achieve mating, and in some cases females have behaviours to resist mating, such as swift and erratic swimming when approached by a male. Some males have even developed sucker-shaped setae on the legs which allow them to grab females and prevent them from escaping during mating.

Dispersability

Adult dytiscids are capable of sustained flight, and often travel some distance to disperse and find new habitats. They generally fly in the evening or at night, and they use reflected light from a water surface as a method of finding a new habitat. They can be confused by artificial reflected surfaces (e.g. glass) or lights, and are often attracted to these sources rather than water.

Juvenile period

Adults generally lay eggs in the underwater stems of plants, using their ovipositor to cut the stem to deposit the eggs. Dytiscid larvae are very aggressive predators which attack a variety of prey. Pupation takes place out of the water. The larva forms a cell in damp soil near the water, and the adults return to the water after emerging.

Hyderodes sp. larva

Hyphydrus sp. larva

Rhantus sp. larva

Hydrology and Salinity

Salinity Tolerance

Gooderham & Tsyrlin (2002) and Chessman (2003) reported that Dytiscidae as a family are quite tolerant of high levels of salinity. However, Bailey et al. (2003) list several examples of specific dytiscids which have very narrow ranges of salinity tolerance and could be eliminated with minor rises in salinity levels.

Flooding Regimes

Alternating periods of flooding and drought could affect dytiscid populations, which need water for survival. The strong flying ability of adults will allow recolonization of aquatic habitats after periods of drought.

Conservation Status

As with most invertebrates, there is insufficient data on Dytiscidae to discuss conservation status in any intelligent manner. Many species are known from only a few specimens, but whether this reflects insufficient collecting or declining population numbers can not even be guessed at.

Uses

There are no records of Aboriginal use of dytiscids as food, although many insect species were eaten by Aborigines (Tindale, 1966). The large dytiscid Cybister explanatus is used as food in Mexico, where they are eaten roasted with salt and in tacos (Ramos-Elorduy & Pino, 1989).

Summary

Dytiscidae share the characteristics of macroinvertebrates that could make them suitable species for including in programs for monitoring water quality (Water and Rivers Commission, 1996; Chessman, 2003; Minnesota Pollution Control Agency, 2004). As of yet, we lack specific examples of their use in such programs.

List of MDB Species

Table 1. Dytiscidae recorded from the Murray Darling Basin (50 species in 19 genera).

References