Lumbriculus variegatus

Lumbriculus variegatus, sometimes known as blackworm (or even California blackworm), is the name given to at least three distinct species of worms which are identical in outward appearance. Until 2009, the species were regarded as one. ^[1][2]

These species of worms live in shallow water marshes, ponds, and swamps, feeding on microorganisms and organic material. They live throughout North America and Europe.

On average, an adult blackworm can have from 150 segments to 250, each of which has the ability to regenerate into a whole new individual when separated from the rest of the animal; in most populations, this is the primary mode of reproduction, and sexually mature individuals are exceedingly rare - in large areas (including Asia, most of North America and the greater part of Europe) sexually mature individuals have never been found.

Characteristics

Among the characteristics of the genus are a green pigmentation of the anterior end, its two-pointed setae, and its curious reflex escape mechanisms. When touched, Lumbriculus will attempt to escape, either by swimming in a helical ("cork-screw") fashion, or by reversing its body. The escape pattern used depends on where the worm is touched: anterior touch elicits body reversal, whereas posterior touch triggers helical swimming. Lumbriculus has a quick reflex that allows it to use its photoreceptors to escape. The photoreceptors can detect shadows and movement. The posterior end lifts out of the water and forms a right angle. It is then exposed to air and is used to exchange oxygen and carbon dioxide, although this exposes its posterior to its enemies. If the photoreceptors detect a shadow or movement, the posterior rapidly shortens in response to a threat.

Reproduction

Regeneration in L. variegatus follows a set pattern. If the regenerating segment originated less than eight segments from the anterior tip, this amount of segments are regenerated; if, however, the segment was originally from a more posterior position, only eight segments are regenerated. Posterior to the segment, a variable number of segments are regenerated, and the original segment undergoes transformation to become suited to the new, often more anterior position. L. variegatus is presumed to be holarctic in distribution, although in East Asia and North America (and perhaps other places as well) it is probably replaced by the other species of the genus (many of which are known from very small areas; Lumbriculus illex, for instance, is known only from three individuals found in a stream to the North of Vladivostok). Mature Lumbriculus can easily be told apart by a number of genital characters, but as few of the species in the genus ever become sexually mature, it is often difficult to know which species you are dealing with. A physical description of L. variegatus is, to a large extent, valid for the entire genus.

References