D. digitata is one of the most common species of naidine oligochaetes. It is a round, segmented worm, ranging in size from 6 to 32 mm, and weighing around 0.06 mg. It is whitish in color, with color hues varying depending on the worm's diet and current stomachal contents. In the anterior part it has a head formed by four ventral chaete-bearing segments, plus a cone-shaped prostomium and small peristomium. It has no eyes. As in other member of the genus, the caudal region (pygidium) is modified into a characteristic ciliated branchial pavillion (also known as branchial fossa), which in D. digitata is well developed, and comprises four pairs of gills.This worm secrete transparent mucous tubes adhered to the substrates they inhabit, and tend to be found aggregated in clumps.
D. digitata is one of the most common and widely distributed naidine species. Easily distinguishable by it's characteristic posterior hand-like branchial pavillion, it can be found in ponds and slow stream reaches, and in the littoral and profundal region of lakes. It feeds mostly on detritus and algae, and reproduces asexually by paratomic fission. It has a characteristic unique helical swimming patern and a 180-degree turn reflex if poked at its tips. It is capable of fully regenerating lost parts after injury or partial predation.
Length: 6-32 mm; segments: 20-105. Dorsal chaetae begin on VI(5), bundles composed of 1 hair and 1 bifid needle, with upper tooth 1-2 times longer than the lower. Ventral chetae all bifid, those of II-V(1-4) longer and thinner than the rest, uper tooth 1.5-2 times longer than the lower, 3-6 per bundle; from VI(5) upper tooth thinner and shorter than lower, 2-5 per bundle, penial chaetae absent. Branchial pavillion usually with 4 pairs of gills, 1 small dorsal and 3 foliate ventral pairs; occasionally 1 or more pairs lacking, or 1-3 pairs cleft forming supernumerary gills.
D. digitata worms usually crawl over the substrate, or build tubes attached to it using mucous secretions. They usually agreggate in clumps before tube-building. Their tubes are usually open on both sides; worms are usually found inside the tubes, with their posterior end extending out and the branchial pavillion fully extended and moving water around the gills. Worms don't leave their tubes often, but they can be forced out by carefully poking the anterior tip of the head with a blunt pin or forceps, causing the worm to back up its anterior end and wriggle tail first out of the tube.
D. digitata has a characteristic escape reflex followed by a unique swimming mode. If touched on the anterior or posterior end, it will turn 180 degrees and start swimming. It swims with a very particular helical motion not normally seen in other annelids. While the escape reflex stimulus and response is mediated by a system of giant-axon fibers lying inside the core of the worm's ventral nerve cord, swimming movement is most likely coordinated by non-giant nerves.
As other annelids, D. digitata grows in length by adding new segments at its posterior end. New segments are formed in a cell proliferation ring that is located right in front of the pygidial branchial pavillion. The rate of growth is strongly influenced by food availability. While it doesnt seem to be a limit to the number of segments a given individual may have, longer worms are very likely to undergo paratomic fission, intercalating a number of segments inside a single mid-body segment and then splitting in two worms.
D. digitata is the most common species of what may be considered a single species complex of four species: D. digitata, D. trifida, D. nivea and D. obtusa. Most of the difference among them is in variations in the length of the needle teeth and details of the branchial pavillion, which may be more indicative of geographical or ecotypical races rather than reproductively isolated species. While resolving the question of whether they are separate species awaits further research, it seems liek for more purposes determination to the level of "species complex" should be enough,
Dero digitata, as other members of the Naidinae, are able to reproduce asexually through paratomic fission. In this developmental process, a new set of anterior and posterior ends differentiates in a fission zone that forms within a single segment, delimitating two daughter worms termed zooids. When development is complete, the zooids dettach resulting in two new worms. D. digitata is described as having a "slow" fission type, in which only one single fission zone is present at a single time; however, it has been a few observed having more than one fission zone, with the second zone occuring in the posterior zooid. During paratomic fission, an indefinite number of segments plus a branchial pavillion form in the anterior part of the fission zone; and 4 segments plus prostomium and peristomium form in the posterior part of the fission zone.
<p>D. digitata is able to survive a complete transverse cut, and to regenerate the missing end. If a portion containing the head is removed, it will regrow a new head: if less than four chaetea-bearing segments are removed, it will regrow that same number (homomerous regeneration). However, if more than four segments are removed, only four segments plus the asegmental tip regenerate (hypomerous regeneration). It takes between 3 and 4 days for a new head to regenerate. A time-lapse video of head regeneration can be found at http://www.youtube.com/watch?v=gmvg-kI8S-4</p><p>If a portion containing the tail is removed, it will regrow a new one, including a branchial pavillion and a posterior growth zone, along with a variable number of segments. This posterior regeneration process takes about 3-4 days, and it grades into normal growth once the growth zone activates and starts making new segments. There is no described relationship between the number of segment removed and regrown during posterior regeneration.</p>
D. digitata is most likely a cosmopolitan species, having been described in every continent but Antarctica.
D. digitata is a meiofaunal species that inhabits epiphytical or epilythical niches in lotic or slow lenthic ecosystems. In lakes, it is most commonly found in the littoral area, but it has also been described from profundal regions. In streams and rivers, it is more common on middle or lower reaches with stony or gravely substrates, but it can be found in muddy deposit in ponds and sluggish bog streams.
D. digitata worms have a mostly detritivorous diet, based on data from gut content analysis. In laboratory cultures, they can be fed using grains, like cracked wheat or rolled oats. Worms feed by protruding the sticky pharynx, which becomes concave when applied to food material; retraction of the pharynx brings the adhered food into the mouth.
Population studies of D. digitata have found that there is a population size anual cycle with yearly peaks in worm abundance. During those peaks, densities as high as 900 ind/m2 have been estimated. Peaks have been found both in spring and early fall. Population size change is achieved mostly through asexual reproduction (see below), with none to more than half the individuals having a fission zone. Density has been found to be positively correlated with water temperature in the field.
As in other naidines, D. digitata propagates mostly through asexual reproduction. It does it through paratomic fission, in which a new set of anterior and posterior ends are formed in a fission zone at a mid-body segment. During fission, a new head formed by prostomium, pygidium and 4 chaetae-bearing segments develops, separated from a new tail formed by pygidial branchial pavillion plus an indeterminate number of segments by a groove-shaped fission plane. D. digitata has a "slow" fission type, and only very rarely shows more than one fission zone in a single worm. Fission rates vary with water temperature and food availability, and are also correlated with population density; at one alkaline bog stream, the mean population doubling time has been estimated in 62.4 days.
Worms mature sexually only during a narrow period of time, and only a small amount of the total population does so. The peak in abundance of sexually mature individuals matches the abundance peak of the whole population; it has been described as taking place in early fall in some localities, and spring in others. In some areas with a spring peak, the species is not found during the following summer months; those are termed winter populations. In others, worms can be found all year round. Gut degeneration has been observed in sexually mature individuals, suggesting that they die after sexual reproduction. "Cocoon" like structures containing embryos and remains from anterior parental segments have been found, but it is unclear whether these result from clitellar secretions, remains of mucous tubes or remains of the parent's cuticle. Observations suggest that eggs may be fertilized post-mortem by liberated sperm, but the evidence is inconclusive. Sexual reproduction is relatively rare compared to asexual reproduction, and it is most likely fulfilling a genetic recombination or drought/freezing survival function rather than a propagation strategy.