Pristina leidyi Smith, 1896
Pristina leidyi are tiny, delicate, segmented (annelid) worms commonly found in pond and stream sediments. They can reach high densities in the field, especially in spring and summer, owing to their ability to reproduce asexually. Asexual reproduction occurs by paratomic fission, in which a new head and tail are intercalated in the middle of the body, forming a transiently linked chain of individuals which are behaviorally and physiologically coordinated. The fission process can be completed in as little as 1 to 2 days. They have excellent regenerative abilities, being capable of regenerating every part of their body.
Pristina leidyi are largely transparent, 2 to 4 mm long, and comprised of ~15 to 35 segments. In each segment, short bristles (chaetae) protrude ventrally and long hair-like bristles protrude dorsally, the dorsal chaetae of segment 2 being characteristically much longer than the rest. These chaetae are used for both locomotory and sensory functions. At the anterior end, a thin, elongated sensory proboscis protrudes from above the mouth. These worms feed on algae and detritus. (Alexa Bely, pers. comm. August 2010)
Pristina leidyi crawl on or within freshwater substrates, anchoring themselves with the bristles (chaetae) that protrude from each segment. Chaetae also serve a sensory function.
Pristina leidyi can regenerate every part of their body. A complete head or tail can be regenerated within just 3 to 5 days. For details of the regeneration process in this species, see Bely and Wray 2001, Bely and Sikes 2010, and Zattara and Bely in press.
Pristina longiseta is very similar in appearance to P. leidyi and some authors have questioned whether they are in fact distinct species (Brinkhurst and Jamieson 1971; Kathman and Brinkhurst 1999).
Pristina leidyi are tiny, delicate, largely transparent annelid worms. "Individuals are typically 0.15-0.20 mm in diameter, 2-4 mm in length, and comprised of ~15-35 segments. An asegmental cap of tissue is present at either end of the worm, the posterior cap being the pygidium, and the anterior cap being the prostomium (elongated into a proboscis in this species). A proliferative region just anterior to the pygidium, the posterior growth zone, produces new segments during normal growth. Fission zones are typically formed between segments 15 and 18." (Zattara and Bely in press)
"Segments have a muscular body wall, composed of a single-layered epidermis underlain by longitudinal and circular muscles, and a spacious coelomic cavity subdivided by septa into segmentally iterated pouches. In each segment, four bundles of chaetae protrude from the body wall, a left/right pair of ventrolateral bundles and a left/right pair of dorsolateral bundles" (Zattara and Bely in press). Ventral chaetae are hooked and relatively short, while dorsal chaetae are long and hair like. The dorsal chaetae of the second segment are much longer than the rest. "The digestive tract is comprised of an anteroventral mouth and buccal cavity which are unciliated followed by a protrusible pharynx, esophagus, stomach (in segment 7), and hindgut, all of which are ciliated. In segments posterior to segment 7, nephridia (ciliated excretory structures) are typically present laterally, beneath the body wall, on one or both sides of the segment. A dorsal cerebral ganglion lies above the foregut in segment 1 and a ganglionated ventral nerve cord runs along the length of the body, with one ganglion per segment. A nerve loop connects the cerebral ganglion and ventral nerve cord: a left/right pair of circumenteric connectives extends anterodorsally from the ventral nerve cord ganglion of segment 1, loops around the foregut, and connects in the cerebral ganglion, forming the cerebral commissure connecting the left and right halves of the brain. Three subepidermal peripheral nerve rings (nr1, nr2, and nr3, from anterior to posterior) occur in all segments posterior to segment 6. Ventrally each nerve ring connects to the ventral nerve cord, and fine extensions from these rings connect to epidermal sensory hair tufts. A main dorsal blood vessel and a main ventral blood vessel each run along the length of the animal, being connected to each other directly at the termini and by smaller transverse vessels along the length of the body" (Zattara and Bely in press).
Individuals are typically 2 to 4 mm in length and 0.15 to 0.20 mm in diameter (Zattara and Bely in press).
Pristina leidyi live in fine sediments at the bottoms of ponds, lakes, and streams (Alexa Bely, pers. comm. August 2010).
Pristina leidyi can feed on algae and detritus.
Pristina leidyi is distributed widely in North America (Brinkhurst and Jamieson 1971; Kathman and Brinkhurst 1999).
Pristina leidyi can reproduce both sexually and asexually, although most generations are asexual.
Sexual reproduction typically occurs in the fall and involves paired mating of sexually mature hermaphrodites. Clutches of fertilized eggs are deposited in protective cocoons. Eggs undergo direct development, producing juvenile worms which crawl out from the cocoons.
Asexual reproduction occurs by paratomic fission, in which a new head and tail are intercalated in the middle of the body. By this process, the original head end of the worm acquires a new tail, and the original tail end of the worm acquires a new head, producing of chain of transiently linked individuals which eventually separate. The region of intercalated tissue is referred to as a fission zone. An individual worm can have multiple fission zones developing at once: typically, subsequently formed fission zones are placed one segment anterior to the youngest fission zone. In actively growing worms, fission zones are typically positioned between segments 15 and 18.
During paratomic fission, most organ systems maintain continuity across the fission zone and linked individuals appear physiologically and behaviorally coordinated. The ventral nerve cord, gut, longitudinal blood vessels, and longitudinal muscles all remain continuous and functional until the very final stages of fission (Zattara and Bely in press). Until worms physically separate, only the anterior-most head ingests food. Until daughter worms separate, the behavior and locomotion of the worm chain appears to be coordinated exclusively by the anterior-most head.
(Alexa Bely pers. comm. August 2010).
Evolution and Systematics
Systematics and Taxonomy
The taxonomic treatment of the higher level groups including the genus Pristina has undergone some confusing changes. Until around 2002 the genus Pristina was placed in its own subfamily, the Pristininae, within the family Naididae. This original conception of the Naididae included only small, freshwater (or brackish water) oligochaetes, all of which could reproduce by fission. Subsequently, Erseus and Gustavsson (2002) and Erseus et al. (2002) proposed that the taxonomic rank of the Naididae should be lowered to a subfamily within the large family Tubificidae, based on its phylogenetic position according to analyses of both molecular and morphological data sets. However, because the family name Tubificidae was published later than the family name Naididae, the International Code of Zoological Nomenclature would normally require that the family name Naididae, the earlier name, take precedence. An appeal to retain the name Tubificidae for the clade comprised of the former Tubificidae plus the former Naididae (as well as several other taxa)--based on the confusion that would be generated by the name change ostensibly required by the Zoological Code--was rejected by the International Commission on Zoological Nomenclature (ICZN) (Erseus et al 2008). Further complicating this story, the former Naididae now appears not to be a monophyletic group because the monophyletic subfamily Pristininae is not recovered in phylogenetic analyses as the sister group to the remainder of the former Naididae (which form another monophyletic group) (Envall et al. 2006; Bely and Sikes 2010). Currently, therefore, the genus Pristina is treated as the sole genus within the subfamily Pristininae within the family Naididae (sensu Erseus et al. 2008).
Several molecular phylogenetic analyses have inferred the position of Pristina relative to other aquatic oligochaetes (Bely and Wray 2004; Erseus and Kallersjo 2004; Sjolin et al. 2005; Envall et al. 2006; Erseus et al. 2010).