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(Ström, 1768)

Overview
Diagnosis: Procerodes littoralis (Ström, 1768) can be recognized easily by its head shape and pigmentation and by its rapid "looping" type of locomotion.
Habitus: Live specimens usually 3-7 mm long and 0.75-1.25 mm wide (Böhmig 1906, Curtis 1908, Wilhelmi 1909, Luther 1961), sometimes measuring up to 9 x 1.5 mm (Ball and Reynoldson 1981). Head broad and slightly convex, with two distinct tentacles. In front of the eyes the head shows a constriction, posteriorly of which the body gradually increases in diameter. The greatest breadth is generally reached somewhat before the rounded posterior end. The colour of the dorsal surface is variable but usually greyish brown, pale brown, or dark brown with a more or less pronounced mottled appearance. In a large number of specimens pigmentation is in the head region restricted to three stripes, which converge between the eyes. Areas between these stripes are unpigmented, giving the appearance of pale stripes of which the lateral ones contain the eyes. Some animals show two narrow pale lines which extend from the two unpigmented patches anterior to the eyes to the copulatory apparatus. These lines should not be confused with the two dorsal nerve cords which occasionally may be visible as two narrow and pale stripes. The ventral surface of the body is much paler than the dorsal surface; occasionally specimens may be completely devoid of pigment.

Alimentary System
The pharynx is large, measuring about one-third or more of the body length. The inner circular muscle layer of the pharynx is generally the thickest of all, but the difference with the outer circular muscle layer is not always pronounced. The anterior ramus of the intestine extends anterior to the eyes and gives rise to a pair of pre-ocellar diverticula. The total number of diverticula on one side of the body is about 18-22 (Böhmig 1906). The posterior rami of the intestine do not fuse in the hind end of the body (Böhmig 1906, Wilhelmi 1909).

Male Reproductive System
The numerous testes are situated dorsally between the diverticula of the intestine, the follicles being irregularly distributed throughout the body. In the pharynx region the vasa deferentia enlarge to form false seminal vesicles which narrow again towards the penis bulb. The vasa deferentia enter the penis bulb separately, uniting only at the base of the penis papilla. The penis papilla is conical and has a ventro-caudal orientation. The papilla is situated in a spacious male atrium which opens into a small common genital atrium. The ejaculatory duct is wide and opens at the tip of the penis papilla. The lining epithelia of the atria and the penis papilla are underlain with circular and longitudinal muscle fibres.

Female Reproductive System
The paired ovaries are situated directly behind the brain, dorsally to the ventral nerve cords. In the hind end of the body the oviducts fuse to form a common oviduct, which opens into the diverticulum of the bursal canal. This diverticulum receives the openings of shell glands. The vitellaria are distributed throughout the body and extend anterior to the ovaries. The bursal canal runs from the large copulatory bursa obliquely towards the common genital atrium. The bursa is surrounded by a layer of interwoven muscle fibres.

Eyes
The pigment cups contain three, occasionally four, retinal cells; eye lenses are absent.

Reproduction
Karyology: The diploid complement consists of 14 metacentric chromosomes, gradually decreasing in size (Ball 1976, Galleni and Puccinelli 1979, 1984).

Life Cycle:

The spherical, unstalked coccoons are about 1 mm in diameter. Cocoons may be found under stones or other submerged objects, e.g. on the inner surface of dead Mussels (Mytilus). Cocoons are laid from April to October.

Ecology
P. littoralis' main biotope are freshwater streams running through the intertidal belt. Here the species occurs under stones or other objects. Further, the species may be found in coarse sand and gravel in the higher part of the intertidal zone, at a depth of 10-20 cm, and in stagnant rock pools (Den Hartog 1968). On the south-west coast of Finland P. littoralis has been found on Wrack (Fucus) at a depth of about 8-8.5 m, rarely at 10 m (Luther 1961). Occasionally, the species may be found on algae such as Kelp (Laminaria) and Sea-lettuce (Ulva) (Johnston 1865, cited in Bock 1926, Wendt 1888). P. littoralis has a great salinity tolerance and the ability to withstand sudden salinity changes.

Distribution
Type locality: Söndmör, Norway (Bock 1926). The species occurs on the Atlantic coast of North America, from Newfoundland to New Haven (Conn.). Along the Atlantic coast of Europe, P. littoralis occurs from Murmansk to the southernmost part of France (St. Jean-de-Luz, Material Examined), on the coasts of the United Kingdom, as well as on those of Skagerrak, Kattegat, Baltic Sea, Gulf of Bothnia and Finland. According to Ax (1959/60) the stated occurrence in the Black Sea (Sernow 1909, Sabussowa 1945) must be considered dubious. The species has also been collected from Iceland (Steinböck 1938, Material Examined).

Material Examined
R.O.M.: C69, Leighton Neck, 28.08.1973, transverse sections on 4 slides; C48.3, Canada, N.S., 8.08.1973, sagittal sections on 3 slides; C46.3b, St. Marthe, Gaspe' Peninsula, Quebec, 25.06.1971, sagittal sections on 3 slides; C46.3a, ibid., sagittal sections on 3 slides; C8b, Rocky Harbour, Newfoundland, 1.08.1969, sagittal sections on 2 slides.
Private collection I. R. Ball: St. Andrews, New Brunswick, 1970, sagittal sections on 3 slides.
Collection J. Wilhelmi, now housed in Z.M.B.: P."wheatlandi": sections of 7 specimens from Massachusetts.
B.M.N.H.: 1948.7.28.13-20, Iceland, 1947, sagittal sections on 2 slides; ibid., horizontal sections on 1 slide; 1948.7.28.1-12, Bay Bulls, Newfoundland, June 1935, sagittal sections on 1 slide.
M.N.H.N: V7-A0718-1, St. Jean-de-Luz, France, 01.1932, sagittal sections on 3 slides; V7-A0718-2, sagittal sections on 3 slides; V7-A0718-3, sagittal sections on 3 slides.

Type Material
No type specimen available. Following Graff and Lang, Bock (1926) showed unequivocally that Ström's (1768) description should be recognized as the first account of the species.

Procerodes littoralis