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Marine Chaetonotid gastrotrich of genus Heterolepidoderma , probably H. ocellatum, from the superficial intertidal benthos of marine estuary Accabonac Harbor at Louse Point launching ramp. The biotope is clean medium grain sand. Imaged in Nomarski DIC on Olympus BH2 using SPlan 40x objective plus variable phone camera cropping on Samsung Galaxy S9+.

Gastrotricha are microscopic (0.06-3.0 mm in body length) free-living, acoelomate, aquatic worms, characterised by a meiobenthic life style. In marine habitats they are mainly interstitial, whereas in fresh waters they are ubiquitous as a component of periphyton and benthos and to a more limited extend also of the plankton. In marine sediments, gastrotrich density may reach 364 individuals/10 cm2 ; typically they rank third in abundance following the Nematoda and the harpacticoid Copepoda, although in several instances they have been found to be first or the second most abundant meiofaunal taxon" (1). "Gastrotricha are considered to have originated in the sea (Kieneke et al. 2008). However, our understanding of how they adapted from marine to freshwater ecosystems is still incomplete" (2).

"In aquatic environments the ecological role of the gastrotrichs is realised within the microphagous,detritivorous, benthic community. Like free-living nematodes, gastrotrichs swallow their food, which is made up of microalgae, bacteria and small protozoans, by means of the powerful sucking action of the triradiate muscular pharynx, and in turn they are preyed upon by turbellarians and small macrofauna. The phylum is cosmopolitan with about 820 described species grouped into two orders: Macrodasyida, with some 250 strap-shaped species, all but two of which are marine or estuarine, and Chaetonotida with some 450 tenpin-shaped species, two thirds of which live in freshwater. Macrodasyida include 7 families and 32 genera, whereas Chaetonotida counts 8 families and 30 genera. However due to the numerous species, and at least three new genera that wait to be described, these statistics should be considered as very conservative, particularly for the Chaetonotida" (1).

"Currently, approximately 820 Gastrotricha species have been described, divided into two orders: Chaetonotida Remane, 1925 [Rao & Clausen, 1970] and Macrodasyida Remane, 1925 Rao & Clausen, 1970. The representatives of Chaetonotida inhabit fresh, brackish and marine waters, while the Macrodasyida species almost exclusively inhabit marine waters. Only five taxa (including two fully described and named species) among all known Macrodasyida have been found in fresh waters (Rutner-Kolisko 1955; Kisielewski 1987a; Garraffoni et al. 2010; Todaro et al. 2012; Araújo et al. 2013; Kånneby et Wicksten 2014)" (2).

"In marine environments, the majority of gastrotrich species are interstitial dwellers, preferring especially fine- to medium-grained, clean sand. Only a small number of known species have adapted to other marine habitats, such as sediments rich in organic matter, rocky bottom, macrophytes or even hydrothermal vent zones (Hummon 1982; Kisielewski 1990; Kieneke et Zekely 2007; Kolicka et al. 2014)" (2).
"Heterolepidoderma is undoubtedly a genus of systematic and biogeographic interest, being one of few gastrotrich genera that include both marine and freshwater species" (3). Genus: Heterolepidoderma Remane, 1927 Typus generis: Heterolepidoderma ocellatum (Mečnikow, 1865) Terra typica: Russia. Scales with a keel. Common; freshwater, brackish-water, marine: epibenthic, periphytic, interstitial.

Heterolepidoderma is a genus of gastrotrichs belonging to the family Chaetonotidae. Heterolepidoderma Remane, 1927 is considered a polyphyletic genus (Kieneke et al. 2008, Kånneby et al. 2012, 2013) (2). "The genus Heterolepidoderma Remane, 1927 contains 36 valid species, 22 of which are freshwater1 (Balsamo et al. 2009; Kånneby 2011; Kånneby et al. 2012) and 14 marine or brackish (Hummon & Todaro 2010; Kånneby 2011; Kolicka et al. 2015). Heterolepidoderma is characterized by having keeled scales, i.e., scales equipped with strong median longitudinal keels (Schwank 1990; Kisielewski 1991), although keeled scales have also been reported in freshwater species for the genera Aspidiophorus, Chaetonotus and Ichthydium (e.g. Kisielewski 1991; Kånneby et al. 2009)"(3). Thus, "the main character which allows classification of a taxon as a member of genus Heterolepidoderma Remane, 1927 is body covering by elongated and keeled scales without posterior notches (e.g. Remane 1927; Schwank 1990; Kisielewski 1997; Todaro et Hummon 2008)" (1,2).

I found a pair of ocellar granules in my specimens which measured 90 um in length when fully extended. There was also a cuticular pharyngeal reinforcement. I suspect that my observation is Heterolepidoderma ocellatum Metschnikoff,1865, a relatively small sized species, with five-lobed head and a pair of ocellar granules. This species is widely distributed in Europe (see e. g. Greuter 1917; Mola 1932; Remane 1935–36; Valkanov 1937; Rudescu 1967; Martin 1981; Schwank 1990; Kisielewski 1998) and is also reported from Canada (Schwank 1990), United States (Schwank 1990), Japan (Saito 1937) and Brazil (Kisielewski 1991) (4). The species is reported in freshwater as well as marine biotopes and listed in the World Register of Marine Species (5). H. ocellatus is "a relatively small sized species, with five-lobed head and a pair of ocellar granules. Sporadic absence of ocellar granules have been reported in some populations (Schwank 1990; Fregni et al. 1998). Two pairs of sensory bristles present, each of the posterior pair anchored by a double-keeled scale. Dorsal body surface covered by elongated elliptical keeled scales, smaller in size in head and neck regions. Interciliary area naked except for a pair of keeled terminal scales and 1–2 transverse rows of 3–4 keeled scales. Pharynx with small swellings at both ends" (4).

1. An overview and a dichotomous key to genera of the phylum Gastrotricha M. Antonio Todaro* and William D. Hummon. Meiofauna Marina, Vol. 16, pp. 3-20, https://www.researchgate.net/publication/236021984