Microstomum edmondi, Atherton & Jondelius, 2018

Atherton, Sarah & Jondelius, Ulf, 2018, Microstomum (Platyhelminthes, Macrostomorpha, Microstomidae) from the Swedish west coast: two new species and a population description, European Journal of Taxonomy 398, pp. 1-18 : 6-8

Microstomum edmondi sp. nov.

urn:lsid:zoobank.org:act:E26B0ECB-C9DE-4FBB-A7D2-178A3C79F20C

Figs 2–3

Diagnosis

Microstomum with animal/zooid body length of 1700/750 µm. Conical pointed anterior end; blunt posterior end with numerous adhesive papillae along rim. Ciliary pits large, bottle shaped. Pigmented eyes absent. Dense field of cilia clearly covering epidermis. Preoral gut extending anteriorly to brain. Mouth distinctly encircled by glands. Protandrous hermaphrodite. Male reproductive system with single large testis. Vesicula seminalis circular to elliptical, 98 µm long, and containing the ends of numerous prostate glands in the distal part. Stylet approximately 67 µm long; shaped as an elongate, narrow funnel, slightly curved in one plane with a short, arched tip. Female reproductive system with single ovary and gonopore. Eggs develop caudally. GenBank accession number for partial COI sequences MF185700 -11.

Etymology

This species is dedicated to Edmond T. Atherton, father of the first author.

Material examined

Holotype

SWEDEN: ♂, Fiskebäckskil, Kristineberg Sven Lovén Center for Marine Research , 58°14′52″ N, 11°27′05″ E, 25 cm, 17 Aug. 2015, marine, eulittoral sand, S.Atherton leg. ( SMNH-Type-8890 , Genbank accession MF185704 ).

Additional material

SWEDEN: 3 ♀♀, 1 ♂, 1 vegetative, Munkedal , 58°27′31″ N, 11°41′10″ E, 15 cm, 15 Aug. 2015, S. Atherton and Y. Jondelius leg. ( SMNH-Type-8898–8902 , Genbank accession MF185700 -3) ; 2 ♂♂, 5 vegetative, same data as holotype (SMNH-Type-8891–8897, Genbank accession MF185705 -11).

Description

Microstomum with vegetative chains up to four zooids long, typically two. Maximum animal/zooid length 1700/750 µm. Body strap-shaped with constrictions between zooids; ratio of body width:length approximately 1: 4 in slightly compressed animal with two zooids. Anterior end conical pointed from level of ciliary pits. Posterior end blunt with row of 5–8-µm-long adhesive papillae along rim ( Figs 2C , 3 )

Pigmented eyes absent. Ciliary pits large and distinct; each with a wide 43 µm pore that abruptly narrows to a 15 µm wide tube; total length 55 µm ( Figs 2A , 3A ). Epidermis noticeably covered with dense field of cilia, 10–15 µm long ( Fig. 2B ). Nematocysts present. Rhabdite bundles to 45 µm long, occurring primarily in the posterior end of the animal ( Fig. 2D ).

Mouth distinctly encircled by glands ( Figs 2B , 3A ). Pharynx spherical, 150 µm in diameter connected to yellow-brown or red-brown intestine. Body colorless. Preoral gut extending well anteriorly to brain.

Protandrous hermaphrodite. Male reproductive system with single large testis connected by short vas deferens to male copulatory apparatus ( Fig. 3A ). Vesicula seminalis circular to elliptical, 98 µm long, and containing the ends of numerous prostate glands in the distal part ( Fig. 2D ). Stylet 67 µm long, shaped as a tube very slightly curved in one plane and narrowing to a short, arched tip; width at base approximately 17 µm and terminal opening 4 µm ( Figs 2D , 3C ). Male pore not seen.

Female reproductive system including single mediolateral ovary and ventral female gonopore ( Figs 2E , 3B ). Eggs develop caudally. Very small testis posterior to ovary present in some animals ( Fig. 2E ).

Remarks

Almost all sexually mature specimens of Microstomum edmondi sp. nov. displayed only male or female sexual organs. Only one individual ( Fig. 2E ) contained both an ovary with a single egg and what appeared to be a small testis, roughly a quarter of the size of the testes of the other male specimens. This individual otherwise lacked any discernible male copulatory apparatus (vesicula seminalis or stylet). Furthermore, animals with male reproductive anatomy were generally composed of two zooids with the sexual organs in the posterior zooid only, and animals with female anatomy were always solitary. Previous life cycle studies on M. papillosum Graff, 1882 and M. spiculifer Faubel, 1974 found that male genital organs first occur in asexually produced zooids that are otherwise well-developed, and sexual development then finishes in solitary individuals ( Faubel 1974, 1976; Hellwig 1987). Thus, M. edmondi sp. nov. has a protandrous hermaphroditic development. Protandrous development also occurs in other species of Microstomum , including M. bispiralis Stirewalt, 1937 , M. lineare (Müller, 1773) , M. papillosum and M. spiculifer ( Bauchhenss 1971; Faubel 1974, 1976; Heitkamp 1982; Hellwig 1987).

In general, M. edmondi sp. nov. can be easily distinguished by the shape of the male stylet. Of the 15 currently accepted species of Microstomum for which sexual anatomy is known, six ( M. bispiralis , M. giganteum Hallez, 1878 , M. groenlandicum Levinsen, 1879 , M. jenseni Riedel, 1932 , M. lineare and M. spiriferum Westblad, 1953 ) have spiraled or coiled stylets, two ( M. dermophthalmum Riedel, 1932 and M. spiculifer ) have strait stylets ( M. dermophthamum with a thicker distal end) and five ( M. crildensis Faubel, 1984 , M. ornatum Uljanin, 1870 , M. papillosum , M. septentrionale Sabussow, 1900 and M. trichotum Marcus, 1950 ) have distinctly and continuously curved or crescent shaped stylets. Furthermore, the stylet of M. edmondi sp. nov. clearly differs from that of M. melanophthalmum Steinböck, 1933 by its size (67 vs 30 µm, respectively), as well as the lack of very wide, almost flat proximal rims and mid-way 90° bend.

Of the species of Microstomum for which sexual anatomy is known, M. edmondi sp. nov. is most similar to M. hamatum Westblad, 1953 . The male reproductive system for both includes a large seminal vesicle and a 60–70-µm-long stylet with similar shape. Both species additionally include individuals with only a single large testis, although animals with smaller, paired testes were also found in M. hamatum ( Westblad 1953) . The stylet of M. edmondi sp. nov., however, can be distinguished by the narrower base, more gradual distal tapering and a small arched tip. Microstomum hamatum has a much broader funnel-shaped stylet ending in an 180° curve that forms a large hook. Other morphological differences include the pointed anterior end, large ciliary pits and lack of dark gray pigmentation in M. edmondi sp. nov. Finally, M. edmondi sp. nov. was collected from shallow, fairly clean marine sediments instead of deeper black mud.

Of the species of Microstomum for which the sexual organs remain undocumented, only eight inhabit marine waters ( M. bioculatum , M. breviceps Marcus, 1951 , M. davenporti von Graff, 1911 , M. lucidum Fuhrmann, 1896 , M. mundum von Graff, 1905 , M. rhabdotum Marcus, 1951 , M. rubromaculatum von Graff, 1882 , M. ulum Marcus, 1950 ). Microstomum edmondi sp. nov. is clearly most similar to M. ulum in that both have conically pointed anterior ends, large ciliary pits and large rhabdite bundles, and both lack eyespot pigmentation. Morphological differences occur in the shape of the posterior end, where a clear constriction sets apart a rounded adhesive tail plate in M. ulum while the posterior of M. edmondi sp. nov. is more paddle-like and blunt, and perhaps in the density of the locomotory cilia. Both species may be found in shallow marine sediments but are described from very distant locales, M. ulum being from the southwest Atlantic near the Island of São Sebastião, Brazil ( Marcus 1950) while M. edmondi sp. nov. was described from the Swedish west coast.

Figures

Fig. 2. Microstomum edmondi sp. nov. A. Entire body. B. Anterior end with focus on glands around mouth.C. Posteriorend with focuson adhesive papillae. D.Anterior end of male specimen.E.Anterior end offemale specimen.Abbreviations:a =auricles (ciliarypits);ap=adhesivepapillae; b=brain; eg= egg; i = intestine; m = mouth; o = ovary; ph = pharynx; r = rhabdite bundle; st = male stylet; t = testis; vs = vesicula seminalis.

Fig.3.Microstomumedmondisp.nov.,compositesketches.A.Entirebodyofmalespecimen.B.Posterior end of female specimen. C. Stylet.Abbreviations:a =auricles (ciliary pits); ap = adhesive papillae; b = brain; eg = egg; i = intestine; m = mouth; n = nematocysts; o = ovary; ph = pharynx; r = rhabdites; st = male stylet;t = testis; vs = vesicula seminalis.