Sphaerodoridium minutum (Webster & Benedict, 1887)

Capa, Maria, Osborn, Karen J. & Bakken, Torkild, 2016, Sphaerodoridae (Annelida) of the deep Northwestern Atlantic, including remarkable new species of Euritmia and Sphaerephesia, ZooKeys 615, pp. 1-32 : 15-18

Taxon classification Animalia Phyllodocida Sphaerodoridae

Sphaerodoridium minutum (Webster & Benedict, 1887) Fig. 7

Ephesia minuta Webster & Benedict, 1887: 728-729, pl. IV, figs 64-66.

Sphaerodoropsis minuta .- Imajima 1969: 153-154, fig. 2; Imajima 2009: 77.- Hartmann-Schröder 1996: 237.- Moreira 2012: 39-41, fig. 13.

Sphaerodoum minutum .- Berkeley and Berkeley 1948: 27-28, fig. 34.

Sphaerodoridium minutum .- Lützen 1961: 415.- Capa et al. 2016: 12.

Material examined.

Lectotype: USNM 393, Eastport, Maine, United States, North Atlantic Ocean, coll. Webster, H. E. Paralectotypes: USNM 1407984 (11 ind. and 4 slides), Eastport, Maine, United States, North Atlantic Ocean, coll. Webster, H. E. Paratypes: USNM 22873 (29 ind., 3 for SEM) Eastport, Maine, United States, North Atlantic Ocean, coll. Webster, H. E.

Diagnosis.

Palps and lateral antennae digitiform, median antenna shorter and digitiform. Tentacular cirri digitiform. Eyes not observed. Parapodia with three (or four) parapodial papillae; compound chaetae with blades 4-5 times as long as maximum width on mid-body chaetigers.

Re-description.

Measurements and general morphology. Holotype 1.1 mm long, 0.6 mm wide and with 22 chaetigers. Body ellipsoid, ovoid in cross-section, with slightly flattened ventrum and convex dorsum (Fig. 7 A–C). Anterior end blunt, mid-body broad, slightly narrowing along posterior segments (Fig. 7A). Segmentation inconspicuous, tegument with transverse wrinkles (Fig. 7 A–C). Preserved specimen lacking pigmentation.

Head. Prostomium with five short appendages, including a pair of digitiform palps in ventral-most position, a pair of digitiform lateral antennae, similar in size and shape as palps, and a digitiform median antenna, shorter than lateral antennae and palps (Fig. 7C, F). A pair of tentacular cirri shorter than lateral antennae and palps, close to lateral antennae (Fig. 7F). A few rounded small papillae scattered around head appendages (Fig. 7F).

Tubercles. First chaetiger with eight dorsal macrotubercles; following chaetigers each with one transversal row of dorsal macrotubercles increasing to 10-12 tubercles per segment from chaetiger 5 (Fig. 7 B–C). Macrotubercles spherical to club-shaped with a short and smooth stalk (Fig. 7 D–E); all macrotubercles similar in shape and size. Additional spherical and sessile papillae in different sizes over dorsum, arranged in 2-3 irregular transversal rows per chaetiger; 20-30 papillae on each mid-body chaetiger (Fig. 7 A–C). Ventral surface with spherical papillae in different sizes, arranged in 2-3 transversal rows in a zig-zag pattern, with about 20 per segment in mid-body; numbers decreasing towards posterior end (Fig. 7A, C).

Parapodia. Parapodia sub-conical, increasing in size towards chaetiger 3 (Fig. 7C), around 2 times longer than wide (Fig. 7G, H). Acicular lobe anterior to chaetae, digitiform to clavate, longer than parapodial papillae and projecting distally (Fig. 7C, G, H). Ventral cirri digitiform projecting 1/2 to 2/3 as long as acicular lobe on anterior and mid-body segments, almost as long as in posterior segments (Fig. 7C, G, H). Parapodia with three spherical to clavate papillae: one on anterio-dorsal surface, one on anterio-basal position (Fig. 7 F–H), and one on the posterior surface (Fig. 7 F–G).

Chaetae. All parapodia with 4-7 compound chaetae, arranged in a curved transverse row around acicular lobe (Fig. 7 F–I). Serrated, long blades, 4-5 times longer than maximum width, with a curved tip (Fig. 7 J–L), similar throughout.

Pygidium. Pygidium terminal, with one mid-ventral digitiform anal cirrus projecting beyond parapodia, and one pair of clavate anal cirri, at base on median cirrus (Fig. 7I).

Internal features. Specimens are all opaque after fixation and preservation and internal features not observable.

Reproductive features. Copulatory organs or eggs not seen in type specimens.

Variation.

Most paratypes show small, digitiform anterior appendages but the anterior end is often retracted and these are not easily discernible. This species seems very homogenous regarding the number and arrangement of epithelial tubercles. The exact number of macrotubercles and papillae is, however, difficult to assess in the type material because some of these are detached or because when macrotubercles are inflated and the stalk allows some reorganization, the exact number of tubercles per segment is imprecise. Parapodial papillae in mid-body segments vary between three and four as in some specimens a second papillae can be observed at the base of the posterior parapodial surface (Fig. 7G). Sexual dimorphism not observed.

Remarks.

The original description is detailed and accurate for most characters, including those more difficult to observe. In most type specimens the dorsal macrotubercles are lost or they fall off when being handled and consequently the total number of tubercles vary from the 10-12 macrotubercles in each transverse row as stated in the original description. Macrotubercles are described as being attached to the body by a "short neck" ( Webster and Benedict 1887), here interpreted as a short stalk. This feature justifies this species to be considered as a Sphaerodoridium and must have been overlooked in later studies dealing with this species, explaining why it was placed in Sphaerodoropsis for a long time ( Berkeley and Berkeley 1948, Imajima 1969, Hartmann-Schröder 1996, Imajima 2009, Moreira 2012). Dorsal cirri were described as present ( Webster and Benedict 1887), most likely referring to dorsal macrotubercles closest to parapodia.

Sphaerodoridium minutum is most similar to Sphaerodoridium guerritai Moreira & Parapar, 2015, described from the north eastern Atlantic, having as many dorsal macrotubercles, but Sphaerodoridium minutum lacks the characteristic papillae on the macrotubercle’s stalk that Sphaerodoridium guerritai possesses, and the latter species has long stalks compared to the short ones in Sphaerodoridium minutum . Further, there are clear differences between the two species in number and composition of parapodial papillae. Sphaerodoridium minutum is also similar to Sphaerodoridium evgenovi Gagaev, 2015, Sphaerodoridium kolchaki Gagaev, 2015, and Sphaerodoridium kupetskii Gagaev, 2015, all recently described from the Arctic Ocean, as they all share a similar arrangement and number of macrotubercles ( Gagaev 2015). The two latter species, Sphaerodoridium kolchaki and Sphaerodoridium kupetskii , have long macrotubercle stalks, hence they are different from Sphaerodoridium minutum . The seemingly short macrotubercle stalks in Sphaerodoridium evgenovi are rather similar to those in Sphaerodoridium minutum . However, Sphaerodoridium evgenovi has chaetae with somewhat shorter blades, lack parapodial papillae on the posterior surface, and also has parapodia much longer than wide compared to Sphaerodoridium minutum ( Gagaev 2015). The three species described by Gagaev (2015) and the one described by Moreira and Parapar (2015) are all similar to each other and show resemblance to Sphaerodoridium minutum . These two papers were published the same year and the authors were most likely not aware of each other’s work. These species should be compared in detail in order to assess the species delimitation ( Gagaev 2015).

Sphaerodoridium minutum has similar number of stalked macrotubercles to Clavodorum polypapillata ( Hartmann-Schröder & Rosenfeldt, 1988) described from Antarctica, and Clavodorum andamanense Bakken, 2002 described from Thailand. In Clavodorum polypapillata the original description overlooked stalked macrotubercles ( Moreira and Parapar 2011), and has been re-described to have a larger number of macrotubercles (12-17 per transverse row, compared to 10-12 present in Sphaerodoridium minutum ) and a large number of ventral papillae (>40 per segment compared to ca. 20 in Sphaerodoridium minutum ). Clavodorum polypapillata further has longer stalks than in Sphaerodoridium minutum ( Moreira and Parapar 2011). Clavodorum andamanense lacks papillae on the dorsum, unlike Sphaerodoridium minutum .

Sphaerodoridium minutum has been reported with a wide geographic distribution ( Berkeley and Berkeley 1948, Lützen 1961, Imajima 1969, 2009, Hartmann-Schröder 1996). In order to clarify the geographic distribution of this species careful examination of specimens reported under this name from other geographic areas is needed.

The validity and clear delimitation of Sphaerodoridium and Clavodorum Hartman & Fauchald, 1971 has been regarded as doubtful. Several authors pointed out the insubstantial generic differences between the two genera ( Bakken 2002, Moreira and Parapar 2011, 2015) and only recently reciprocal monophyly was not assessed in the first phylogenic hypothesis of the family ( Capa et al. in 2016). There are currently 10 species of Sphaerodoridium and another 10 of Clavodorum described from world-wide areas and from shelf to abyssal depths ( Capa et al. 2014, Moreira and Parapar 2015, Gagaev 2015). As the delimitation of the two genera is not clear, and species representing the two genera are phylogenetically nested in a group ( Capa et al. 2016), it will be necessary to assess all described species in both genera when comparing similarities.

Distribution.

This species is known from the New England region of the US. It has been reported from the North Atlantic (e.g. Hartmann-Schröder 1996), North Pacific (e.g. Berkeley and Berkeley 1948) and Japan ( Imajima 1969, 2009). The emer gence of descriptions of new species similar to Sphaerodoridium minutum suggests a review of the true identity of reported specimens is needed.