Modiolus cimbricus Ockelmann & Cedhagen 2019, sp. nov.

Modiolus cimbricus sp. nov.

urn:lsid:zoobank.org:act: FA5D6226-648B-44C5-A5BE-1E4D0F97BA67

Figs 1–2

Mytilus adriaticus – Petersen 1888: 126.

Modiola adriatica – Petersen 1893: 71. — Jensen & Spärck 1934: 75. — Jørgensen 1946: 288.

Modiolus adriaticus – Muus 1973: 84. — Rasmussen 1973: 272. — Christensen 1978: 49. — Wikander 1981: 73. — Bondesen 1984: 13. — Larsen et al. 2007: 425.

Diagnosis

Dissoconch shell substance thin and light. Shell shape in lateral aspect somewhat elongated, anteroventrally upturned often approaching a parallelogram, dorsal margin behind umbones straight in adult specimens; shell margin postero-dorsally stretched and narrowly rounded posteriorly, mid-ventrally straight or nearly so. Umbones strongly prosogyrate not much elevated above the dorsal margin. Shell surface with sharp growth lines and often with low and uneven commarginal ribs. Periostracum yellowish and varnished turning brownish in larger specimens. Short and simple byssal hairs on the periostracum occur mainly around the siphonal region. They are easily lost just as are grains glued to the shell. Shell inside glossy, neither adductor muscle scars nor pallial line are discernible. Ligament is sub-internal, not strong and its length between 35–40% of the adult shell length. The prodissoconch I is 400 µm. The species is edentulous after the nepioconch stage. The nepioconch usually turns whitish when it remains in adult specimens. Largest diameter of nepioconch is most often between 0.6–0.8 mm. Shell colouration varies greatly. Red only, red with blue (violet or purple), blue only, and presence of opaque white, usually as blotches, or absence of white. Red is by far the most common colour expressed as rays, strongest dorsally and posteriorly on the shell. Uncoloured specimens appear to be very rare. A well-developed bundle of byssus, strong, hair-like protein threads, originate from the posterior end of the foot and are used for attachment to sandgrains that form a covering around the shell. This species is referred to the genus Modiolus.

Etymology

The species name, ‘ cimbricus ’, is derived from the name of a people, the Cimbricans, believed to have been at home in the northern Jutland, Denmark.

Material examined

Holotype (Fig. 1)

DENMARK • a pair of cleaned valves (dimensions (mm): dissoconch shell length 13.3, height 7.3 and width 6.0, length of ligament is 4.9, and largest diameter of the nepioconch is ~0.65) (shell colour red rays on most of the shell, and with a diffuse violet sheen on central region of the shell); Øresund, between Denmark and Sweden, Ellekilde Hage; near 56°05′ N, 12°30′ E; 14–16 m depth; 21 Aug. 1965; KWO leg. and det.; sediment coarse sand; NHMD.

Paratypes

DENMARK • 54 pairs of cleaned valves (individuals taken alive 6.2–12.8 mm long and with the same colour pattern as the holotype); Øresund, northern part, off Odinshøj; 14–18 m depth; 21 Jul. 1965; KWO leg. and det.; sand; NHMD.

Additional material

Material with soft parts still present is available from different localities in the Kattegat. It is very difficult to specify the number of specimens, because most of samples consist of valves that are mixed in a glass tube. Deposition of the type series and additional material is at the Natural History Museum of Denmark. The new species was often kept in aquarium in the Marine Biological Laboratory, Helsingør, University of Copenhagen between 1958 and 1990.

DENMARK • several shells, including an individual within sand clump, in a glass tube; S Kattegat; before 11 Jul. 1990; KWO det.; bred in aquarium; NHMD • 5 tubes without label; spring 1995; KWO leg.; NHMD • Hornbaek bugt; 31 Aug. 1966; KWO leg. and det.; NHMD • Kattegat, stn 492x; C.G.Joh. Petersen leg.; KWO det.; NHMD • Samsø Flak, east; 12 Aug. 1986; KWO det.; NHMD • Kattegat, Kobberenden, Kanonbaaden Hauch, stn 303; 6½ fathoms depth; 8 Sep. 1885; C.G.Joh. Petersen leg.; KWO det.; NHMD • Kattegat, stn 136x, 303x, 492x; 1885; C.G.Joh. Petersen leg.; KWO det.; NHMD • Kattegat, Hellebaek; 1860; C.F. Lütken leg.; KWO det.; NHMD • Øresund, northern part, off Odinshøj; 16–17 m depth; 1 Dec. 1981; KWO det.; coarse sand, shells; NHMD • Kattegat, Fornaes lighthouse, Kanonbaaden Hauch, stn 328; 18 Sep. 1885; C.G.Joh. Petersen leg.; KWO det.; NHMD • Øresund, Hellebaek; 12 m depth; 11 Oct. 1960; KWO leg. and det.; NHMD • northern Øresund; 31 Aug. 1983; KWO leg. and det.; NHMD • Frederikshavn; 14 Jan. 1984; KWO leg. and det.; NHMD • Frederikshavn; 1949; G. Thorson leg.; KWO det.; NHMD • Anholt, Forskningsstyrelsen stn 48-1 and 48-4; 3 Sep. 1985; O. Norden Andersen leg.; KWO det.; rocky bottom; NHMD • same data as for preceding, Forskningsstyrelsen stn 57-1, 57-2, 58-2, 58-3, 58-4, 58-5, 62-1, 62-3 and 62-18; 29Aug. 1985; O. Norden Andersen leg.; KWO det.; rocky bottom; NHMD • same data as for preceding, Forskningsstyrelsen stn 60-2 and stn 60-5; 30 Aug. 1985; O. Norden Andersen leg.; KWO det.; rocky bottom; NHMD • same data as for preceding, Forskningsstyrelsen stn 62-5; 19 May 1985; O. Norden Andersen leg.; KWO det.; rocky bottom; NHMD • same data as for preceding, Forskningsstyrelsen stn 65-4; Aug–Sep. 1985; O. Norden Andersen leg.; KWO det.; rocky bottom; NHMD • Kattegat, Samsø, western Flak; Aug. 1986; KWO det.; sandy bottoms; NHMD • Kattegat, Øresund, off Odinshøj; 12–15 m depth; 11 Aug. 1966; C. Nielsen leg.; KWO det.; NHMD • various samples; Kattegat, Øresund; 12–22 m depth; 1958–1977; KWO leg. and det.; sandy bottoms with gravel; NHMD • E of Fyns Hoved; 55°37.5′ N, 10°39.2′ E; 16.5 m depth; M. Jensen leg.; KWO det.; stones, sand; NHMD • Kattegat, Trindelen; 5½ fathoms depth; C.G.Joh. Petersen leg.; KWO det.; NHMD • Kattegat, Hirtsholmene; 24 Jul. 1950; P.L. Kramp leg.; KWO det.; shell gravel; NHMD • Kattegat, between Samsø and Bolsaren; 6 Jun. 1876; G. Winther leg.; KWO det.; NHMD • Kattegat, Kanonbaaden Hauch, stn 79; 10 Sep. 1884; C.G.Joh. Petersen leg.; KWO det.; NHMD • Frederikshavn, NE of Hirtsholmene; 5 Aug. 1981; 10–12 m depth; KWO leg. and det.; coarse sand and shell gravel; NHMD • Frederikshavn, NE of Hirtsholmene; 7 Aug. 1969; KWO leg. and det.; sand; NHMD • same data as for preceding; 24 Jun. 1967; 6–8 m depth; KWO leg. and det.; sand; NHMD • Frederikshavn; 25 Aug. 1965; 5 m depth; J. Knudsen leg.; KWO det.; gravel; NHMD • Laesø; KWO det.; NHMD • Frederikshavn, NE of Hirtsholmane; 16 Aug. 1950; KWO det.; gravel; NHMD • Frederikshavn; 8 Jul. 1967; C. Nielsen leg.; KWO det.; NHMD.

SWEDEN • Kosterfjorden, N of Saltö; 5–15 m depth; AW leg.; KWO det.; NHMD • Kosterfjorden, Tjärnö; AW leg.; KWO det.; NHMD • Bonden, Kristineberg Marinebiological Station; 27 Jan. 1964; NHMD • Bohuslän; A.W. Malm leg.; KWO det.; NHMD • (labelled Modiolus adriaticus); Skagerrak, Bohus-Malmön, Kattesand; 1970; H.G. Hansson leg. and det.; in fishing net; GNM Gen. Kat. Nr. 2009- 22139 • (labelled Modiolus adriaticus); Skagerrak, Kosterfjorden, Kostergrundet; 15 fathoms depth; 2 Aug. 1968; AW leg. and det.; sandy bottom; GNM Gen. Kat. Nr. 69-12.448 • (labelled Modiolus adriaticus); Skagerrak, Koster; 16 fathoms depth; 4 Aug. 1869; A.W. Malm leg. and det.; GNM Moll. 5022 • S. Lovén leg.; AW det.; SMNH • AW leg. and det.; SMNH.

Comparison with other modioline bivalves in Skagerrak, Kattegat, and the English Channel

Morton (2015) gave an overview of the systematics of the superfamily Mytiloidea. Characters were compared with information from the following sources: Jeffreys (1863), Dautzenberg (1895), Jensen & Spärck (1934), Tebble (1976), Hayward & Ryland (1990, 1995), Poppe & Gotto (1993), Huber (2010) as well as our material from museum collections.

Modiolus cimbricus sp. nov. differs from M. adriaticus by its smaller size as its maximum size only reaches about half that of M. adriaticus. The ventral margin is less concave than in M. adriaticus. Its shell is often slightly more elongated and the periostracum slightly darker. The byssal hairs on the periostracum short and simple, but absent in M. adriaticus. Shell surface with sharp growth lines and often with low and uneven commarginal ribs, contrary to a sculpture of fine but irregular concentric lines, but indistinct growth lines in M. adriaticus. Ligament is sub-internal contrary to the narrow, external, deeply inset ligament in M. adriaticus. It also differs by its endobyssate lifestyle, contrary to M. adriaticus, which lives epibyssate, at least as adult.

A taxon described as Modiolus ovalis Sowerby, 1859 was synonymized with Modiolus adriaticus, but retained as a distinct variety by Jeffreys (1863: 117). The form ovalis differs from Modiolus cimbricus sp. nov. by reaching more than the double maximal shell length; by its more cylindrical shell shape; its proportionally straighter and longer ventral margin; its more evenly rounded dorsal margin; its darker shell colour – horncolour to brownish-yellow with fewer, darker and less obvious coloured rays. Dark individuals may even lack rays (M. a. var. ovalis 17636 NHMD). Its umbo reaches the anteriormost part of the shell. The form ovalis is Lusitanian and does not occur in the Skagerrak, Kattegat or the North Sea.

Modiolus gallicus Dautzenberg, 1895 is a poorly understood species from the French Atlantic and Channel coasts. It is synonymized with Modiolus barbatus (Linnaeus, 1758) by some authors or regarded as a variety of that species. It differs from Modiolus cimbricus sp. nov. by its larger shell size; thicker shell; more cylindrical shell shape; more concave ventral margin; thicker periostracum with prominent growth lines and covered with numerous long byssal hairs; darker and more uniform colour without violet or pink rays. As a Lusitanian form, it does not occur in the Skagerrak, Kattegat or the North Sea.

Modiolus cimbricus sp. nov. differs from M. modiolus by its smaller maximal size, thinner and brittle shell. Its anterior margin projects well beyond the umbo, but less so in M. modiolus. Its lighter periostracum has fewer and smaller periostracal spines, contrary to the pale brown periostracum in small individuals of M. modiolus. Its soft parts are red with brown gills, contrary to the orange-red soft parts, speckled with white and often tinted with brown, as well as its red and white foot in M. modiolus. The ligament is sub-internal contrary to external and deeply inset in M. modiolus.

Modiolus cimbricus sp. nov. differs from M. phaseolinus Philippi, 1844 by its smaller maximal size. Its anterior part projects well beyond the umbo. The shell outline is more trapezoid or oblong. The siphonal end exhibits a proportionally smaller semi-circle outline and is often more pointed than in the large, rounded end of M. phaseolinus. The ventral margin is straight or nearly so, contrary to concave in M. phaseolinus. It has a more prominent umbo. Its periostracum is thinner with fewer and much smaller byssal hairs. Soft parts light red with brown gills, contrary to a reddish-brown body in M. phaseolinus. Ligament is sub-internal contrary to external and deeply inset in M. phaseolinus.

Synonymy of Gibbomodiola

We regard Modiolus adriaticus as the closest relative of Modiolus cimbricus sp. nov. The first species has been moved to the genus Gibbomodiola Sacco, 1898 (Huber 2010; WoRMS 2019). Sacco (1898) described Gibbomodiola as a subgenus of Modiolus. His description is very short and with general characters which do not allow any clear distinction from other genera or subgenera. Other characters are impossible to see on his plate. Cox et al. (1969) retained it as a subgenus and used Sacco’s characters formulated as “elongately ovate, transversely arcuately gibbous, posteriorly broadly expanded”. Huber (2010) characterized the genus as modioliform, gibbous, smooth without periostracal hairs, but did not discuss it any further. The fact that Modiolus cimbricus sp. nov. has short and simple byssal hairs on the periostracum should exclude it from the genus Gibbomodiola. Jeffreys (1863: 116), who was a very careful observer, also reported “slight hairs” on the periostracum of M. adriaticus, but this character is not mentioned by other authors. Consequently, the validity of the genus Gibbomodiola is dubious and needs to be clarified. We therefore synonymize Gibbomodiola with the genus Modiolus.

In the same publication Sacco (1898: 41, pl. XI, figs 34–35) described the type species Gibbomodiola taurarcuata, a fossil from the Oligocene and wrote that it resembles the recent species Modiola arcuatula Hanley, 1843 (a synonym of Arcuatula arcuatula (Hanley, 1843)) from SE Asia and Modiola nitens Gould & Carpenter, 1857, a species of uncertain identity, sp. inquirendum, from California. Huber (2010) included other species in this genus: Gibbomodiola adriatica (Lamarck, 1819); Gibbomodiola albicosta (Lamarck, 1819) and Gibbomodiola biradiata (Hanley, 1843), all of them with photographs. Under the genus Modiola, Huber (2010) wrote “whether the 35 species recognized as Modiolus indeed all belong here, needs to be resolved by future workers”. This is also true for the species previously included in Gibbomodiola.