Dynamene bidentata (Adams, 1800)

Vieira, Pedro E., Queiroga, Henrique, Costa, Filipe O. & Holdich, David M., 2016, Distribution and species identification in the crustacean isopod genus Dynamene Leach, 1814 along the North East Atlantic-Black Sea axis, ZooKeys 635, pp. 1-29 : 4-6

Dynamene bidentata (Adams, 1800)

Restricted synonymy.

Oniscus bidentatus : Adams (1800).

Naesa bidentata : Leach (1815).

Dynamene bidentata : Holdich (1968a, b, c, 1969, 1970, 1971, 1976); Kussakin (1979); Harrison and Ellis (1991).

An extensive synonymy was given by Holdich (1968a, c) for citations prior to 1968.

Material examined.

Specimens have been examined from 129 locations in the NE Atlantic, mainly from the British Isles, Channel Islands, France, Spain, Portugal and Morocco - see the Suppl. materials 1 and 2. A number of literature records have been included where the diagrams clearly indicate this species. In addition, there are 76 records from the NBN database.

Key morphological characters.

Body convex; in stage 8 males the pleotelsonic boss is large and bilobed, the two halves are separated by a wide v-shaped groove; the arms of the bidentate process taper to a point, and are sparsely rugose dorsally (Fig. 2 A–B). In stage 7 females the pleotelsonic dome is smoothly rounded in side view and the pleotelsonic foramen is open and flush with the edge of the pleotelson (Fig. 3 A–B). In populations from Atlantic coasts the smooth outline of the pleotelsonic dome in females and juveniles is key to separating this species from Dynamene magnitorata and Dynamene edwardsi , where it is keeled in side view. Further details are provided by the scanning electron microscope pictures of the posterior body of a stage 8 male and a stage 7 female in Holdich (1976).

Size.

Adult males (stage 8) typically 7.0 × 3.0 mm, although specimens 10 mm in length have been seen; pre-ovigerous females (stage 7) typically 6.0 × 2.9 mm.

Life-history.

There are eight life-history stages in both males and females ( Holdich 1968b). Sexual dimorphism becomes apparent in stage 6 males with the appearance of a very small bidentate process, this increases in size at the seventh, and is fully developed by the eighth and terminal stage (Figs 1D, 4-lower row 6-8). This process is absent from juveniles and females (Figs 1A-B, 4-upper row 6-8, 2 A–M). Juveniles and females up to and including stage 7 are very similar to each other morphologically. At the moult to stage 8 females become ovigerous and are very similar morphologically between the species. Their mouthparts are strongly metamorphosed, and they die after releasing their broods ( Hansen 1905, Holdich 1968b, 1971). Stage 8 males live for two breeding seasons, at least in the British Isles, and remain in their cryptic habitat for the entire period without apparently feeding ( Holdich 1971). Those in their second year are recognizable from the growths of algae, and sometimes serpulids, on the pleotelson.

Habitat.

All stages can be found on a wide variety of mid- to lower littoral algae, and also in rock pools in the upper littoral zone. Fenwick (pers. comm., July 2016) has found this species commonly amongst lower shore and sublittoral coralline algae in Cornwall, and he has also recorded adults from under large lower shore pebbles. Stage 7 females and stage 8 males move from the algae into cryptic habitats, such as crevices and empty barnacle tests, particularly Balanus perforatus , to breed ( Holdich 1970, 1976). Stage 7 females moult into stage 8 females within such a habitat and reach peak numbers in April/May each year ( Holdich 1968b).

Colour.

Some degree of camouflage in the algal habitat is given by green, yellow and brown ‘uniformis’ phenotypic varieties, and this is enhanced by the development in some individuals of patterns of white or red, dorsal, non-adaptable chromatophores ( Tinturier-Hamelin 1962, 1967, Holdich 1969, Arrontes 2009). In the past some workers have given specific status to the red and green colour varieties, e.g. rubra and viridis (see Holdich 1968c). Adult males are particularly colourful when found amongst red algae on the lower shore, with the margins of the body segments and uropods bordered in orange.

Geographical distribution.

The distribution of this species shown in Holdich (1970, 1974) has been extended by the present study. It occurs from the Shetland Islands to Tarfaya in western Morocco and Tenerife and Gran Canaria in the Canary Islands, which are the only two records of the species in Macaronesia (Fig. 5A). Within this range Dynamene bidentata occurs in the north, northwest (including the outer islands), west and south coasts (as far as the Isle of Wight) of Great Britain, around Northern and Southern Ireland, the Channel Islands, northwest France, Atlantic Iberian Peninsula and in northwest Africa. Arrontes (1991) cites Dynamene bidentata as being the most abundant isopod species on shores in northern Spain. It is the only species present in the British Isles (with the exception of a single record of Dynamene magnitorata in southern England). It is particularly common in SW England and SW Wales, especially where the large barnacle, Balanus perforatus is present. There is one recent record for north-eastern England, which may be the result of a stranding, as are records for The Neth erlands, where it is not considered indigenous ( Holthuis 1956). The closest record to the Mediterranean of Dynamene bidentata is Tarifa, in southern Spain ( Guerra-García et al. 2011, Izquierdo and Guerra-García 2011, Guerra-García et al. 2012, Torrecilla-Roca and Guerra-García 2012).

Remarks.

Maggiore and Fresi (1984) described Dynamene bidentata from the Gulf of Naples (publishing descriptions and figures), and several authors (e.g., Castelló and Carballo 2001, Castellanos et al. 2003, Junoy and Castelló 2003) have used Maggiore and Fresi’s (1984) observations to justify their findings of Dynamene bidentata in the Mediterranean. Yet, examination of the single specimen found by Maggiore and Fresi (1984) showed that it was in fact a Dynamene magnitorata .

A lot of confusion regarding the identification of Dynamene bidentata was caused by Torelli (1930) who figured what she called Dynamene bidentata (a stage 8 male and a stage 8 ovigerous female), from the Bay of Naples, Italy. Omer-Cooper and Rawson (1934) used Torelli’s figures to illustrate Dynamene bidentata from Britain, which was then proliferated in some British identification guides, e.g., Barrett and Yonge (1964), although this has been corrected in more modern guides, e.g., Hayward and Ryland (1995). Pauli (1954) also used Torelli’s figures to illustrate Dynamene bidentata from the Black Sea. Holdich (1968a) collected material from Naples and decided that Torelli’s figures were in fact of a new species, commonly found in the Bay of Naples, which he named Dynamene torelliae Holdich, 1968. However, Kussakin (1979) decided that Dynamene torelliae was in fact synonymous with Dynamene bicolor (Rathke, 1837). This species was in fact unknown to Holdich at the time of his studies.

Databases we have consulted indicate that Dynamene bidentata commonly occurs around Northern and Southern Ireland. However, we could only find one modern published record, i.e., de Grave and Holmes (1998) from Lough Hyne in County Cork.

Unlike most other isopods, stage 8 male Dynamene bidentata do not have appendix masculina on the endopods of the second pair of pleopods, this is also the case in the other Dynamene species. This phenomenon has also been noted by Messana (2004) in Sphaeroma terebrans Bate, 1866. It is very difficult to observe mating in Dynamene due to the cryptic habitat of the adults. It is probable that sperm are released directly into the marsupium as the eggs are laid.