Dynamene magnitorata Holdich, 1968

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 : 10-12

Dynamene magnitorata Holdich, 1968

Restricted synonymy.

Dynamene magnitorata : Holdich (1968).

Dynamene bidentata : Monod (1932); Maggiore and Fresi (1984).

Dynamene magnitorata : Holdich (1968a, 1970, 1976).

Material examined.

Specimens were examined from 52 locations in the NE Atlantic, and four countries in the Mediterranean - see the Suppl. materials 1 and 2. A number of literature records have been included where the diagrams clearly indicate this species.

Key morphological characters.

Body convex; in stage 8 males the pleotelsonic boss is large, bilobed, with the two halves separated by a narrow groove; the arms of the bidentate process are of similar width along their lengths and are dorsally tuberculate ( Holdich 1976, fig. 3A, B; Fig. 2 C–D in this paper). In stage 7 females the pleotelsonic dome is keeled in side view and the pleotelsonic foramen is flush with the edge of the pleotelson (Fig. 3 C–D). 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). The females of this species are very difficult to separate from those of Dynamene bicolor . See keys to stage 8 males and stage 7 females.

Size.

Adult males (stage 8) typically 4.25 × 2.25 mm, pre-ovigerous females (stage 7) typically 4.0 × 2.0 mm.

Life-history.

A comparison of the life-histories of Dynamene bidentata and Dynamene magnitorata from two Atlantic coast locations was made by Holdich (1976). Only a limited number of Dynamene magnitorata specimens were available but it showed that this species has a similar sequence of seasonal events (see description for Dynamene bidentata ). However, whereas Dynamene bidentata stage 8 males live for two breeding seasons, those of Dynamene magnitorata may only live for one.

Habitat.

A mid- to lower littoral and shallow sublittoral species, although sometimes recorded from deeper water. Its range occasionally overlaps that of Dynamene bidentata . Juveniles are found associated with a wide range of littoral and shallow water algae, particularly Corallina sp., Rhodomenia palmata , Chondrus cripspus and Gigartina stellata . Adults have been found in empty tests of Balanus crenatus , amongst ascidians, and in channels within sponges (including those associated with eel grass beds). In the Roscoff region (northern France) adults were frequently found within the encrusting sponge, Halichondria sp. In the Azores ( São Miguel island) adults have been found sublittorally in the empty tests of Megabalanus azoricus , as well as intertidally among algae on the islands of Terceira, São Miguel and Santa Maria. On Fuerteventura (Canary Islands) adult males were caught using a surface dip net. In the Chafarinas Islands off Mediterranean Morocco they have been recorded from 0.0m down to 20.0 m on a variety of algae. Like Dynamene bidentata ( Harvey et al. 1973), Dynamene magnitorata adults were found to have a tolerance to high air temperatures, i.e., 38 °C ( Holdich 1976). However, survival at 5 °C was much lower for Dynamene magnitorata compared to Dynamene bidentata ( Holdich 1976) and this may be the reason it has not colonized more northerly regions.

Colour.

Individuals exhibit a wide variety of colours, often matching the colour of their background, the predominant colours being coralline-pink and brown, rather than the greens and yellows seen in Dynamene bidentata . Individuals sometimes exhibit polychromatism caused by white, dorsal, non-adaptable chromatophores, as seen some other species ( Holdich 1969, 1976).

Geographical distribution.

The distribution of this species shown in Holdich (1970) has been extended by the present study. It has been recorded from southern England (a single specimen only that may be the result of a stranding), the Channel Islands, around the coasts of Brittany, the Atlantic Iberian Peninsula and northwest Africa, the islands of the Azores, Canary Islands and Madeira in the Macaronesian archipelagos, and in the Mediterranean along the European and African coasts, and also Egypt (Fig. 5E).

Remarks.

Almost all the Dynamene specimens found in the Azores during recent field work belonged to Dynamene magnitorata . However, Dynamene was less prevalent in the benthic community when comparing with Canaries and Portugal (pers. obs., unpublished data). Maggiore and Fresi (1984) described Dynamene bidentata from the Bay of Naples, but in fact examination of the specimen showed it to be a male Dynamene magnitorata . If the author’s had compared an actual Dynamene bidentata with their specimen then they would have realized this, particular as it is so much smaller than any known Dynamene bidentata specimen. Dynamene magnitorata has only rarely been recorded in the Mediterranean, i.e. twice in Spain, and once in each of Egypt, Italy, Monaco and Tunisia, although it was found to be common on the Chafarinas Islands off Morocco ( Castellanos et al. 2003) (see Suppl. material 1).