Dissodactylus

Dissodactylus is a complex which gathers closely phylogenetically dependant crabs and sharing behavioral and biogeographic characteristics. It counts 13 species distributed in 2 groups Monophylétique S corresponding each one to a distinct kind: Dissodactylus (9 species) and Clypeasterophilus (4 species). This complex exclusively counts symbiotic species of irregular sea urchin S - they are moreover the only symbiotic species of 'Pinnotheridae of irregular Echinidea -; for a species however ( Dissodactylus schmitti ), symbiosis is suspectée but not shown since the species is known only by one collected isolated individual “fortuitously”. The hosts concerned are as a majority of the clypéastéroides and belong to the family of the Mellitidae (case of 8 species of Dissodactylus ) or to that of the Clypeasteridae , and more particularly with the kind Clypeaster (case of the 4 species of Clypeasterophilus ). Among the Dissodactylus , the species D. primitivus has the appearance of an exception since it is associated with Spatangoïdes and more particularly with two species, grown Meoma ventricosa and Plagiobrissus . Occasionally, certain species of Dissodactylus of the Atlantic ( D. crinitichelis , D. latus , D. mellitae ) can join Clypeaster subdepressus .

List species by area

List species

According to ITIS:

Dissodactylus borradailei Mr. J. Rathbun, 1918
Dissodactylus crinitichelis Moreira, 1901
Dissodactylus encopei
Dissodactylus latus Griffith, 1987
Dissodactylus mellitae (Mr. J. Rathbun, 1900)
Dissodactylus primitivus Herdsman, 1917

Morphology

The crabs present morphological adaptations related to their of symbiotic lifestyle and more particularly with “the thorny habitat” than the sea urchin constitutes. These adaptations relate to the dactyls, the Chélipèdes, the Céphalothorax. The Dactyles are bifides, which facilitates the fixing with the prickles. The Chélipèdes are at the same time cutting and crushing, able to divide Net and to crush the prickles; they also take part in the maintenance of crab on its host. The céphalothorax is convex and smooth, which facilitates the displacement of crab on its thorny host; it is moreover particularly solid (calcified well at all the adult stages), a character making it possible to resist abrasion by the sediments and the prickles.

Food behavior

The majority of the species of the complex Dissodactylus feeds by filtering the water which circulates on the surface of their hosts (circulation maintained by the host); the detail of this mode of food is not described. Certain species are coprophages as Dissodactylus lockingtoni which is nourished deposit produced by its host. Other species are ectoparasitic and take fabrics or appendices of their hosts by using their chélipèdes; these ectoparasites nourish themselves podions and of the pedicellariae ( D. calmani ), or of the prickles and the tegument ( D. crinitichelis , D. mellitae , D. primitivus , D. nitidus ).

Characteristics of the vital cycle

The fruitfulness of the species of the complex Dissodactylus (200-300 eggs/female) is lower than that of the other species of Pinnotheridae (5800-8000 eggs/female). The larval development was studied only at 8 of the 13 species of the complex Dissodactylus . Its characteristics are the following ones: a number of stages Zoe S different according to the species and of their geographical location. Thus, the species of the Pacific pass by 4 stages Zoés; those of the Atlantic present 3 stages Zoe S except for Clypeasterophilus stebbingi which has 4 of them.
Enfin, Dissodactylus mellitae , which is found in the Atlantic, presents a larval development which fluctuates according to the individuals: it passes by 3 (87% of the cases) or 4 stages Zoe S (13% of the cases) before arriving at the stage Mégalope. The number of zoés stages of the ancestor of the complex Dissodactylus is not known and one thus does not know which of these characters is derived. The metamorphosis of the stage Mégalope in stage young crab is induced by the host; the symbions would thus colonize their host during this last larval stage. Only the species D. primitivus could make exception to the rule bus of the researchers showed during experiments undertaken in laboratory that this species is metamorphosed in young crab without being in the presence of its host.

Biogeographic and evolutionary characteristics

The species of the Dissodactylus complex are distributed on both sides Isthme of Panama, 6 species are localized in the Pacific Is (since Bay of California until the North of Peru); the 7 others are found in the Atlantic Western and distributed from Massachusetts to Brazil. The 2 kinds Dissodactylus and Clypeasterophilus count species on the 2 sides of the Isthmus. The majority of the species would have appeared before the formation of the Isthme of Panama. There exist however species-sisters located one in the Pacific and the other, in the Atlantic whose evolution results from a Vicariance related to the formation of the Isthme of Panama: they are the following pairs: D. mellitae (Atlantic) - D. glasselli (Peaceful) and D. primitivus (Atlantic) - D. schmitti (Peaceful).

The following scenario is proposed to explain the evolution of the species of the Dissodactylus complex. The ancestral form of Dissodactylus was to be cryptic, the made-to-order of the current littoral crabs ( Parapinnixa spp and Sakaina spp) which belong to the group-brother of the Dissodactylus , which hide under stones or in the tufts of algae or in the tubes of polychètes (Terebellidae). It is imagined that the ancestral form of Dissodactylus was to hide under stones or the sea urchins clypéastéroides without making great differences between these two types of hiding-places. Then, the crabs had to take refuge preferentially under the sea urchins which ensured at the same time the lodging but also cover! The first hosts used undoubtedly were of the clypéastéroïdes of the kind Clypeaster : they were very abundant in the infralittoraux habitats where the ancestral form of Dissodactylus lived (Oligocène). Clypeaster is an old kind, with distribution pantropicale, present in the Caribbean and the Gulf of Mexico du during the Oligocène and of the Miocène. With the appearance of Mellitidae (lower Miocène), the crabs had with their range of new hosts; the separation of the ancestral forms in 2 clades major is undoubtedly related to the colonization of Mellitidae by a symbiotic form of Dissodactylus of Clypeaster . Association with spatangues is regarded as a derived character: it is relatively recent and D. primitivus would have evolved/moved starting from a symbiotic species of Mellitidae.

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