Friday 25 June 2021

Sechelleptus arborivagus: A new species of arborial Millipede from Mayote Island, the Comoros.

The Millipede genus Sechelleptus was created in 1980, to contain a single species, Sechelleptus seychellarum, a species from the Seychelles, which had previously been assigned to the genus Iulus, which is now only considered to contain species from Europe. The genus remained monotypic (i.e. only had a single species) until 1992, when a number of species from Madagascar were added to it, with species from Mauritius, Zanzibar, the Comoros, and Tanzania subsequently being added. To date a single species has been described from the Comoros, Sechelleptus variabilis, although when that species was first recorded in 2007, the authors did note that they believed a second species to be present, but were unable to describe it as they has found only female specimens (Millipede taxonomy has traditionally relied upon description of the male reproductive structures, and while genetic classification methods have now been applied to the group, these were not available in 2007).

In November 2019, during the local rainy season, an expedition organised by the Direction de l’Environnement de l’Aménagement et du Logement de Mayote (Mayote is a Department of France in the Comoros Islands, with the other Islands of the group making up the Republic of Comoros), visited a forest fragment located between 500 and 550 m above sealevel on Mount Tchaourembo in central Mayote, where they collected two mature males of a large Spirostreptid Millipede, corresponding to the unknown females previously collected. The new species appears similar to Sechelleptus variabilis, but is much larger; even the gonopods (male sexual organs used in the classification of Millipedes) appear like a scaled-up version of those seen in the smaller species, although close morphological examination did reveal some differences between the two taxa. 

In a paper published in the European Journal of Taxonomy on 16 June 2021, Didier Van den Spiegel and Arnaud Henrard of the Biological Collection and Data Management Unit at the Royal Museum for Central Africa, and Aurore Mathys, also of the Biological Collection and Data Management Unit at the Royal Museum for Central Africa, and of the Royal Belgian Institute of Natural Sciences, formally describe the Mount Tchaourembo Millipede as a new species.

Distribution of the new species, Sechelleptus arborivagus and previously described Sechelleptus variabilis, on the island of Mayote. Van den Spiegel et al. (2021).

Genetic material was taken from seven of the new specimens, as well as one of the females collected previously, using a Macherey-Nagel NucleoSpin tissue kit, and compared to previously obtained sequences from closely related species, including Sechelleptus variabilis, using the Bold Systems tool and BLAST web application of GenBank.

This methodology strongly supported the idea that the new specimens are a separate species within the genus Sechelleptus, and that the previously collected female was also a member of this genus. A a sub-adult female collected at Mont Combani on Mayotte apparently represents a third species, being most closely related to an undescribed specimen previously collected from Madagascar.

The new species is named Sechelleptus arborivagus, where 'arborivagus' refers to the ecology of the species, which has always been observed climbing trees. It is a medium-sized, tree-dwelling Millipede with relatively long legs, similar to Sechelleptus variabilis, with which it shares the structure of the male first leg and rather simple gonopods (specialised appendages used in reproduction). However, the two species do differ in the structure of the gonotelopodite, which is simple in Sechelleptus variabilis but branches near the tip in Sechelleptus arborivagus.

Sechelleptus arborivagus in their natural habitat. (A) Two females showing different colour. (B) Female on tree. Scale bars are 3 cm. Van den Spiegal et al. (2021).

Sechelleptus arborivagus is more-or-less a uniform brown colour, with the exceptions of the metazonae (sclerotic plates that make up part of the dorsal surface of each segment), which are a lighter (sometimes redder) brown, but have a darker border posterior. The head of this species is smoth, with an eye patch made up of about 60 ommatidia (individual units of a compound eye). The clypeus (broad plate at the front of the head) has for setae (hairs), two on each side, above the labrum (flap-like structure that lies immediately in front of the mouth). The antennae are moderately long, and have protruding black rings. 

Sechelleptus arborivagus, male paratype (BE_RMCA_MYR.Dip.22875). SEM views. (A) Ommatia, frontal view. (B) Antennae, ventral view, arrows pointing to sensilla basiconica. (C) Lateral view of collum, arrows pointing to the anteroventral angle 80–90°. (D) Head, ventral view. (E) Mandible, ventral view. (F) Mandible, frontal view (F). (G) Clypeus, frontal view. (H) Limbus. (I) Defensive gland. (J) Walking leg, lateral view. (K) Tarsal claw, lateral view. Abbreviations: dg, defensive gland; me, mentum; od, odontomere; pl, pectinate lamellae; pm, prementum; pad, tibial pad; ps, psectomere; se, sectile edge. Scale bars: (A)–(B), (F) 200 μm; (C)–(E), (G), (J) 500 μm; (H) 5 μm; (I) 1 mm; (K) 100 μm. Van den Spiegel et al. (2021).

All known specimens of Sechelleptus arborivagus were collected in trees, and most of them in a single forest fragment. These Millipedes were always found singularly, never in pairs or groups, and females were much more common than males, with a sex ratio of about six-to-one. These Millipedes have large ommatidia, long legs with strongly curved tarsal claws, and produce copious amounts of defensive secretion when irritated, all of which have been considered to be adaptations to an arboreal life-style in other Millipedes.

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