Spider Tortoise

Species Description Photo: Christina Castellano The Malagasy Spider Tortoise (Pyxis arachnoides) is the smallest of four endemic tortoise species that inhabit Madagascar. It has a domed shell that is mostly black in coloration, but includes yellow lines radiating from the centers of each scale. It is the only tortoise in the world with a hinged lobe on the bottom portion of the shell, or plastron. This lobe may have evolved either as a defense against dehydration, or a means of protection against predators (Pritchard, 1979).

The Spider Tortoise can reach lengths of up to 200 mm and weigh over 400g. There are modest differences between males and females. Female tortoises have a flattened plastron and miniature tail, whereas males exhibit a slightly concave plastron and enlarged tail (Pritchard, 1979).

The common name of this species varies across its range. It is often referred to as Tsakafy, Kapila, and Zakapy in southern Madagascar. The generic name Pyxis refers to the box-like form of the shell, while the species name arachnoides refers to the linear pattern across the scales, which is reminiscent of a spider’s web (Pritchard, 1979).

Taxonomy

Bell first described the genus Pyxis in 1827. Subsequently, three subspecies were recognized: the Northern Spider Tortoise (Pyxis arachnoides brygooi), Common Spider Tortoise (Pyxis arachnoides arachnoides), and Southern Spider Tortoise (Pyxis arachnoides oblonga). Differentiation between them is based mainly on the flexibility of the lobe on the plastron and coloration (Bour, 1979).

The Spider Tortoise is most closely related to the Flat-tailed Tortoise (Pyxis planicauda), a species that inhabits forests and woodlands on the west coast of Madagascar. Together they are considered to be close relatives of the giant tortoises of the Indian Ocean islands. The Malagasy tortoises may have floated northward with ocean currents towards the Aldabra archipelago around 11 mya and subsequently gave rise to these giants (Caccone et al., 1999).

Distribution

The Spider Tortoise only exists on the island of Madagascar. It occurs within a narrow band of dry forest about 10-50 km wide along the southern and western coastlines. Its range-wide distribution extends from the area north of the village of Morombe located in the west to the Cap Sainte Marie Special Reserve positioned at the southern most point of the island, and east almost to the port city of Fort Dauphin. The subspecies have nearly distinct distributions; however, their exact ranges are not well-known. Preliminary surveys suggest that the northern subspecies has the smallest distribution of the three and that the common form has the largest (Durrell et al., 1989).

Habitat

The Spider Tortoise is restricted to the ecoregion known as the Spiny Forest, which extends southwards from the Mangoky River along the west coast to the slopes of the Anosyennes Mountains in the southeast. It is the most arid region in Madagascar with the average annual rainfall being usually less than 500 mm. The climate is dominated by wet and dry seasons with most precipitation occurring between November and April, though January and February are the wettest months of the year. The mean annual minimum and maximum temperature ranges are 15-21°C and 30-33°C, respectively (Donque, 1972).

Vegetation communities can be highly variable throughout the Spiny Forest primarily due to dissimilarities in soil types. Sandy soils support taller and more densely packed plants than calcareous soils that support dwarf species. Plant families characteristic of this region include the Didiereaceae and Euphorbiaceae that exhibit extreme adaptation to the aridity of the region and possess succulent and reduced leaves, thorns, and enlarged trunks and branches. The vegetation along the southwest coastline is usually around 3-6 m tall (DuPuy and Moat, 1996).

Very little is known about the ecology of the Spider Tortoise. It inhabits a highly seasonal environment and the amount of vegetation cover preferred by individuals can vary between wet and dry seasons. Tortoises seek shelter beneath shrubs and trees during periods of increased temperature and precipitation. This species is most active during the wet season, mainly between November and April, and tends to aestivate partially, or completely buried underground for extended periods during the dry (Walker et al., 2009).

Movement and Home Range

Information on the movements and home range of this species is currently unavailable.

Diet

Little is known of the foraging habits and diet of the Spider Tortoise. Limited reports suggest that it consumes young leaves on low-lying plants and cow dung containing insect larvae (Glaw and Vences, 1994).

Reproduction Female Spider Tortoises lay a single large egg per clutch during the wet season; although, the number of clutches deposited each year remains unknown it may be multiple (Durrell et al., 1989). Hatchling tortoises have been recorded most frequently during February and March, which suggests that emergence from nests occurs around this time. It has been suggested that this species becomes sexually mature between eight and 12 years old in the wild (Walker et al., 2004).

Threats to Survival

The survival of the Spider Tortoise is threatened by illegal collection, over-harvesting for local consumption and habitat loss. This species is illegally traded in international pet markets, which has increased substantially within the last decade. Much of the Spiny Forest has been cleared for agriculture, livestock grazing, and charcoal production. Taboos that once prevented Malagasy tortoises from being disturbed, or collected by local tribes’ people have eroded. The Spider Tortoise is listed as Endangered in Madagascar and Critically Endangered by the IUCN (Pedrono, 2008).

Literature Cited;

Bour, R. 1979. Les tortues actuelles de Madagascar (Republique malagache): liste systematique et description de deux sous-especes nouvelles (Reptilia-Testudines). Bull. Soc. Et. Sci. Anjou, N.S., T.X:141-154.

Caccone, A., Amato, G., Gratry, O.C., Behler, J.L., and Powell, J.R. 1999. A molecular phylogeny of four endangered Madagascar tortoises based on mtDNA sequences. Molecular Phylogenetics and Evolution 12:1-9.

Donque, G. 1972. The climatology of Madagascar. In Battistini, R and Richard-Vindard, G. (Eds.). Biogeography and Ecology of Madagascar. Junk (The Hague): pp. 87-144.

Dupuy, D., and Moat, J. 1996. A refined classification of the primary vegetation of Madagascar based on the underlying geology: Using GIS to map its distribution and to assess its conservation status. In Lourenco, W. R. ed. Proceedings of the international Symposium on Biogeography of Madagascar. Editions de l'Orstrom, Paris: pp. 205-218.

Durrell, L., Groombridge, B., Tonge, S., and Bloxam, Q. 1989. Pyxis arachnoides Madagascar spider tortoise; Tsakafy; Kapila. In Swingland, I.R. and Klemens, M.W. eds. Conservation Biology of Tortoises. IUCN, Gland, Switzerland: pp 103-104.

Glaw, F., and Vences, M. 1994. A Field Guide to the Amphibians and Reptiles of Madagascar. Moos Druck, Leverkusen and FARBO, Koln.

Pedrono, M. 2008. The Tortoises and Turtles of Madagascar. Natural History Publications, Borneo.

Pritchard, P.C.H. 1979. Encyclopedia of Turtles. T.F.H. Publications, Inc., Neptune, New Jersey.

Walker, R.C. J., Rix, C., and Woods-Ballard, A.J. 2004. The export of the endangered Madagascar spider tortoise (Pyxis arachnoides) to support the exotic pet trade. Herpetological Bulletin 90:2-9.

Walker, R.C.J. 2009. Comprehensive survey of spider tortoise distribution in Madagascargets underway in the north Turtle Survival Alliance Newsletter. August 2009.