Dodecolopoda is a Monotypic taxon genus of sea spider (class Pycnogonida) in the family Colossendeidae.
Discovery
This species was first described by the Scottish
Zoology William Thomas Calman and
Isabella Gordon of the British Museum (Natural History) in 1933. The original description is based on a male
holotype found at a depth of 219 meters below the surface off the coast of MacRobertson Land. Calman and Gordon named this species for
Douglas Mawson, who obtained this specimen on his voyage to the
Antarctic region in 1931.
Phylogeny
Phylogenetics analysis based on molecular data places
Dodecolopoda in a
clade with the ten-legged sea spiders in the genus
Decolopoda, which emerge as the closest relatives of
Dodecolopoda.
This analysis also indicates that both
Dodecolopoda and
Decolopoda are nested within the eight-legged genus
Colossendeis in a phylogenetic tree.
Thus,
Colossendeis is
Paraphyly with respect to these two polymerous (i.e., extra-legged) genera. This paraphyly would normally make both
Dodecolopoda and
Colossendeis junior synonyms of
Decolopoda, the oldest name, and require all three genera to merge under the name
Decolopoda. To avoid this disruption, authorities keep these genera under different names and retain
Colossendeis as a paraphyletic genus.
The molecular evidence also indicates that the polymerous clade including Dodecolopoda and Decolopoda is nested within a Monophyly group containing the "longitarsal" species in the genus Colossendeis. The sea spiders in this "longitarsal" clade feature legs in which the three most distal articles (claw, propodus, and tarsus) taken together are at least three-quarters the length of the fourth most distal article (second tibia). In "brevitarsal" species of Colossendeis, the three distal articles are instead much shorter relative to the second tibia.
Description
The species
Dodecolopoda mawsoni is notable for not only its twelve legs but also its gigantic size. The holotype, for example, measures from tip to tip of its legs when fully extended. These legs are about the same length except for the first pair, which are shorter than the others. The leg of the fourth pair on the holotype measures 240.3 millimeters (9.46 in) long. The ventral surface of the second most proximal article (second coxa) of each leg features a minute genital pore.
The sea spider's long legs act as stilts, holding its body well above the seabed surface below.
This species is the only sea spider to combine giant size with six pairs of legs, as the only other species with twelve legs (
Sexanymphon) is small.
The polymerous genera Dodecolopoda and Decolopoda both resemble their eight-legged relatives in the genus Colossendeis.
The polymerous genera Dodecolopoda and Decolopoda both differ from their close relatives in the genus Colessendeis not only by featuring more legs but also by retaining chelifores as adults.
The closely related genera Dodecolopoda and Decolopoda share other distinctive traits. For example, both genera feature a long proboscis that curves downward and is distally inflated. Furthermore, both genera feature legs in which the third most distal article (tarsus) is much longer than the second most distal article (propodus).
The sea spiders in Dodecolopoda not only have more legs, however, but also are larger than the species in Decolopoda. Furthermore, in Dodecolopoda, the most distal leg article (claw) is shorter relative to the propodus, and the propodus is shorter relative to the tarsus, compared to the corresponding articles in Decolopoda. In a representative leg of Dodecolopoda, for example, the claw is less than half as long as the propodus, which is less than half as long as the tarsus.
Locomotion
While walking,
Dodecolopoda moves its legs in a metachronal pattern, with the movements of each leg lasting for a nearly equal amount of time and these strokes moving in regular waves along the body. This sea spider, like its close relatives in the genus
Decolopoda, uses all of its legs in walking with a metachronal rhythm. These polymerous species walk with more coordination and precision than eight-legged sea spiders, which use a more casual gait. Octopodous sea spiders with long legs exhibit a more imprecise gait, lifting some legs or letting them drag while other legs take steps for several cycles, with strokes of irregular duration and legs sometimes overlapping or tangling with each other.
Distribution
Sea spiders in the genus
Dodecolopoda are found in the
Indian Ocean and
Atlantic Ocean regions of the
Southern Ocean and are
Endemism to the Antarctic region.
Specimens have been found off the South Shetland Islands, the Palmer Archipelago, and
Enderby Land, as well as in the
Ross Sea, caught at depths ranging from 146 to 549 meters below the surface.
The distribution of this genus is circumpolar in the Antarctic zone.
