Physical Description

T. cymodoce can reach sizes of up to 20mm carapace width, with females generally larger than males (Patton, 1966, Huber, 1983). The dorsal carapace is typically purplish-blue in colour, although this can vary in intensity between brownish-blue and light violet. A transverse row of orange dots occur across the middle of the dorsal carapace and there is a thin red line running along the anterior edge of the carapace, particularly between the eyes Chelipeds are orange and covered by small white hairs (tomentum). Small red-orange square reticulations are found on the upper surface of the propodus of chelipeds. Dactyls are brownish-black in colour fading from tip toward propodus. Walking legs and ventral surface areorange in colour. See Castro (1997) and figure 2.

Figure 1: Labelled diagram showing the main body sections of T. cymodoce

Figure 2: Labelled image of an adult female T. cymodoce illustrating its colouration

The carapace of T. cymadoce, like all craps of the genus Trapezia, is trapezoidal in shape and smooth and encases the cephalothorax (Castro et al., 2004). The anterior (frontal) margin of carapace has four acute teeth (epibranchial teeth) between the eyes. They have two small pairs of antennae located between the eyes and above the rostrum. As with all decapods, 5 pairs of thoracic appendages are present; the first pair (chelipeds)containing the claws and the rear four pairs used for walking (walking legs). The chelipeds are of similar size as are the walking legs. The last pair of walking legs contain 6 – 8 transversal rows of setae on their inner margin, used in feeding (Castro, 1997). In males, somites 2-5 of the abdomen are fused (Castro et al., 2004).

Figure 3: Labelled close-up image of the top/dorsal side of the anterior region of the crab (head)

Species in the genus Trapezia are very similar morphologically and so are often distinguished based on colouration (Castro, 1997, Castro et al., 2004, Patton, 1966). These organisms form heterosexual pairs, where each member of the pair is of the same colour. From this it has been determined that species are separated by colour rather than there being a single species with a number of colour variations (Huber, 1983, Castro, 1997, Patton, 1966, Patton, 1974). Therefore, T. cymodoce is distinguished by its colouration along with a number of morphological traits, used when distinguishing it from T. ferruginea (a very similarly appearing species) (Castro, 1997):

1) Carapace is purplish blue and has a transversal row of orange-red spots

2) Upper and outer surface of chelipeds has well developed tomentum (covering of fine hairs)

3) The dark colouration of the dactylus extends to only about two-thirds the length of the dactylus

4) A suture (rigid joint between exoskeleton plates) is present between the second and third thoracic stemites (sections)

5) Epibranchial teeth are acute (spine-like)

6) The last pair of walking legs contain 6 – 8 transversal rows of setae on their inner margin