Giant Chiton

Appearance

The white Plaxiphora chiton reaches a common size of about 95mm, with a minimum and maximum length of 40–100 millimetres and a width of 25–38 millimetres. The shell of this large chiton is dark green to brown, humped and oval shaped, with eight rough valves. Its girdle is leathery, brown with darker bars and with long bristles.

Naming

This species of Plaxiphora chiton was discovered by Dr. Charles Boyden on wave-exposed rocky shores of New Zealand. It was distinguishable enough from other chitons that it was described as a new species. These chitons do not have bristle-tufts on their girdle, and are low shore chiton. They are also the largest giant chiton species found on Australia's coasts.

Distribution

This species of low-shore chitons is native to south-western Australia, including Queensland, New South Wales, Victoria and Tasmania.Plaxiphora albida and "Onithochiton quercinus" are endemic to Southeast Australia, where they are the most abundant chitons. Both chitons grow to about 80 mm in length and are usually found around the surface of rocky shores that are exposed. The specific community is a low-shore algal type which is most abundant with the species "Pyura stolonifera." The sex ratio of the chiton is consistently 1:1 for males and females.

Reproduction

The gonodosomatic index of Plaxiphora albida showed fluctuations throughout the year. Smaller gonads are characteristic of immature gametes and a lower gonadosomastic index. Contrarily, larger gonads show more mature gametes and exhibit a bigger gonadosomatic index. Gonads in Plaxiphora albida are around 6–7mm. This number rises to 9–10mm when there are mature eggs and sperm present. For the Plaxiphora albida, January was the month which showed the most mature gametes and an increased size of the gonads. The gonads showed a dip in the index size during late summer and early autumn. By April, the gonads of the chiton are reduced immensely in size, staying this size for the remainder of the year.

Predators

As rocky intertidal organisms, Chitons directly face threats caused by ecological shifts: temperature, tidal action, and pH. The organism's resistance to the physiological changes due to the negative impacts is largely determined by its valves. The valves determine the integrity of the shells, of which when at their threshold, could fracture and compromise the defense structure of the organism. While the fractures on the valves can be caused by a multitude of factors, there is a positive correlation between the thickness of the valves and force required to fracture.

Although Chitons face variety of threats from the marine ecosystem, their ability to be abundant for the last 500 million years, suggest that their resistance to ecological pressure is robust. Scientists have accredited the agile movements of Chitons up and down the rocky niche to have partaken in their longing survival.