I’m paired up with Yinghui for this assignment, and the pair of creatures we got were the fire sea urchin (mine), and the urchin crab (Yinghui)!
As there are a number of urchins who look like they’re on fire ahaha, I decided on the astropyga radiata, who are well-known for being ferried about by carrier crabs
range of motion and anatomy
Couldn’t find a diagram specifically for the species I’m researching on so here’s a general diagram. Urchins can’t be that different right.
Astropyga radiata is a large urchin with a test diameter (diameter of it’s hard shell) of up to 20 cm, flattened or slightly concave on the upper side. The spines are up to 4 cm long and are grouped in five vertical clusters in between which are V-shaped areas with no spines corresponding to the interambulacral plates (like plates that are…bones..from what I understand).
These bare areas are red with lines of iridescent blue dots while the colour of the rest of the test and spines varies from reddish brown to purple, dark brown or nearly black. The spines are long and hollow; they are of two kinds, the shorter being venomous.
Astropyga radiata is mostly nocturnal and feeds by grazing on algae. The mouth is at the centre of its bottom surface where there are five powerful teeth in an arrangement known as an Aristotle’s lantern. It is surrounded by lips of softer tissue, with numerous small, embedded bony pieces. The jaw apparatus consists of five strong arrow-shaped plates known as pyramids, with a hard tooth pointing towards the centre of the mouth. Specialised muscles control the protrusion of the apparatus and the action of the teeth, and the animal can grasp, scrape, pull and tear very effectively.
This urchin is light sensitive and can angle its spines towards an approaching threatening object.
Generally, urchins move slowly through the use of their tube feet on it’s underside.
Watch from 0:41 to see the tube feet in action!! Like little suction cups that wave around.
two key senses for its survival
- Its sense of touch for its tube feet in order to move around
- ????? it cant see or hear or smell……maybe it can taste but its not necessary for survival……..
Relationship between the urchin and the crab
One of SYMBIOSIS! The crab’s scientific name is dorippe frascone, btw.
AAHAHAH I found this cute educational cartoon about their relationship. The song just summarises everything:
(The urchin is grossly small here compared to real life though)
|The urchin crab sits on top of the carrier crab, who…carries..it..across the ocean floor like a good pal.
|Benefits for the urchin
|Urchin gets to travel around faster, thus being transported to new feeding grounds. More food!! Mmmm 🙂
|Benefits for the crab
|The urchin makes the crab look more predatory, and protects it from predators like fish, with its long and venomous spines. It also isn’t too heavy to carry!