Pea crab

[3] The bodies of the female pea crabs are often translucent and show the inner organs and gonads as yellow and red, with the males being a "more yellowish-grey with patches of brown".

[4] The pea crab relies solely on its host for food, safety, and oxygen.

[3][4] Other hosts, in addition to oysters, include sea urchins and sand dollars.

[7] A study by New Zealand researchers Oliver Trottier and Andrew Jeffs from the University of Auckland shows this behaviour on a similar parasitic pea crab, Nepinnotheres novaezelandiae.

[8] Pinnotheres is Greek for "guard of Pinna" and pisum is Latin for a pea, in reference to the shape of the crab.

A pea crab (yellow in color) (in Moss Landing , California )
Male pea crab entering a green-lipped mussel hosting a female crab, infrared video [ 8 ]