TThe parasite for today is a parasitic isopod belonging to the family Gnathiidae - the larvae of this particular species feed upon the requiem shark (Carcharinus melanopterus). There are many different species of gnathiids parasitising many different species of fish, and they have an interesting life-cycle which involve "protelian parasitism" where only the juvenile stages (called a praniza) are parasitic, while the adult stages are free-living. They go through several stages of development, alternating between feeding and non-feeding developing stages (when they are engorged with blood) before reaching sexual maturity.
They are almost like a functional equivalent of ticks for fishes - they wait in ambush for a passing host, and when one arrives, it climbs onboard, sucks blood for a few days until full, then drops off to develop into the next stage. And like ticks, they can also act as vectors which can transmit blood parasites between the fishes they feed upon.
The photo shows a pair of third-stage pranizae, scale bar is 1 mm and it came from this paper:
Coetzee, M.L., Smit, N.J., Grutter, A.S., Davies, A.J. (2009) Gnathia trimaculata n. sp. (Crustacea: Isopoda: Gnathiidae), an ectoparasite found parasitising requiem sharks from off Lizard Island, Great Barrier Reef, Australia. Systematic Parasitology 79:97-112
Contributed by Tommy Leung.