The harmfulness of parasites to their host is not always so straightforward, there are often many factors which contribute to the pathology of an infection. The parasite we are looking at today is Gnathia auresmaculosa - a type of blood-sucking crustacean with an interesting life cycle (which you can read about in this post from last year). These little gnathiids are like ticks of the sea, clinging onto passing fish and gorging themselves on blood before dropping off to continue developing. For adult fish, a few gnathiid here and there is probably not a big deal, but for growing juveniles, that is another matter.
Settlement is a critical transitional stage for coral reef fishes, and that is also when they are most vulnerable to parasites like G. auresmaculosa. A recent study by the lab group of Dr. Alexandra Grutter revealed just how costly these ticks of the sea can be to juvenile fishes. Dr. Grutter and her colleagues found that juvenile damselfish which have been fed on by just one of those little blood-suckers exhibit significantly decreased swimming ability, far higher oxygen consumption rate, and are about half as likely to survive than uninfected fishes.
So if you happen to find yourself on a beautiful tropical reef, take a moment to think about all the little baby fishes which are swimming for their lives through the gauntlet of gnathiids - they never mentioned that in Finding Nemo!
Grutter, A.S., Crean, A.J., Curtis, L.M., Kuris, A.M., Warner, R.R. and McCormick, M.I. (2011) Indirect effects of an ectoparasite reduce successful establishment of a damselfish at settlement. Functional Ecology 25: 586-594