The parasite we are featuring today is Nicothoë astaci, the "lobster louse." Despite its name, it is not a "louse" (true lice are insects) as such, but rather a copepod (a type of crustacean), just like the salmon lice we have previously featured on this blog. But whereas salmon lice are well-studied due to their economic impact on salmonid fisheries (especially on farmed fishes), far less is know about the lobster louse. Despite having been recorded on the European lobster (Homarus gammarus) since the 1950s, to this day there is very little known about this parasite, including the type of pathology it causes, its complete life-cycle, or even what the male of the species looks like (parasitic copepods often have cryptic or dwarf males which are very elusive).
The paper we are looking at today is taking the first step to rectifying that situation. The photo (from the paper itself) depicts larval stages of N. astraci, with the arrows indicating the oral cone,the structure this parasite uses (along with its front pairs of legs) to attach itself to the host's gill filament and feed on its blood. While the larval stage looks like a rather ordinary copepod, as it matures into an adult, it morphs into what looks like a miniature boomerang with a pair of stretched out "wings" on either side, and a pair of bulbous egg sacs dangling from its rear end. The attachment and feeding activity of the lobster louse can cause pronounced physical damage to the lobster's gill filaments.
As with any kind of infection, you would expect to see some kind of cellular response. While the innate immune systems of invertebrates like lobsters are not as sophisticated as the adaptive immune system of vertebrate animals such as ourselves, they can present a formidable challenge to any would-be intruder (to see an example of what the cellular defence of a crustacean can do to a parasite, click here). Basically, the crustacean's equivalent of blood cells wrap themselves around the parasite or pathogen and initiate the process of melanization, where the intruder becomes entombed in a hardened capsule of melanin (the pigment which determines our skin colour). The researcher did find signs of melanization and other cellular disruption throughout the gills of infected lobsters, but none of it was near the lobster louse's attachment point.
So the lobster's immune system recognizes the presence of an intruder, but is unable to pinpoint and focus its wrath on the parasite. The authors of this paper suggest that this indicates the lobster louse is able to somehow interfere with the lobster's defensive mechanism so that it can blood-feed in peace. The mechanism through which the lobster louse disrupts this particular aspect of host physiology is yet to be uncovered, along with much of the parasite's ecology and life-cycle. Hopefully, with further research on this host-parasite system, this situation will change in the future.
Image from the paper.
Wootton EC, Pope EC, Vogan CL, Roberts EC, Davies CE, Rowley AF. (2011) Morphology and pathology of the ectoparasitic copepod, Nicothoë astaci ('lobster louse') in the European lobster, Homarus gammarus. Parasitology 138:1285-1295.