Columbicola columbae

You would think that of all living things, parasites would have the least need to move around. After all, it is sitting in its ideal habitat and is already (in a way) surrounded by food. Why would it need to go anywhere else? But most parasites usually reside at a very specific part of the host's body - at some stage, it would have had to makes its way there somehow, even if it stays in one spot after that. Furthermore for some parasites, where they live on the host is not the same as where they eat, so they have to commute regularly in order to get their meal ticket.

Photo by Vince Smith at phthiraptera.info

One such parasite is the humble pigeon louse (Columbicola columbae), which is usually found hanging out on the wing feathers of pigeons. It has evolved a narrow body that allows it to fit snugly between the barbs of the flight feathers and safe from the preening beak of the host. But while wing feathers are a nice place to seek shelter, they do not make for such an appetising meal - they are far too tough for C. columbae to chew on. So when the pigeon louse gets hungry, it needs to make a move to the body region where the more palatable, downy feathers are found.

So how does C. columbae find its way from the wing to the body? It's not like it can just look up Google Pigeon or something like that and get directions. Well, based the study we are featuring today on this blog, they use temperature to find their way.

Like us, birds are homeotherms - which means they keep a consistent body temperature, regardless of the outside environment. But even for a homeothermic animal, the temperature is not consistent across the body. For example, the temperature at the wings and tail of a pigeon is about 32 °C (89.6 °F), whereas the body region temperature is approximately 36 °C (96.8 °F). So are the lice using temperature differentials across parts of the pigeon's body as a cue for navigation? To find out, a pair of researchers did a series of experiments to determine what temperature the lice preferred under different circumstances.

They did a choice experiment where they put some pigeon lice in a glass petri dish with one end resting on top of a heated metal block. They also did another experiment where they placed some lice on a piece of filter paper sit on a heating apparatus that they built to generate a radial temperature gradient. In both experiments, they recorded where the lice moved to and found while the lice did respond according to the temperature differences, it was also dependent on whether they were hungry or not.

Lice which had a full belly prefer to hang out at 32 °C (wing region temperature), but those that have been experimentally starved for 18-20 hours tend to move to where it is 36 °C (body region temperature). But if down feathers are so tasty, why don't they just hang out there all the time? While the pigeon's main body is covered in tastier feathers, it is also more exposed to the murderous beak of a preening host. Whereas on the wings, the skinny body of C. columbae allows it to tuck itself between the barbs of the pigeon's flight feathers, and stay safe and sound.

So some lice like it hot, but only if they are hungry.

Reference:
Harbison, C. W., & Boughton, R. M. (2014). Thermo-orientation and the movement of feather-feeding lice on hosts. Journal of Parasitology 100: 433-441.

Calyptraeotheres garthi

There are many parasites that castrate their hosts - the parasitic barnacle that feed on the velvet belly lantern shark, the nightmarish Sacculina that takes over the body of a crab and turns it into a baby-sitting zombie, or the nematode that sterilise queen hornets and turn them into mobile nurseries.

Limpet without (left) and with (right) C. garthi
Image from Fig. 2 of the paper

There are two main ways that a parasite might stop its host from having babies. It can manipulate host physiology and suck up resources that would have otherwise gone into growing and maintaining the host's reproductive organs, which then simply shrivel up from being starved of nutrients. Alternatively, a parasite might actively occupying the space where the reproductive organs or resulting broods would normally be, displacing any would-be eggs and/or offspring.

Pea crabs are tiny crabs that specialise in making a living inside the body of marine invertebrates like various molluscs and echinoderms. The species featured in the study we are looking at today is Calyptraeotheres garthi, which lives inside limpets on the coast of Argentina. The researchers that conducted this study started noticing that most limpets that have crabs tend not to produce any egg sacs during the reproductive seasons, so they tried to find why that is by examining limpets from the field and by raising both crab-free and crab-infected limpets in the lab to compare their reproductive output.

Out in the wild, about a third of the crab-free limpets carried eggs during the breeding season, and a few of the limpets infected with smaller male or juvenile crabs managed to produce at least some eggs. But the limpets that were harbouring fully-mature female crabs had no eggs at all. This was similar to what they observed in their captive limpet population - while half of the uninfected limpets could spawn and produce a brood, none of the crab-laden limpets managed to do so.

Even though the crab-infected limpets did not produce any eggs, they had intact ovaries which were filled with oocytes (egg cells) ready to go. And when the researchers they remove crabs from infected limpets, they quickly recovered. Within a week or two after crab removal, those limpets started producing eggs again. So how was C. garthi stopping the limpet from producing a brood? Is the crab hogging all the nutrients and leave none to the developing eggs?

These limpets feed by collecting phytoplankton (floating, single-celled algae) into a mucus string around the gill fringe, and that is what C. garthi feeds on - pilfered strings of algae-loaded slime from its host. While you might think this free-loading would be taking a major toll on the limpets, they do not seem to be too affected by this. Crab-infected limpets carried on feeding and digesting at the same rate as crab-free ones, so the little crustaceans was not affecting the limpet's usual energy intake - at least not to a level that the host cannot compensate.

Calyptraeotheres garthi with a stomach full of phytoplankton
Image from Fig. 4 of the paper

So C. garthi is not stealing much nutrient from the hosts, but is it actually adding caviar to its green salad and treating itself to the limpet's eggs? After all, it is in the perfect position to snack on a serving of freshly-produced eggs. But when the researchers examined field-collected limpets that harboured crabs but still managed to squeeze out a few eggs, none of their egg sacs showed no signs of damage by the crab, which means C. garthi were only interested in one thing - sweet green slime strings.

Despite not being a severe physiological drain, their physical presence occupy the spot where the limpet would carry its brood of eggs. So while the limpet can still carry on feeding and digesting as normal, it gets brood-blocked by the crab.

The relationship between the limpet and the crab is made even more complicated by seasonal changes. During summer, the larger limpets that are infected with C. garthi are healthier than crab-free limpets, but in winter the situation is reversed. However, on top of that, during winter, only the larger limpets with crabs suffer a decline in health, while those below a certain size threshold gets away with carrying around a food-stealing crab without any severe consequences.

From our perspective, under certain circumstances, it might actually seem beneficial to have a pea crab, seeing as crab-harbouring limpets seems to be healthier during certain times of year. But from an evolutionary perspective, this pea crab is extremely harmful - by preventing its host from reproducing, it is effectively terminating that individual limpet's genetic lineage - all just for a mouthful of green slime.

Reference:
Ocampo, E. H., Nuñez, J. D., Cledón, M., & Baeza, J. A. (2014). Parasitic castration in slipper limpets infested by the symbiotic crab Calyptraeotheres garthi. Marine Biology, 161: 2107-2120.