Octopicola superba

When it comes to reproduction, most living things can be classified along a scale. At one end, you have the r-strategists (many insects and molluscs) that produce a prodigious number of offspring but few survive to adulthood. And on the other end are the K-strategists that produces only a few progeny, but to invest a lot of resources into each to ensure they are more likely to reach maturity (for example, elephants, humans, etc).

SEM photo of female
Octopicola superba from here

There is a cost/benefit trade-off inherent with being on either side of the scale because as a r-strategist, you might be producing a lot of progeny, but most of them will probably die before they get to reproduce themselves. While on the K-strategist end, by investing so much resources into each individual young, you can only afford to produce a few of them. The reproductive strategy of different organisms all fall somewhere along that continuum between low quality mass production or high quality but infrequent output, and different circumstances call for different strategies.

Textbook often use parasites as key examples of r-strategists, as a model of organisms that producing prodigious number of offspring. Indeed some internal parasites are well-known for their reproductive capacity - for example, the female blood fluke Schistosoma mansoni produces 300 to 3000 eggs per day, while tapeworms like Diphyllobothrium dendriticum can produce tens of millions of eggs per day. But not all parasites opt for quantity over quality.

The study we are featuring today examined the reproductive capacity of the parasitic copepod Octopicola superba, which, as its name indicates, lives in the common octopus. As far as a parasite goes, this crustacean seems rather innocuous and does not really cause much harm to its host. Octopicola superba can be found all over the body of the octopus but most of them are located on the skin and gills. Even though it is a parasite, it has a reproductive strategy which brings it closer to being a K-strategist.

Each female O. superba produces a clutch of only a few dozen eggs per season; if a female was to produce more than about forty eggs in a clutch, she starts reaching the upper limit of her reproductive capacity and the size of each egg (which reflects how much resources is invested into it) begins to shrunk as the brood imposes too much of a drain. This reproductive capacity varies considerably between individual; the most productive copepods are able to produce over twice as many eggs as the least productive ones, while some produced eggs that were almost twice as big as those produced by others.

Octopicola superba's reproductive strategy also shifts during different seasons; in winter, they produced a larger clutch of smaller eggs, whereas in summer they produce a smaller clutch of bigger eggs. Such season shifts has been observed in other parasitic copepods, though for O. superba, the reason for them doing so remains unknown. Despite these seasonal and individual differences, overall O. superba is certainly low-key when it comes to reproduction - even the most fecund female had just above sixty eggs in a clutch and the rest mostly produced between thirty to forty eggs.

So why has this parasitic copepod evolved to produce so few eggs compared with parasites like tapeworms and blood flukes that pump out thousands or even millions of eggs on a daily basis? It might have something to do with the habits of its host.

Octopus tend to be territorial homebodies that likes to stay in their little corner of the sea. Previous analyses indicate that hosts with such sedentary habits tend to select for parasitic copepods that produce larger eggs. Unlike one infecting more mobile animal (like a fish), parasites of sedentary animals cannot rely upon their host's routine daily movement to bring them into contact with new hosts. Therefore, they must do so under their own steam. By investing more into each egg, the female O.superba ensures each of her babies are better equipped for the long journey to find a new home, even if it means she can only produce just a few dozen of them at a time.

With offspring, you can only invest so much into them - at some point, they are on their own

Cavaleiro, F. I., & Santos, M. J. (2014). Egg number-egg size: an important trade-off in parasite life history strategies. International Journal for Parasitology 44:173-182

Cucumispora dikerogammari

Invasive species can be very disruptive - cane toads, rabbits, water hyacinth, and zebra mussels are just a few well-known examples of species that have been introduced to areas outside of their original geographic range and have caused extensive ecological disruption in their new home. One of the hypotheses for why some introduced species become so successful when they arrive at a new region is called the "enemy release hypothesis". In their new home, introduced species run amok as they are no longer hounded by their usual foes that would otherwise keep their population in check.

Top: A heavily infected amphipod
Bottom: Spores of C. dikerogammari
Photo from here

Dikerogammarus villosus is an amphipod (a little, shrimp-like crustacean) from the Ponto-Caspian that has invaded western and central Europe, and is now also found in the United Kingdom. They might only grow up to a little over an inch long, but they are voracious little predators that eat everything smaller than themselves, including each other. Released from their usual predators and parasites, D. villosus rips through the freshwater life of its new neighbourhood. But they have not completely escaped from their past foes; one parasite has managed to come along for the ride, and it is a microsporidian called Cucumispora dikerogammari.

As far as the parasite goes, Cucumispora dikerogammari is a pretty nasty one. It invades the host's muscles, reproduces prolifically and eventually kills the host by overwhelming it with sheer numbers. There is some concern that this parasite can spill over into the native invertebrates and add insult to injury to the local stream life. But on another hand, a new study shows that this parasite might be one of the few things holding back this voracious invasive amphipod from causing even more destruction.

A group of scientists from France conducted a study to looked at how C. dikerogammari affects the activity levels and appetite of D. villosus. They observed the behaviour of both infected and uninfected amhipods in a water-filled glass tube and noticed that amphipods at a late stage of infection that are visibly "filled to the brim" with parasite spores are actually more active than healthy amphipods or those that are not visibly parasitised because they are at a much earlier stage of the infection.

Close-up of a C. dikerogammari spore from here

Furthermore, they also presented amphipods with midge larvae (also known to some as "bloodworms") to see how many they ate. Both infected and uninfected D. villosus pounced on those insect larvae, but the heavily infected amphipods ate far less than the healthy ones. For whatever reason, this parasite seems to cause D. villosus to lose its appetite, and given this crustacean's reputation of eating everything that it can get its claws around, this may have significant ecological ramifications. It could mean that C. dikerogammari may be subtly reducing the impact these amphipods have on the areas where they have been introduced.

But why would heavily-infected D. villosus, which would have much of their muscle mass already converted to parasite spores by C. dikerogammari, be more active? Well, it could just be an odd manifestation of the disease, but if it is, it is certainly a useful one for this parasite - as it depends upon cannibalism for transmission to new hosts. Dikerogammarus villosus are rather homely creatures and usually prefer to stay under a shelter and wait for potential prey to wander by. By getting their host out and about, C. dikerogammari might increase the chances that its host will either run into one of its cannibalistic buddies, or die out in the open where it can be scavenged by other D. villosus.

It seems that for this little invasive amphipod, no matter how far you go, you can never really run away from your past (foes).

Reference:
Bacela-Spychalska, K., Rigaud, T., & Wattier, R. A. (2013). A co-invasive microsporidian parasite that reduces the predatory behaviour of its host Dikerogammarus villosus (Crustacea, Amphipoda). Parasitology 141: 254-258.