Corynosoma cetaceum

image from here

In the last post we met Acanthocephalus rhinensis - an acanthocephalan which lives a pretty normal life (for a thorny-headed worm) - it spends its adult life anchored to the intestinal wall of its eel host, absorbing the nutrient-rich slurry of the intestinal content through its body surface. Today, meet Corynosoma cetaceum - it is yet another acanthocephalan, but that's about where its similarity with A. rhinesis ends. Corynosoma cetaceum lives inside the stomach of dolphins, and it is one prickly customer. As well as having the signature thorny proboscis (see the lower right picture), its entire body is covered with a spiky coat of wickedly-sharp spines (see picture on the upper left showing spines extending well pass the proboscis) which would put a hedgehog to shame.

Whereas in other acanthocephalans the proboscis plays the main attachment role, in C. cetaceum uses its entire body to cling on. The study which forms the basis of today's post looked at differences in the spines of male and female C. cetaceum, and found a high degree of divergence between the sexes. While female worms are smaller, overall they have much longer spines than males. In fact only in females do the spines grow significantly during maturation from larva (known as a cystacanth) to adult. In contrast, the body spines of adult male C. cetaceum remains more or less the same length as they were as cystacanths.

image composed from here and here

This seems odd, because being smaller, the females are actually at less risk of being dislodged (less surface area exposed to the dragging flow of the stomach content) - so why the longer spines? One possibility raised by the researchers is that perhaps the males simply depend upon attachment mechanisms other than body spines - but compared with females, the male worms have smaller proboscis and hooks too. Alternatively (and more likely), perhaps female worms need to stay in the host for longer than the males in order to produce and release eggs. There are indirect data which indicates female C. cetaceum live longer than their male counterpart - this is inferred from what is known for other acanthocephalans, and the sex ratio of C. cetaceum populations found in the stomach of dolphins which is skewed towards having more females.

There are further, as yet unsolved mysteries relating to C. cetaceum. As mentioned at the start of this post, the stomach is a very different habitat to the intestine. The life of parasites living in the intestine is fairly leisurely, being bathed a steady flow of nutrient-rich slush composed of finely-digested food infused with a cocktail of the host's bodily secretions. In stark contrast, the stomach is an extremely harsh environment. It is where early stages of digestion takes place - where chunks of food are mashed up and soaked in harsh digestive juices. The content of the stomach is composed largely of chyme - an acidic mixture of partially digested food and acid which is not all that nutritious for parasites like acanthocephalans which absorb nutrients through their body surface. In addition, carnivorous marine mammals consume huge quantity of food whenever the opportunity arises; this results in unpredictable and heavy flows of food through the stomach which makes for an extremely turbulent environment that can easily dislodge any parasitic worms (see this paper).

Of all the places in the digestive tract that C. cetaceum can occupy, why has this species evolved to live in such an inhospital environment?

Reference:
Hernández-Orts, J.S., Timi, J.T., Raga, J.A., García-Varela, M., Crespo, E.A. and Aznar, F.J. (2012) Patterns of trunk spine growth in two congeneric species of acanthocephalan: investment in attachment may differ between sexes and species. Parasitology 139:945-955.

P.S. Attention parasite appreciators! Both Susan and I will be attending parasitology conferences happening on our respective continents in July and we will be tweeting about them. So as if this blog isn't already enough, you can your 140 characters or less fix of parasitology goodness on Twitter - you can find me on Twitter @The_Episiarch and Susan @NYCuratrix. I will be tweeting the Australian Society for Parasitology conference 2-5 July, while Susan will be tweeting the American Society of Parasitologists conference 13-16 July.