MARINE PARASITES – The Rhizocephalan


All living organisms require a source of energy to maintain being alive. Take that source of energy away and the organism simply dies. Plants are autotrophs and produce their energy, sugar, from the light energy they collect through their leaves. This process is commonly known as photosynthesis. Consumers, on the other hand, eat their energy, with primary consumers being herbivorous and secondary consumers being omnivorous or simply carnivorous. For the classical model of ecosystems any animal you can think of would fall somewhere within this category; enter parasites.

‘A parasite is an organism that lives on a host and derives its nutrients from it’ – a good working definition that I will use within this article. Anyone reading that statement carefully though would quickly realise that some predators could be considered parasites and this requires slightly more clarification to our working definition.

A predator consumes its nutrients with full intension of killing its prey. By contrast, it is not in the interest of parasites to kill off their prey at all, after all, they are getting free daily meals! There is a third group of organisms which do kill off their host, normally as a side effect, and these are known as parasitoids. The best way to consider this its to imagine a sliding scale, predators on the left, full parasites on the right and parasitoids sitting in the middle. It really is a sliding scale and the animal chosen determines where it lies on that scale.

As with any ecosystem, the marine environment is full of parasitic organisms. It is important indicator of the positive health of an ecosystem to have a wide diversity of parasites even though we tend to think of their presence as negative. Here is an example of a local parasite and its unique lifestyle choices.

Barnacles form part of the Subphylum Crustacea and are related to common shore crabs. They are sessile animals which form colonies on any suitably submerged surface and the worst that can be said for them is that they are incredibly abrasive to our skin. But, one type of barnacle, Rhizocephalan, lacks appendages, internal organs, muscles and have almost no nervous system. The name literally means “root head” and the female rhizo larva is ready to settle on host within days of being born.

Females inject themselves into crabs or prawns through the openings in the shell where the fine hairs protrude. They make their way into their lymphatic system where they grow and take over the hosts bodily functions. Eventually, both male and female crabs will be sterilised by the rhizo as an energy saving measure. After all, the parasite wants as many nutrients as possible for itself. To avoid predation, it will take over the crab’s motility system and head for deeper waters to ensure that it survives long enough to reproduce.

Without any specific mouth parts, the Rhizocephalan literally drinks all the nutrients it requires for its own growth from a root like structure which it grows inside its host. Eventually, the female becomes so large that she erupts out of the hosts abdomen and entices males to her. Throughout this period the host crab acts as if they are pregnant and will care for the parasite as if they were its own eggs. But what about the males I hear you say? Rhizocephalan already has that covered. After tapping into the brains of the crab, it tricks them into thinking they are all female hence invoking female behaviours like egg brooding. In this respect this parasite is fairy unique with respect to how much it actually takes over the hosts bodily functions and bends it to serve its own purposes.

The male rhizocephalan is attracted by the body of the female bulging out of the crab and fuses with her. This allows the female to fertilize her eggs and she then moves the host crab back to shallow waters where she invokes the crab to release its eggs, or her eggs in actual fact. Even though the crab instinctively feels that’s they are spawning the reality could not be more different as the action is in fact releasing thousands of new parasite larvae into the water column. These larvae will be ready to parasitize more crabs within a few days and so the cycle repeats.

So fortunately, we are not shore crabs and are therefore safe from such a terror, right? Actually, as it turns out scientists from the University of Bergen have found that this parasite is evolving at an incredible rate and new species were found inhabiting sharks. After some careful work it appears that the parasite is now replicating what it was doing to crabs within its new-found host. This opens up the question of how adaptable this parasite could possibly be. Could they make the jump into a human host? It is highly unlikely at present but that being considered, if a family member starts acting a bit strange on the beach this summer it might be worth checking them out for a possible Rhizocephalan infection!