"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

February 14, 2019

Petromyzon marinus (revisited)

Today we're featuring a guest post by Darragh Casey - a student from 4th year class of the Applied Freshwater and Marine Biology' degree programme at the Galway-Mayo Institute of Technology in Ireland. This class is being taught by lecturer Dr. Katie O’Dwyer and this post was written as an assignment about writing a blog post about a parasite, and has been selected to appear as a guest post for the blog. Some of you might remember Dr. O'Dwyer from previous guest post on ladybird STI and salp-riding crustaceans. I'll let Darragh take it from here.

What makes huge sharks jump skywards? Perhaps, the answer to this question is the ancient sea lamprey, Petromyzon marinus.

Image from Figure 1 of this paper
No one is quite sure about what makes the basking sharks of our oceans breach and leap like their predacious cousin, the great white shark. Many theorise this phenomenon is the shark’s action to rid itself of various menacing parasites from their bodies. It could be the case that the annoyingly adapted sea lamprey is proving one rowdy passenger too many, hence, pushing these sharks over the edge, or, in this case, the waterline.

Sea lampreys are one of the most noticeable and common ectoparasites observed on the second largest fish in the sea, the basking sharks. Interestingly, it’s not until the lampreys become adults that they begin to bother larger fish in the ocean, in fact, they don’t even enter the ocean until they’re adults.

Prior to becoming fully metamorphosed they will have spent the last 3 – 5 years of their lives burrowed in the sediment of rivers, filter feeding on organic matter in the water column, and then they transform to become parasitic wanderers. Once they find a suitable host they use their oval shaped sucking mouth and many small teeth to grasp on and feed on the tissues and blood of an unsuspecting donor.

When the victim is the basking shark, the lamprey show their unique abilities to full power. First off, they have to penetrate the hard dermal denticle armour of sharks, which is no mean feat! The next problem they face is the high urea levels in the tissues and the blood of basking sharks. To cope with this potentially toxic level of urea in their host’s blood, the lamprey has a fantastic capability to dispel the urea whilst feeding, using this ability for their survival as described by Wilkie and colleagues. The lamprey also use lamphredin, a chemical in their saliva with anti-clotting properties, to prevent wounds from healing while feeding.

A pair of sea lamprey feeding on a basking shark, from Fig. 1 of this paper
The resulting damage from sea lamprey, especially in great numbers, can be very negative on the basking shark. They deprive the sharks of some of their urea, which is vital for osmoregulation to keep constant pressure in their bodily fluids, and they leave the sharks with open wounds which can become infected, and who knows what could happen then? However, it is more likely, that the sharks, only experience minor lamprey-related health deficiencies.

After a few years, the lampreys will eventually jump ship from their aggravated marine host and return to riverine habitats to find a suitable ally to mate with, spawn, and die soon after. In doing so, they set the foundations for a new generation of lampreys to hassle the basking sharks of the oceans for many years to come.

Are the sea lamprey such a nuisance to these sharks that they decide to momentarily leave the water in an attempt to shake them off? It’s hard to know for certain but one thing is for sure, if blood draining parasitic fish were to latch on to me I would be trying to leave the ocean pretty fast too.

Johnston, EM., Halsey, LG., Payne, NL., Kock, AA., Iosilevskii, G., Whelan, B. and Houghton, JDR. (2018). 'Latent power of basking sharks revealed by exceptional breaching events’. Biology Letters. 14: 20180537

Wilkie, M., Turnbull, S., Bird, J., Wang, Y., Claude, J. and Youson, J. (2004). ‘Lamprey parasitism of sharks and teleosts: high capacity urea excretion in an extant vertebrate relic’. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology. 138: 485-492.

This post was written by Darragh Casey.

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