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The-Episiarch's avatar

Cleaner Placoderms

So lately I've been reading about some recent research into the evolution of cleaner-mutualism, of which the relationship between cleaner-wrasse and their client fishes on the Great Barrier Reef is the best-studied example (though recently there have also been some interesting results coming out of studies on the cleaner gobies found at the Caribbeans). It occurred to me that such ecological interactions most likely (if not definitely) had existed in the ancient past.

Like any modern animals, it would be expected that placoderms would have been plagued by various infectious organisms, including ectoparasites. All the major groups that represent most of the ectoparasites found on today's fishes (such as flatworms and arthropods) had already evolved in the Devonian, and it is probably expected that placoderms would be infected with similar (morphological and ecologically, even if not taxonomically) parasites. And just like today, these ectoparasites would have presented a mobile buffet to any smaller animals that can exploit it. Through convergent evolution, such cleaner-mutualism relationships (like other interspecific interactions) would have evolved independently on multiple occassions throughout the history of life on this planet.

Clearly others have thought the same too, John Meszaros :iconnocturnalsea: has done a wonderful painting of an ichthyosaur being cleaned by belemnites ([link]).

I have depicted a pair of my hypothetical "cleaner placoderm" as small arthrodires with specialised mouthparts that end in a forcep-like beak which allows them to pluck off any ectoparasites which are stubbornly attached to their client's body. The client in question is a Latocamurus coulthardi. Modern ectoparasites can be found attached to many different parts of a fish's body, ranging from their lips, to inside the mouth, on the gill filaments, on the skin, on the fins, or even in the eye. However, given the armoured plating of placoderms, perhaps more ectoparasites tend to be found in "soft sites" such as the gills or the eyes compared to modern fishes...unless there were specialised ectoparasites that were able to anchor themselves to the "head shield" by partially boring into the armour plating of placoderms...

Perhaps the great extinction of the placoderms at the end of the Devonian was accompanied by the great extinction of thousands of species of specialised parasites that once lived on the placoderms themselves...
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Wouldn't shield-boring parasites leave visible marks on the fossils? Has anyone looked?
The-Episiarch's avatar
Yep, and that's kind of what I was implying. Either they aren't there (and the parasites only bore into the softer parts like gills or eyes), or no one has found them yet.
senoritafish's avatar
Nifty concept - and I'm sure you're correct, those kinds of relationships have got to be pretty longstanding.

Coindentally, the fish I take my user name from is a cleaner wrasse that lives off the coast of California (although they have a reputation for being rather lazy about the job). Not nearly so colorful as your placoderms, though. ;)
The-Episiarch's avatar
I decided to do a quick search on senorita fish (Oxyjulis californica), they're pretty cool. Too bad there hasn't been more recent studies done on them - the most comprehensive study I can find on them in relations to their cleaning behaviour dates back to 1971.
Psithyrus's avatar
Lovely colors! I am a big fan of mutualisms :)
The-Episiarch's avatar
Thanks! In most books, the placoderms have such dull colours and there's no real reason for it. Placoderms once occupied a lot of the niches that are now occupied by bony fishes, and look at all *their* arrays of colouration - from the drab to the extravagant!
Sinande's avatar
I love this. Such a great concept. Keep 'em coming! :clap:
commander-salamander's avatar
Great idea! I also really rather like the colouring on the Latacamurus.

Another option = Cleaner shrimps. On another note I spent a bit of time in Fiji and got to see a cleaning station in action. I wasn't allowed to join the queue though.
The-Episiarch's avatar
That's right. There hasn't been as much work done on cleaner shrimps as there has been on the cleaner wrasses. However, the same crew that had been responsible for doing a lot of the research on cleaner wrasses and their interactions with client fishes have also started started looking at the behavioural ecology of cleaner shrimps in the last few years.

They found that apparently, the shrimps do a little dance at their cleaning stations in order to advertise to potential clients that they are hungry and ready to clean!
commander-salamander's avatar
It makes sense if you happen to belong to one of the tastiest groups of animals around that you can do better for another creature than just fill their stomach! What gets me about that is the other party understands the dance, I wonder if it is instinctive or learned.
The-Episiarch's avatar
Well, interestingly enough, some insight on your question has recently come in the form of a study published in Journal of Evolutionary Biology on cleaner gobies which (Lettierei & Streelman 2010) indicate it might have all started with colourful stripes.

Using fish models painted in different colour patterns, they found that the blue stripes actually deterred predation. The results of the experiment indicates that for the ancestral cleaners, the conspicuous stripes actually started out as defensive signal advertising chemical defences, but as the predator-prey relationship between the gobies and their predators evolved, the role of that signal and the nature of the interaction changed.

As some of the gobies evolved to take up occasional ectoparasite snacking as an alternative foraging strategy and develop a tolerance towards the presence of potential predator, the signal the gobies communicated to potential predators went from "back-off, I taste bad" to "keep still, I'll clean you". During this transition period, the signal might have continued sent out a dual message - "back-off, I taste bad" to predators, and "keep still, I'll clean you" to fish which are less risky and were thus potential clients.

I can't really summarise it much more beyind that without taking up too much space, but if you are interested in checking out the paper, it's:
Lettieri, L and Streelman, J.T. 2010. Colourful stripes send mixed messages to safe and risky partners in a diffuse cleaning mutualism. Journal of Evolutionary Biology 23:2289-2299

The authors has a PDF copy of the paper that you can access for free here: [link]
commander-salamander's avatar
Fascinating! I really need no encouragement to look up a paper, the trouble I find is stopping. One of the perks of studying a BSc is having access to the library ;)
The-Episiarch's avatar
Yeah, I've always considered it one of the perks of the job being a scientist. I don't know if many of my peers thought similarly, but I consider being able to access all the latest scientific publications being a perk of the job during my PhD. I voraciously read any scientific papers I could find, with the mindset that "I might never get the chance to do this EVER again!"

Fortunately, it never came to that, and now I have an academic job where I "have to" keep up with the latest literature! Oh what a chore... ;-)
Albertonykus's avatar
Good thought! I'm not a fish person, so I've never thought of this, but it makes sense to me!
The-Episiarch's avatar
Well, I spend a lot of my time thinking about the spectrum of symbiotic relationships, and cleaning mutualism happens to be one of them.

The thing I've noticed about cleaning mutualism is that while it seems to be relatively common in marine systems, often with an intricate degree of coevolution and adaptations, evidence for cleaning mutualism in terrestrial system appears to be rare, despite ample opportunities for it to have evolved.

Or it could be that people haven't looked hard enough in the right places.

Some might cite oxpeckers as an example, but there are now evidence to suggest that they are actually opportunistic blood feeders which preferentially feed upon bloated ticks (by which time the damage is already done to the host), and even prolong the healing time of open wounds to access the blood of their host. So they actually cause more harm than good. As for the supposed Egyptian Plover and Crocodile cleaning mutualism, as far as I can tell, the reports appear to be anecdotal.
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