Part 4: (3/14/24) How do animals fake each other out, and why?

[Dan Russell]

Thursday, March 14, 2024

  Mussels are remarkable... 

California mussel (Mytilus californianus)

... I hope I've convinced you of that.   We've learned about the amazing mimicry behavior of mussels--they use both their mantles (soft tissue at the edge of the shell) and create conglutinates (long mucus-like secretions in which they tuck some glochidia), both of which look like prey items to lure fish to take a bite, but end up spraying the larval glochidia into their mouth and onto the gills.  

The point of all this fakery and elaborate mimicry is to get their larval baby mussels (aka glochidia) onto the gills where they can clamp on for a temporary ride on the host fish. The big question you are probably asking yourself now is this...   

3. The point of all this mussel-bound fakery is to get the glochidia into the gills of fish.  Are the glochidia actually parasites?  Or are they just hitching a ride on the fish's gills? 

The big question here is to determine if the glochidia are actually parasites on the fish, or if they're just along for the ride.  As we've learned in the last two posts, getting the precise word to describe what you're searching for is often a huge help.  So, let's look for the right word by doing this search: 

     [ a word describing an organism that gets a free ride on a host ] 

With a little looking at those results I learned the exact definitions of: 

Commensalism ... in which members of one species gain benefits while those of the other species neither benefit nor are harmed

Mutualism... in which both organisms benefit from each other; 

Amensalism... where one is harmed while the other is unaffected

Parasitism... where one is harmed and the other benefits, usually in order to obtain nutrients

AND 

Phoresy ... in which a phoretic animal latches onto a host animal for the purpose of dispersal. 

The word phoresy derives from the Greek phorein, which means ‘to carry’. Typically, the attacher is an animal, such as a nematode or mite, with limited ability to travel great distances on its own is happy to get a lift from a highly mobile host, such as a fly or bee.  (More details at the Wikipedia entry for phoresy.) 

So, now we have a very precise term to search for what we want:  Are glochidia parasites or phoronts?  

Next query: 

     [ mussel glochidia parasite phoresy ] 

This takes us to several great resources. Here's the first one:  

“Plasticity, Paralogy, and Pseudogenization: Rhabdoviruses of Freshwater Mussels Elucidate Mechanisms of Viral Genome Diversification and the Evolution of the Finfish-Infecting Rhabdoviral Genera” (Full citation below.)  

 This paper tells us that 

“These larvae [glochidia] attach to the gills or fins of the fish, encyst and derive nutrients from fish blood and tissues, complete metamorphosis and excyst, then drop off to begin their sessile, filter-feeding life stage”   Later on it says that “Freshwater mussel larvae attach to fish and feed on tissues and blood..."    

HOWEVER... the next paper listed in the SERP is this one: "Adaptations to host infection and larval parasitism in Unionoida." 

And this paper says:  

"Freshwater  mussel  larval  parasitism  of  fish  is  unique  among  bivalves.  The  relationship  is primarily  phoretic  rather  than  nutritive;  only  the  smallest  glochidia  and  the  haustorial [a parasite that penetrates the tissues of the host]  larva  grow substantially while on the host. Growth of the smallest larvae suggests a lower functional size limit of ~150 μm for the juveniles... "  

Uh oh.  We've got a difference of opinion.  The first paper is written by virologists looking for a way that mussels transmit viruses to fish, so their focus is really on mussel-to-fish blood contact.  While everyone agrees that the glochidia create little cysts while attached to the gills, it's kind of a relative thing--do they actually pull nutrients from the host fish or not?  In the "Adaptations" paper, they say (basically) that "sure, there's a little extraction, but not much..."  

I'll spare you the entertaining hours I spent reading papers in benthology (that is, is the study of the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream--that is, the benthos), but will summarize it by saying that there is a debate about how parasitical the glochidia are in reality.  Some researchers find that their is nutrient exchange between the host fish and the mussel (see "Glochidia Infection" in the citation list below) where they write that: 

"Although the relationship between glochidium larvae of freshwater mussels Unionoida and their obligatory fish hosts has been frequently described as phoretic commensalism (glochidia benefit from transportation services by fish, with no harm to fish) or even symbiotic (both partners benefit), increasing evidence has indicated that the relationship is parasitic. Stable isotope analysis, during the glochidial attachment to the fish host, indicates the acquirement of nutrients from the fish." 

So the debate is really about how many nutrients the glochidia pull out of the host fish and whether or not the mussels are really primarily along for the ride to other locations in the river or stream.  

The last paper I'll cite is this one, "Effects of parasitic freshwater mussels on their host fishes: a review."

“No specific glochidia structures have been observed to extract nutrients from the host, but stable isotope analysis does show such transfer. It is hypothesized that this transfer results from the glochidia digesting the host tissue captured within the initial bite. Denzymes used in the process of breaking down the captured tissue may seep out of the glochidia and digest some surrounding host tissue. Passive absorption of compounds from the intercellular space and blood plasma has also been proposed.”  

So.. maybe it's not such a big deal, just a little bite.  

But as always, use caution when asking LLMs about a topic even as esoteric as this one.  

Q: do Freshwater mussel larvae attach to fish and feed on tissues and blood?  

Gemini: 

“While freshwater mussel larvae do attach to fish, they don't feed on tissues and blood… The glochidia is NOT parasitic and doesn't harm the fish.  

Here's a breakdown of the interesting relationship between freshwater mussels and fish:

Parasitic dependence: Mussels rely on fish as hosts for their glochidia to survive and develop.

Temporary attachment: Glochidia latch onto the fish's gills, fins, or other external body parts for a few weeks. This attachment is relatively harmless to the fish.

Nutritional source: During this period, the glochidia obtain nutrients not directly from the fish but rather from the surrounding water.

This unique adaptation is crucial for freshwater mussels as it ensures the dispersal of their offspring. After several weeks, the glochidia transform into juvenile mussels and detach from the fish, settling at the bottom of the waterway to mature into adults.” 

ChatGPT: 

“Freshwater mussel larvae, known as glochidia, do attach to fish, but their method of feeding and interaction with their host fish is a bit more nuanced than simply feeding on tissues and blood. When glochidia are released into the water by the adult female mussel, they must attach to the gills, fins, or body of a suitable host fish to complete their development. This parasitic stage is crucial for the lifecycle of freshwater mussels.

During this stage, the glochidia encyst on the host fish's surfaces and undergo transformation into juvenile mussels. While encysted, they may derive nutrition from the fish, but rather than actively feeding on the fish's tissues and blood, they are thought to absorb nutrients directly through their tissues, benefiting from the fish's circulatory system indirectly. The interaction is generally not harmful to the fish, and most fish hosts recover without significant harm once the larvae detach and complete their metamorphosis into juvenile mussels.”

Interesting difference.  In this case, ChatGPT is a lot closer to the nuances and subtleties of reality.  (And I would say that Gemini just gets it wrong.)  

SearchResearch Lessons 

Over all of the past few posts, several lessons have become clear.. 

1. Get the most precise term you can find.  Learning the speciality (and precise) terms "mimicry" and "glochidia" get you to the best results fast.  When they're available for your research, learn and use them.  (Such as, phoresy.)

2. Pay attention as you read--you'll pick up important concepts and terminology.  As we read through the SERP results, we often found words and ideas that we could use to zero in on what we were really looking for!  

3. Remember: Sometimes simple questions have complicated answers.  It seems like an obvious question--do mussel larvae parasitize their host fish?  But the answer is nuanced... yes... but!  

4. Caution when asking LLMs... they might really get it wrong, or at least not-nuanced.  

I hope you've enjoyed this little excursion into the sometimes startling world of mussels and mimicry behavior.  I certainly did!   (And I'm grateful I don't have a glochidia infection!)  

Keep searching.  

-------- 

“Plasticity, Paralogy, and Pseudogenization: Rhabdoviruses of Freshwater Mussels Elucidate Mechanisms of Viral Genome Diversification and the Evolution of the Finfish-Infecting Rhabdoviral Genera” Goldberg, T. L., Blevins, E., Leis, E. M., Standish, I. F., Richard, J. C., Lueder, M. R., ... & Bishop-Lilly, K. A. (2023). Journal of Virology, 97(5), e00196-23.

"Adaptations to host infection and larval parasitism in Unionoida." Barnhart, M. Christopher, Wendell R. Haag, and William N. Roston.  Journal of the North American Benthological Society 27.2 (2008): 370-394.  https://www.journals.uchicago.edu/doi/full/10.1899/07-093.1

"Glochidia Infection of Endemic Fishes from Lake Prespa, N. Macedonia" Blazhekovikj-Dimovska, D., Stojanovski, S., Taskinen, J., Smiljkov, S., & Rimcheska, B. (2022). Hydrobiology, 2(1), 36-43

"Effects of parasitic freshwater mussels on their host fishes: a review." Rock, Sebastian L., et al.  Parasitology 149.14 (2022): 1958-1975.  

Mussels on the beach in northern California (P/C Dan)

--

Daniel M. Russell 

Check out my new book:  The Joy of Search (MIT Press, Winter, 2019) 

Available in fine bookstores (and online) everywhere.