One of the most incredible of all natural phenomena is when completely different species have somehow adapted to be able to work together to make each others' lives easier... We see this "symbiosis" across a range of different organisms: ants protecting aphids within in their colonies, clownfish living inside stinging anemones, so-called 'good bacteria' that live inside the human gut and aid digestion. All around the world, millions of pollinators work in symbiosis with all kinds of different plants to keep the ecosystem flourishing every day. And we see these symbiotic relationships in the shark world too. So when do sharks form such relationships with another organism? Are the symbioses all the same? And why do they do it?
Symbiotic relationships between different species are very complex and rarely clearly defined because they are so unique. There are a huge range of different relationships that sharks engage in with other creatures, all with different costs and gains to the participants. Along the spectrum, they can range from outright parasitic relationships, to those that are neither good nor bad for one member, to those that benefit both parties (Brunnschweiler, 2006; Fuller & Parsons, 2019).
Freeloaders
Starting at the most extreme end of the spectrum - "parasitism" is when one animal exploits another for their own gain. In this situation the relationship is beneficial to one party but actually damaging to the other (Fuller & Parsons, 2019)
For example, grey reef sharks (Carcharhinus amblyrhynchos) sometimes parasitise whitetip reef sharks (Triaenodon obesus) when they are hunting in a group together because they rely on the smaller whitetips flushing food out of their hidey holes, but will then sweep in and steal the meal away from them. This is known as "kleptoparasitism" (Labourgade et al, 2020). To learn more about this topic head over to Every Shark for Themselves.
External parasites that latch onto a shark's skin can cause injuries and skin damage that leaves them open to bacterial and fungal infections. Salmon sharks (Lamni ditropis), for example, have been seen scratching themselves against pieces of floating litter to try to dislodge their "ectoparasites" (Du Preez et al, 2022).
A classic example is that of sharksuckers (Echeneis naucrates), which regularly attach themselves to sharks' skin in order to hitch a ride. Even if they cause only mild physical injuries to their hosts, sharksuckers create drag - slowing the shark's swimming speed or reducing how nimble they are - so they could potentially affect the shark's ability to catch food. They are also clearly irritating, as you can often see sharks twitching, rubbing themselves on rocks or jumping out of the surface of the water, in an attempt to dislodge these pests. For instance, blacktip sharks (Carcharinus limbatus) perform evasive manoeuvres when sharksuckers latch on and scientists have observed captive bull sharks (Carcharhinus leucas) swimming more quickly to try get them off (Ritter, 2004; Brunnschweiler, 2006; Fuller & Parsons, 2019)
On Benefits
In the middle of the symbiosis spectrum are "commensalistic" relationships. With these, one party benefits from the interaction, but it has no impact - neither good nor bad - on the other member (Fuller & Parsons, 2019).
There are many examples of commensalistic interactions sharks have with other critters. The most famous is that of pilot fish (Naucrates ductor) with sharks, especially oceanic whitetips (Carcharhinus longimanus), which are rarely seen without their striped entourage. These little fish swim around with the big shark, which provides them protection from predators and easier access to food, as they clean up the little bits that their large pal misses. This doesn't seem to affect the shark really and they are completely tolerant of the little fishes' presence (Fuller & Parsons, 2019).
There are also several examples of different sharks being in commensalistic relationships with various schooling fishes: spinner sharks (Carcharhinus brevepinna) or blacknose sharks (Carcharhinus acronatus) with round scad (Decapterus punctatus) for example. In these associations the fishes benefit from protection and feeding opportunities, but the interactions seem to have no impact on the sharks at all (Fuller & Parsons, 2019).
The Feeling's Mutual
Finally, the type of symbiosis that is beneficial for both parties is known as a "mutualism". In these associations, each party gains something they need from the other animal. The most famous example of this is that of sharks with cleaner wrasses (Labroides species), where a shark will attend a site rich in these cleaner fishes, and allow them to peck about on their skin and even in their mouths. This "cleaning symbiosis" benefits the cleaner fish, who gain a tasty meal, and the shark gets all it's mucus, dead skin and parasites eaten away, making it more comfortable and healthy. Win win (Fuller & Parsons, 2019; Armstrong et al, 2021).
These cleaning relationships are critical to a their health and so many species of sharks are clients of these little beauticians: silky sharks (Carcharhinus falciformis), bull sharks (C. leucas), Galapagos sharks (C. galapagensis), scalloped hammerheads (Sphyrna lewini) and sicklefin lemon sharks (Negaprion acutidens) to name a few. In fact, when they are not able to visit their local cleaners for a time, these sharks can grow less quickly, and suffer general poor health, and higher rates of fungal and parasitic infections (Armstrong et al, 2021).
On a larger scale, these relationships are also powerful drivers in oceanic systems, as the locations of these cleaning stations affect how big, mobile animals use their habitats. Even highly migratory species like manta rays (Mobula birostris), pelagic threshers (Alopias pelagicus) and whale sharks (Rhincodon typus), will actually change their courses in order to detour to cleaning stations for their regular check-ups (Armstrong et al, 2021).
Reef manta rays (Mobula alfredi), for instance, completely change their movement ecology in order to attend reef-based blue-streak cleaner wrasse (Labroides dimidiatus) cleaning stations. Whilst normally "pelagic" (living offshore in open oceans), these enormous rays come right inshore and make a beeline for these cleaning sites, prioritising getting their beauty treatment over all other activities, including mating and even feeding. They will come back to the same station again and again, suggesting they have excellent memories and are able to create detailed mental maps in order to find their way back (Fuller & Parsons, 2019; Armstrong et al, 2021).
Three's A Crowd
As you can tell from all the complex terminology and the fine-grained nature of how we define these associations, symbiotic relationships are quite complex. It gets even more confusing when you consider that these relationships can involve more than two parties (Norman et al, 2022).
Take for example the three-way symbiosis between whale sharks, sharksuckers and "copepods" (small marine crustaceans). As the sharksuckers hitch a free ride to their foraging areas onboard the shark, there is an element of parasitism at play in this situation, but as they also eat the other parasites on the whale shark's skin, their relationship is best described as a mutualism (Norman et al, 2022).
Additionally, the whale shark-copepod relationship is also very complicated! Copepods are often located at the front of the shark in areas where they are only creating very minimal drag, so potentially they do not cause any harm to the shark (commensalism?). However, the copepod cannot survive without it's shark host, and they eat the shark's skin (parasitism?), but they also feed on the shark's mucus and dead skin (mutualism?). So this relationship has features of all three different symbioses, from parasitic all the way to mutualistic. It's fascinatingly complex and probably dynamic; with the value the relationship holds for each party shifting continuously depending on the place and time (Norman et al, 2022).
Who knew the relationships between marine critters could be as complicated as those of humans!
References
Armstrong AO, Armstrong AJ, Bennett MB, Richardson AJ, Townsend KA & Everett JD (2021). Mutualism promotes site selection in a large marine planktivore. Ecology and Evolution. Access online.
Brunnschweiler JM (2006). Sharksucker–shark interaction in two carcharhinid species. Marine Ecology, 27:1. Access online.
Du Preez C, Gartner H, Watts J, Clark L, Du Preez S & Norgard T (2022). Salmon Shark (Lamna ditropis) scratching behaviour using floating anthropogenic debris. The Canadian Field-Naturalist, 136:3. Access online.
Fuller LN & Parsons GR (2019). A note on associations observed between sharks and teleosts. Southeastern Naturalist, 18:33. Access online.
Labourgade P, Ballesta L, Huveneers C, Papastamatiou Y, Mourier J. (2020). Heterospecific foraging associations between reef-associated sharks: first evidence of kleptoparasitism in sharks. Ecology, e03117. doi:10.1002/ecy.3117. Image Credits: Laurent Ballesta.  Access online.Â
Norman BM, Reynolds SD & Morgan DL (2021). Three-way symbiotic relationships in whale sharks. Pacific Conservation Biology, 28:1. Access online.
By Sophie A Maycock for SharkSpeak
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