As scuba divers we really do live among the fish. Sometimes you hear statements like “we are just guests underwater”. But in reality we are more. Some fish even think we are one of theirs. A bold claim! How can I make such a claim?
If you have dived the tropics and observed sharks, manta rays or marine turtles, you have probably seen remoras, or sharksuckers. These are odd fishes – they stick to these large marine vertebrates, and get free rides. The remoras have a dorsal fin which looks unlike that of any other fish. Evolution has transformed this fin into an adhesive disc. It has ridges which point backwards, so that the sharksucker sticks to a forward swimming fish. Take a look:
Here is a pair of happy sharksuckers on a turtle, seen in Dauin just last week:
Why do the remoras stick with sharks, rays and turtles? They are hydrodynamic parasites (that’s how Weihs, 2007, put it). They get free rides, which is a big deal underwater. It takes much more energy to move in the water than on land, since water has so much more resistance against movement (drag) than air. Also, fish need to have water run over their gills to extract oxygen from it. There are two ways of achieving this: fish can pump water over their gills, by spreading and retracting their gill covers. This goby does that:
Good job little goby!
Or, they can swim fast, and have the forward locomotion push water over their gills, in through their mouths, out through the gill covers. Sardines are known to do that. Now both ways of respiration take energy, the pumping does and so does the swimming. Wouldn’t it be a good idea to steal that energy from another fish? By letting that fish swim for you? That’s exactly what remoras and sharksuckers do. They latch onto other fishes (and on turtles, dolphins and whales), let them do the swimming, and get the fresh, oxygenated water pushed over their gills for free. This impedes the swimming of their hosts by increasing their drag in the water, and dolphins seem to jump out of the water to get rid of remoras (Weihs, 2007).
But it’s not all one-sided. The remoras also act, at least sometimes, as parasite removers, making the interaction between host and remora mutualistic symbiosis. Fertl and Landry (2009) write that “remoras feed on parasitic copepods, which constitute the bulk of their diet”, thus helping their hosts. But “They also feed on zooplankton and smaller nekton; food scraps from meals of their hosts; and sloughing epidermal tissue and feces of the host.” Yummy!
Why do the dolphins want to get rid of remoras? Possibly in this case the problem with the increased drag outweighs the advantage of the parasite removal, which might be more of an issue in fishes.
How does all of that relate to the claim that the fish think we are one of theirs? Well, look at this picture of our diving instructor Laura’s leg. A sharksucker thought her leg was a good place to attach.
The fish must have thought of her as a citizen of the Ocean, like a turtle or a manta ray. Unfortunately for the sharksucker Laura stayed in one place and did not swim to save it energy for respiration. Yes, if you dive, you become a part of the marine ecosystem, to the degree that fishes want to engage in a symbiosis with you.
Weihs et al., Mechanics of remora removal by spinning dolphins. Marine Mammal Science, 2007, 23(3): 707–714
Fertl and Landry, Encyclopedia of Marine Mammals (Second Edition), “Remoras”, 2009, Pages 942–943