The horrible natural history of remoras
I first became aware of the existence of remoras long before I became a diver. I think it was in a Jacques Cousteau documentary: I have a distinct memory of Cousteau divers removing remoras that were riding the back of a giant manta (although I have been unable to find the film online). At the time, I bought into the idea that they were giant ectoparasites and carried this view with me for many years.
After I had become a diver myself, I started seeing more and more of these extraordinary fish. On a research dive in Fiji I encountered dozens of them at 40m as I was examining an underwater sewage discharge. A little disoriented and mildly narced I remember thinking to myself 'Where the heck are their sharks?'
I didn’t give remoras another thought until diving in Belize several years later. Two remoras kept circling me and I noticed that, somewhat ironically, one of the remoras had another, much smaller one, attached to it. The big guys kept coming in close as if they were going to attach to me. I was more interested in photographing them than being a host. They soon left me alone and headed towards my dive buddy, Nicole.
Nicole started flailing at them with hands and fins to keep them away. 'Over-reaction,' I thought to myself. When we got back to the dive boat I asked Nicole why she was so excited about a pair of remoras. 'One of the bastards bit me' was her response and she held out her wrist. A series of scratch marks were oozing blood. This piqued my interest as I mentally filed away the information. The remoras in question were Echeneis naucrates, also called shark suckers.
Humans have apparently known about remoras for a long time. Remora, in Latin means delay, a reference to their supposed ability to slow down ships. The generic name Echeneis is derived from the Greek echein 'to hold' and naus 'ship' and Linnaeus was obviously aware of these old stories when he named the genus in 1758.
The earliest known reports of using remora for fishing appear to be from the Spaniard Peter Martyr d’Anghera who was a prominent figure at the court of King Ferdinand. He published a series of books in 1511 and in one of these he recounts, in considerable, if somewhat fanciful detail, the use of remora in the West Indies:
'The most extraordinary thing is that it has at the back of its head a sort of tough pocket. As soon as the fisherman sees any fish swimming near the barque, he gives the signal for attack and lets go the little cord. Like a dog freed from its leash, the fish descends on its prey and turning its head throws its skin pouch over the neck of the victim, if it is a large fish.'
Additional reports detail how large fish and animals such as manatees are also caught using remora.
Fisher folk in the Indian Ocean have also been using remoras to catch turtles for centuries. This was brought to Western attention as early as 1787. The Swede, Andrew Sparrman sailed to the Cape of Good Hope. In a French translation of a book he wrote, the following quote appears (obviously translated into English):
'They carry on a very singular method of fishing for turtles. They take alive a fish called Remora, and fixing two cords, one to its head and one to its tail, they then throw it into the depths of the sea in the region where they judge there ought to be turtles, and when they perceive that the animal has attached itself to a turtle, which it soon does, they draw into them the Remora and with it the turtle. It is said that this manner of fishing is also carried on in Madagascar.'
So how did the fishermen obtain their remora? They either collected them as a lucky accident when they caught fish with them attached or they caught them as juveniles in nets along with other reef fish. They were then kept in cages in the sea and fed on a regular basis.
Their sucking disc on the top of the head develops early, when the youngsters are barely a centimetre in length and becomes fully functional at around 3cm. It enables the remora to attach to either rough or smooth surfaces. For many years, the origin of the sucker was debated. Recently the discussion has been put to rest.
Dave Johnson, a scientist at the Smithsonian National Museum of Natural History and Ralf Britz at London’s Natural History Museum studied larval remoras. These are scarce in world collections, typically only being caught in plankton tows and often over-looked.
Johnson spent seven weeks at the National Museum of Nature and Science in Tokyo going through plankton collected between the 1950s and 1980 during a long-term study on the spawning grounds of tuna and billfish. They painstakingly digested the muscle using the enzyme trypsin, stained the young fish to reveal cartilage (blue) and bone (red) and finally stored them in glycerine. This provided a see-through animal with all the bones revealed in exquisite detail. From development sequences they were able to confirm what had been suggested centuries ago but never properly researched – the sucking disc is derived from the dorsal fin.
But Dave Johnson notes that tiny remora larvae which have not yet developed a sucking disc have large hooked teeth that protrude from their lower jaw and are very rare in the plankton tows. He theorises that perhaps they use these to hang in fish gill cavities until the disc develops.
Recent research by Georgia Tech Research Institute scientists Jason Nadler and Allison Mercer has given us new insight into how the disc works. The disc has a series of lamellae which the fish can raise or lower and there are perpendicular rows of small spines (spinules). A fleshy ring of connective tissue around the outside of the disc provides a seal. The disc is operated by white muscle tissue and this suggests that it is a passive device. In other words, once a seal is made the fish does not have to expend any energy to remain attached. Forward motion of a host makes the seal tighter. To release, the remora swims forward. Further research may show us how to produce similar devices for adhesive-free attachment – something much superior to the rubber suction cup.
Young remora may be free-living and inhabit reefs. There is one report of a remora acting as a cleaner fish but they attach to hosts at a relatively small size.
There are currently eight recognised 'shark suckers' in three genera. Echeneis naucrates is probably the best known of these and is frequently seen on sharks and turtles. Remora remora is one of the largest in the family and seems to favour larger hosts such as giant mantas. No doubt this was the remora that the Cousteau team killed in my flashback.
Remora australis, the whale sucker is found almost exclusively on whales, particularly blue whales, but they will also attach to dolphins. José Martins Silva-Jr and Ivan Sazima studied whale suckers on spinner dolphins in the Fernando de Noronha Archipelago off the Brazilian coast. They observed them eating dolphin faeces and on two occasions cleaning dolphin wounds. Small (less than 10 cm) whale suckers were seen on dolphins throughout the year of study that suggests breeding may occur year-round as well. The authors speculated that where there were two shark suckers on the same dolphin they were likely to be a mated pair. Despite the specific name, the whale sucker is found in all of the world’s oceans.
The manta sucker Remora albescens is mostly found on mantas and will apparently enter and perhaps reside in, a manta’s mouth or gill cavity.
So are remoras parasites or commensals? It really depends on which species you are talking about.
The manta sucker may well be a parasite as its stomach contents have contained very few manta parasites and a lot of manta food.
But the other species all seem to be commensals. Yes, they create a bit of drag on their host but they clear away sloughing skin and scales and eat parasites. They also opportunistically feed on food scraps left by their host.
But most of the remoras have a dark secret. The majority of their food items seem to consist of faecal matter produced by the host. It can’t be particularly nutritious but the free-ride lifestyle evinced by remoras probably doesn’t expend many calories a day.
When I was at the end of the sewer pipeline, all those remoras weren’t looking for shark hosts as I first envisaged. They were in the equivalent of remora heaven just hanging around waiting for the next person to flush a toilet.