6. Jun, 2021

The persistent Remora

In ancient times, the remora  was believed to stop a ship from sailing. In Latin, remora means "delay", while the genus name Echeneis comes from Greek ἔχειν, echein ("to hold") and ναῦς, naus ("a ship"). Its average size is around 40 cm but some species may even reach 80 cm. 

Species Although the taxonomic list of  Remora in Fishbase  mentions various species, it remains unclear if they reflect a genetical differentiation, synonyms, their local names, or the specific host they attach to. For example, next to the Common remora (Remora remora)  there are also Echeneis naucratesRemora albescens and Remora australis. The Common remora is often found on sharks, Echeneis naucrates on smaller fish such as tuna dolphins, and swordfish, R. Albescens that prefers mantas may even enter and perhaps reside in, a manta’s mouth or gill cavity. R. Australis is found almost exclusively on whales, particularly blue whales.

Anatomy of the disc In the common remora or ‘suckerfish' from the family of  Echeneidae, the frontal dorsal fins have evolved to enable them to adhere by suction to smooth surfaces. The suction cup (or disc) on top of the head is an amazingly effective adaptation,  allowing the remora to spend their lives clinging to a  host animal such as a whale, turtle, shark, or ray.  The oval-shaped disk is a modified dorsal fin that has split and flattened to form two symmetrical series of transverse, plate-like fin rays called disk lamellae. Suction under the disc is achieved by rotation of the lamellae when the disc is in contact with the host – this creates a relatively negative, sub-ambient pressure space under the disc. The disc also contains a fleshy-soft outer lip for suction, while the lamellae inside the disc carry tooth-like tissue projections (spinules), which the fish raises to generate friction against various host bodies to prevent slipping during attachment. The entire disc is operated by white muscle tissue ensuring that once a seal is made with the outer lip by creating a vacuum, it remains firmly attached.

From an evolution perspective, the adhesive disc evolved from dorsal fin elements, with an increase in lamellar number as a function of selection for enhanced shear adhesive power to the type of skin of the host. Although the oval disc has probably evolved from the dorsal fin spines typical of other fishes, the softer tissue, like the muscles controlling the disc suction, could be related to adaptations of the remora's cranial veins. These are highly modified and repositioned in comparison to those of other vertebrates,  lying more in front directly under the oval disc.  The suggestion is that these veins have functional importance associated with the adhesive mechanism (see Flammang  and Friedman for more detailed accounts).

Behavior When attached to their hosts, remoras appear to swim upside down. They feed on parasitic copepods, food scraps from meals, and sloughing epidermal tissue and feces of the host. In the Bahamas, they often choose the lemon shark as their favorite target. Sharks may not always appreciate their presence and have been seen acting irritated or even aggressively to remora when they become too obtrusive. A remora desperately seeking a host, may sometimes even cling to a naked spot on a  scuba diver passing by (that almost happened to me in the picture above, taken at Tiger Beach).  Some  African/Asian cultures use remoras to catch turtles. A cord or rope is fastened to the remora's tail, and when a turtle is sighted, the fish is released from the boat; it usually heads directly for the turtle and fastens itself to the turtle's shell, and then both remora and turtle are hauled in.