Morphological and Behavioral Characteristics of the Giant Oarfish, by Hunter Greenway

Across the tropical and temperate coasts of the world, you can find monstrous oarfish (Regalecidae) that may appear to be water serpents. These animals have long silvery bodies that can reach over 50 feet (longest bony fish in the world).  A crimson red dorsal fin spans the entire length of the fish with a long occipital crest is on the head. What I’m describing might sound like something of a legend or fairy tale, but these creatures have been observed for hundreds if not thousands of years and can still be found today. Giant Oarfish (Regalecus glesne) reside at depths of 105-3,000 feet in epipelagic and mesopelagic waters (Benfield et al. 2013) between 55 degrees north and south of the equator. Although occasionally currents or storms can push oarfish close to shore where they’re disoriented and in an unfamiliar environment, they aren’t evolutionary equipped to deal with and beach themselves in the ordeal (Roberts, 2012). It’s when we start looking closely at the morphological and behavioral characteristics of giant oafish when they’re beached or swimming in the open ocean that we find traits truly fascinating.

Illustration of a sea serpent found in Bermuda, 1869.  (Ellis 1994)

Morphologically, the oarfish is an oddity. The Giant Oarfish has a long, slim silvery body with no scales.  The lateral line dips from above the eye to down along the abdomen (Psomadakis et al, 2008). The dorsal fin is the main means of locomotion and uses counter propagating waves to maintain its position in the water. The oarfish can grow up to 56 feet in length, but is typically less because of its tendency for self-automization of the tail section. In fact this ability to self-automize in fish can only be found in the Regalecus genus. Furthermore the Giant Oarfish has only one very long pelvic ray on each fin (to which they get their name) with a large adornment at the end which has no clear function. Another curiosity about giant oarfish is that they produce the largest eggs of any pelagic fish found in the Yucatan peninsula area (Leyva-Cruz et al. 2016). Adding onto this their mouth is small and is usually described to be toothless although some have been found with small conical teeth (Psomadakis et al., 2008).

The Giant Oarfish is a filter feeder that strains zooplankton and krill out of the water. In support of this (Psomadakis et al. 2008) found that the gill rakers were “long with minute spines projecting on the inner and outer margins” which would hint towards it being a filter feeder. (Psomadakis et al. 2008) also stated that the stomach contents of the two oarfish he dissected contained only Mediterranean krill which hints at its diet. Additionally it’s been stated that oarfish can consume juvenile squid and small fish (Kuris et al. 2015), but if so it’d likely be the fault of said prey species for getting too close to the oarfishes mouth. In order to feed efficiently it must situate itself vertically and gulp water over its gills (Paig-Tran, 2014). 

Underwater photograph of two oarfish. (Benfield et al. 2013)

You might think that hovering vertical in the water column only using undulations of the dorsal fin to move might make them slow and vulnerable to predation, but in fact they have several creative anti-predation strategies.  The first of which is that they have the ability to swim away like an eel, which was observed in 2011 when one escaped from an ROV by rapidly swimming away (Benfield et al. 2013). Additionally they are capable of self-autonomy in the same way as many lizards are, although this is debated on whether it’s meant for predator avoidance or a way to minimize body metabolism requirements (Roberts, 2010). It should also be noted that ribbon fish (Regalecus russelii), a closely related cousin of the Giant Oarfish, are believed to have the ability to produce electrical shocks for potentially warding off predators and/or communication. Also people that have tried to eat them find that their meat is gelatinous and distasteful (Helfman, 2015). In fact according to Helfman, 2015 he’s observed no animal attacking or wanting to scavenge upon oarfish except for cookie cutter sharks. This is contradicted though by the fact that oarfish have been observed in the stomach contents of sharks off the coast of Florida (Hutton, 1961). Also the act of staying vertical in the water might be a strategy to reduce their silhouette to predators beneath them. In addition to this oarfish have vibrant blue bands going across their side which appears to be a method of counter-illumination or disruptive camouflage so that the Giant Oarfish might appear to be the same color as the water above them when viewed from below. Watch a video of an oarfish swimming.

Giant Oarfish are truly fascinating for many reasons. One of which is that they can reach monstrous proportions and are alien looking compared to most other fish, also they’re so difficult to find that more people have probably stepped foot on the moon than have touched one in its natural habitat even though they occur pretty much everywhere except for in polar regions. They’re also interesting in terms of its array of adaptations for avoiding predation which likely includes counter-illumination, maintaining a vertical stance, and repulsive tasting meat. The Giant Oarfish is a charismatic fish that should be studied more in order to understand the mechanics of its morphology and behavior.

 

References

Benfield, M. C., S. Cook, S. Sharuga, and M. M. Valentine. 2013. Five in situ observations of live oarfish Regalecus glesne (Regalecidae) by remotely operated vehicles in the oceanic waters of the northern Gulf of Mexico. Journal of Fish Biology 83:28–38.

Ellis, R. 1994. Monsters of the sea. Knopf, New York.

Helfman, G. S. 2015. Secrets of a sea serpent revealed. Environmental Biology of Fishes 98:1723–1726.

Hutton, R. F. (1961). A pleurocercoid (Cestoda: Tetraphyllidae) from the oar‐fish, Regalecus glesne (Ascanius), with notes on the biology of the oar‐fish. Bulletin of Marine Sciences of the Gulf and Caribbean 11, 309–317.

Kuris, A. M., A. G. Jaramillo, J. P. Mclaughlin, S. B. Weinstein, A. E. Garcia-Vedrenne, G. O. Poinar, M. Pickering, M. L. Steinauer, M. Espinoza, J. E. Ashford, and G. L. P. Dunn. 2015. Monsters of the Sea Serpent: Parasites of an Oarfish, Regalecus russellii. Journal of Parasitology 101:41–44.

Leyva-Cruz, E., L. Vásquez-Yeomans, L. Carrillo, and M. Valdez-Moreno. 2016. Identifying pelagic fish eggs in the southeast Yucatan Peninsula using DNA barcodes. Genome 59:1117–1129.

Paig-Tran, M. SciFri. 2014. YouTube. Accessed 1 Apr 2018.

Psomadakis, P. N., Bottaro, M., Doria, G., Garibaldi, F., Giustino, S. & Vacchi, M. (2008). Notes on the Regalecus glesne occurring in the Gulf of Genova and in Liguro‐Provencal waters (NW Mediterranean) (Pisces, Lampridiformes, Regalecidae). Annali del Museo civico di Storia naturale “G. Doria” 99, 549–571.

Roberts, T. R. (2012). Systematics, biology, and distribution of the species of the oceanic oarfish genus Regalecus (Teleostei, Lampridiformes, Regalecidae). Mémoires du Muséum national d'Histoire naturelle 202, 1–266.