Recent Examinations Of The Oarfish, Regalecus glesne, From The North Sea.
D. E. WHITTAKER
The oarfish is a mesopelagic fish found in all the oceans of the world, but whose biology is little known. At least two species are known, though often confused with each other. Strandings or captures of the fish are more frequent in certain particular areas of the globe, including the gulf of Mexico where two species are met with, off the coast of South Africa, and in the British Isles, the North east coast, with most of the records occurring between the Firth of Forth and Flamborough Head, with the beaches about Amble, in Northumberland, and Whitby, in Yorkshire, as the particular hot spots.
PLEASE REPORT ALL FINDS OF THIS FISH FROM THIS COASTAL ZONE TO THIS SITE SO THAT ALL SPECIMENS CAN BE RECOVERED AND EXAMINED
Because of the especial association of the oarfish Regalecus glesne with this stretch of coast, and the scant biological knowledge that has been extracted from those British fish that have occurred along it during the last two hundred and fifty years, every effort has been made to act quickly when learning of Regalecus findings and to endeavour to secure for examination the few specimens that have occurred during the survey period 1967-(2009).
Nearly fifty British records have accumulated since the first fish was recorded from the beach at Whitby in 1759, and although the fish usually excites interest and publicity when found in a fresh and particularly pristine condition, events in recent years have shown that this is not always the case, and fish have been observed and left on the beach without any attendant publicity. It is likely that many examples have not been reported from the area because, especially if somewhat incomplete or degraded, they were not deemed to be anything out of the ordinary by the finders.
All remains of this fish found on beaches on this stretch of coast are of interest, and whatever their condition, should be photographed thoroughly in situ, and then removed from the beach, cut into sections if necessary, and frozen down for examination. Care should be taken to retrieve all fragments of fins. Specimens should not be washed.
Despite the number of occurrences, very few of the British fish have been critically examined, and the biology of the fish occurring in the eastern North Atlantic is virtually unknown. From recent examples examined at Scarborough it appears that the majority of the British occurrences have been of young fish, either immature or approaching maturity, and spawning condition, for the first time. The recent British examples are also the first to be examined for internal parasites, and to be examined for evidence and methodology of age determination of the oar-fish.
Four fish have been examined:-
1) A fish found by Mr. Wood at Sandsend near Whitby in 1981, preserved at Scarborough within a few hours of being found and later despatched to the British Museum.
2) A fish found stranded at Skinningrove near Whitby in 2003 by Mr Rob Herring and Ms Val Fletcher.
3) A fish recovered near Amble in February 2009 by Mr Mick Bould, who immediately reported the fish and carefully froze down the specimen, and to whom we express grateful thanks.
4) A fish in almost pristine condition found stranded at Tynemouth in February 2009, a short distance from the Blue reef Aquarium, and immediately recovered and frozen down by staff from the Aquarium, to whom grateful thanks are due for the opportunity to examine and compare this fish with the previous examples.
MORPHOLOGY OF REGALECUS, AND ITS POSTURE IN THE WATER COLUMN
The morphology and anatomy of the oarfish is remarkable in many respects. The body is elongate, being strongly laterally compressed, with a broad dorsal fin that extends the entire length of the body, tapering away only near the end of the body, which itself terminates in a pointed, blade-like fashion. Consistant with a fish inhabiting mesopelagic depths, but which probably ascends to much higher in the water column on a regular basis, the body is a glossy silver from the dorsal to the ventral margins, without however, any trace of dorsal countershading below the dorsal fin. The head is also brilliantly silver with steely blue reflections, while the box-like front of the head, or face, is black, as is the inside of the mouth. A few narrow, scattered oblique slashes of black pigment are present on the body, or occasionally on the side of the head, while the fins are a translucent red. Another species has steely blue spots and blotches along the silver body.
The fish is thus beautifully adapted to a most unusual mode of existence, only revealed in the last few decades, of hanging vertically, face up, in the water column, its brilliantly silver body rendering it invisible laterally to predators, by reflecting the deep blue of its surroundings. From above, the narrow profile of the fish is made invisible, against the dark depths, by the obtuse black face of the fish, while potential predators above the fish are themselves in silhouette against the low green-blue light penetrating from above, to the upwardly staring oarfish. The red fins are similarly invisible in the spectra penetrating to mesopelagic depths.
A fairly recent encounter, filmed near the surface by U.S. Navy divers, revealed the fish to be capable of swimming rapidly either up or down, by means of the undulating dorsal fin while the body was held vertically rigid, and when alarmed it quickly descended into the depths by means of the same sinuous waves of the dorsal fin, but without any change of orientation of the rigidly held body. The fish was first observed in this posture from the deep-sea submersible “Deepstar” in the 1960’s.
While this vertical mode of life was entirely unsuspected by earlier biologists, in retrospect, and with the benefit of what is now slowly being learned of the oarfish, it now appears somewhat remiss that earlier studies of the singular morphology of the oarfish had not at least led to the suggestion of such a vertical orientation in the water column.
FINS, OCCIPITAL CREST AND FEEDING BEHAVIOUR OF REGALECUS
The pelvic fins are extremely elongate, each consisting of a single substantial ray, which again are frequently broken off and missing in most specimens examined. This ray appears to have evolved from a fusion of a number of fin rays, having a more complex structure, being perforated along its length for individual blood vessels and nerves supplying the large compressed sensory organ held at its extremity. These pelvic rays with their distal organs also appear to be capable of diverse orientation relative to the long axis of the fish, and are provided with substantial muscle blocks at the pelvic girdle.
A remarkable and as yet unexplained feature of the oarfish, is the crowning of the head by great prolongations of the anterior rays of the dorsal fin into long plumes, which are usually broken and incomplete in stranded specimens, or are sadly lost during handling and recovery of the fish.
The crest is known to have been in perfect condition in at least three of the fish stranded on the North-east coast of England in recent years, including a fish stranded at Amble on December 28 2008, which was unfortunately initially left on the beach to become rapidly degraded by the attentions of beach walkers. It was finally cut into sections by anglers, and later discarded by them into the harbour at Amble. The crest was also intact on the Whitby 1981 fish, and that at Tyneside 2009, but on both occasions all or part of the crest was lost on recovery.
This occipital crest, which has rarely been examined intact, and hardly ever documented, is composed usually of twelve to fourteen rays arranged into two groups, but it is unknown what other variation, if any, is present between individual fish or with advancing age, or if any sexual dimorphism is exhibited by the crest.
The first five rays are extremely fine and very flexible, and are united by fin membrane for a third of their length, with the remaining lengths of the rays free from each other, with a narrow ribbon of membrane extending along each, to the tip of the ray, where it tapers to the point. Each membrane is pale red, with a row of deep crimson spots along its length. This part of the crest was retained complete in the fish stranded at Tyneside in 2009.
The second part of the crest, of seven to nine rays, is more substantial, these rays being more rigid and brittle, and thus the more likely to be broken off, in stranded specimens, or by rough handling during recovery. These rays may extend to more than one metre in length in a fish of 3.7metres length, and are entirely free of each other, with a projecting series of semicircular tags of deep crimson fin membrane occurring at regular intervals along their length.
The first part of the crest is capable of being depressed through 110 degrees, from being erect above the head, to point forward in front of the eyes of the fish, the gentle slope of the immediate forehead of the fish accommodating these rays to so overlie each other, while the second part of much longer, free rays comprising the rest of the crest, can also be depressed to point in front of the fish.
A possible explanation of the structure of the crest, given the ability of the rays to be positioned in front of the eyes, together with the vertical, face-up posture of the fish, is that Regalecus is exploiting the phenomenon of bio-luminescence by small mesopelagic fishes and members of the plankton, and that the function of the crest relates to feeding behaviour of the oarfish on such items of prey.
Regalecus is known to predate heavily on euphausiid crustaceans, and which were present in volume in the stomachs of three of the fish examined above. Many euphausiids are known to flash spontaneously with bioluminescence from their downwardly projected photophores, and to keep up a dull glow from their photophores for long periods. The oarfish is widely believed to predate also on small mesopelagic fishes, cephalopods and mysids.
Large numbers of small mesopelagic fishes exhibit ventrally placed photophores, including members of the Sternoptychidae, Photichthyidae, Myctophidae, Opisthoproctidae, Malacosteidae, Stomiidae, Melanostomiidae, etc., while many small deep water squids not only have such photophores, but together with deep-water mysids also eject clouds of bioluminescent material when alarmed.
Possibly, the long rays of the occipital crest may act as a stimulatory or agitative structure above and around the fish, causing such organisms to reveal themselves by greater luminescent displays, or perhaps to aggregate due to stimulus and excitement, caused by movement of the rays, during feeding activity by Regalecus.
Such prey items are only likely to be confirmed in oarfishes recovered over ocean depths or on suitable coasts adjacent to such deep water, and cannot be expected to be found in fishes recovered on the Northumberland, Durham, and Yorkshire coasts of the North Sea.