Recent Examinations Of The Oarfish, Regalecus glesne, From The North Sea.

Head of Regalecus glesne stranded at Sandsend near Whitby in 1981; notice the remnants of the occipital crest.

Head of Regalecus glesne stranded at Sandsend near Whitby in 1981; notice the remnants of the occipital crest.

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.

Regalecus from Sandsend, laid out in Mr Wood's yard at Whitby before preservation.

Regalecus from Sandsend, laid out in Mr Wood's yard at Whitby before preservation.

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.

Gill arch of Regalecus, 2009, showing gill rakers, the heart and red muscle associated with the pelvic fins.

Gill arch of Regalecus, 2009, showing gill rakers, the heart and red muscle associated with the pelvic fins.

Regalecus found on consecutive days at Amble and Tynemouth before dissection and preservation.

Regalecus found on consecutive days at Amble and Tynemouth before dissection and preservation.

Dissection of Regalecus from Amble, showing the bright orange liver, the stomach gorged with euphausiids, and the pink ovaries.

Dissection of Regalecus from Amble, showing the bright orange liver, the large mass of pyloric caecae, the stomach gorged with euphausiids, and the pink ovaries.

Regalecus found at Skinningrove near Whitby in 2003

Regalecus found at Skinningrove near Whitby in 2003

Nudibranch molluscs ( Sea-slugs ) of the Yorkshire Coast & Central North Sea

D. E. WHITTAKER

The nudibranchs belong to that great group of animals known as the Mollusca, which includes the snails, slugs, clams, chitons, limpets, octopus and squid. The bodies of the nudibranchs are without a shell, and the gills, variously carried as veils, branched excresences or plumes on the different species, are therefore totally unprotected, hence the name of the group, the Nudibranchiata, or ‘naked gilled’ molluscs.  Popularly known as sea-slugs, many of these delicate creatures are among the most beautiful and colourful sea-creatures of the British Isles, while tropical species are even more spectacular.

Most British species are small and require careful and laborious searching under boulders on the sea-shore, or among the mass of hydroids and weed brought up in trawl nets. The number of species confirmed as occurring on the Yorkshire Coast is comprehensive, although several other species that are undoubtedly  present in the area have so far evaded observation, due to the difficulties of sampling these molluscs. There have been few workers of this group on this coast, and historical records are few and principally the work of Dr. Irving, who worked the rock pools at Scarborough during the first decades of the twentieth century, while Dr Chris Todd worked the group while a student at the now defunct  Wellcome Marine Laboratory of Leeds University at Robin Hood’s Bay in the mid 1970’s. The present author’s records began during shore work in the early 1960s, extending to include offshore material later in the decade. Of particular interest are the accruing records of the occurrence of nudibranchs in the stomachs of  local fishes.

Reference material in the Archive consists of preserved specimens and photographic records, but close-up video is  now being used to record the animals in situ when found.

Aegires Punctilucens

Aegires Punctilucens - Specimen from the South Bay, Scarborough, Yorks.

Confirmed Yorkshire & Central North Sea species

Dendronotacea

Tritonia hombergi Cuvier, 1803

Tritionia lineata Alder & Hancock 1848

Tritonia plebeia Johnston, 1828

Lomanotus genei Verany, 1846

Lomanotus marmoratus (Alder & Hancock, 1845)

Scyllaea pelagica L., 1758

Dendronotus frondosus (Ascanius, 1774)

Doto coronata (Gmelin, 1791)

A rare North Sea species, this specimen was taken on the shore at Burniston Bay, near Scarborough, Yorks.

Aeolidiella alderi - A rare North Sea species, this specimen was taken on the shore at Burniston Bay, near Scarborough, Yorks.

Doto cuspidata Alder & Hancock, 1862

Doto dunnei Lemche, 1976

Doto fragilis (Forbes, 1838)

Doto hydrallmaniae

Doto koenneckeri Lemche, 1976

Doto pinnatifida (Montagu, 1804)

Doto tuberculata Lemche 1976

Embletonia pulchra Alder & Hancock, 1851

Doridacea

Goniodoris castanea Alder & Hancock, 1845

Goniodoris nodosa (Montagu , 1808)

Armina loveni - Formerly regarded as a rare species around the British coast, this species is common off the Yorkshire Coast; this was the first example found here in 1976.

Armina loveni - Formerly regarded as a rare species around the British coast, this species is common off the Yorkshire Coast; this was the first example found here in 1976.

Okenia aspersa (Alder & Hancock, 1845)

Okenia leachi (Alder & Hancock, 1854

Okenia elegans (Leucart, 1828)

Ancula gibbosa (Risso, 1818)

Acanthodoris pilosa (Muller, 1879)

Adalaria proxima (Alder & Hancock, 1854)

Onchidoris bilamellata (L.,1767)

Onchidoris depressa (Alder & Hancock, 1842)

Onchidoris inconspicua (Alder & Hancock, 1851)

Onchidoris muricata (Muller, 1776)

Onchidoris indet. (a small, purple-spotted species taken on the shore near Scarbough

Onchidoris oblonga (Alder & Hancock, 1845)

Onchidoris pusilla (Alder & Hancock, 1845)

Polycera quadrilineata - Common species on the Yorks. coast, sometimes occurring in large numbers on the shore.

Polycera quadrilineata - Common species on the Yorks. coast, sometimes occurring in large numbers on the shore.

Onchidoris sparsa (Alder & Hancock, 1846)

Aegires punctilucens (Orbigny, 1837)

Limacia clavigera (Muller, 1776)

Polycera quadrilineata (Muller, 1776)

Palio dubia (M. Sars, 1829)

Palio nothus (Johnston, 1838)

Eudoridoidea

Cadlina laevis (L., 1776)

Rostanga rubra (Risso, 1818)

Dendronotus frondosus , an adult mollusc, fully expanded.

Dendronotus frondosus, an abundant sea-slug off the Yorkshire coast; small juveniles may also occur on the shore.

Archidoris pseudoargus (Rapp, 1827)

Discodoris planata ( Alder & Hancock, 1846 )

Jorrunna tomentosa (Cuvier, 1804)

Arminacea

Armina loveni (Bergh, 1860)

Janolus cristatus (Chiaje, 1841)

Janolous hyalinus (Alder & Hancock, 1854)

Hero formosa (Loven, 1841)

Aeolidacea

Coryphella browni Picton, 1980

Coryphella gracilis (Alder & Hancock, 1844)

Coryphella lineata (Loven, 1846)

Corryphella verrucosa (M. Sars, 1829)

Aeolidiella glauca, a common off-shore nudibranch.

Aeolidiella glauca, a common off-shore nudibranch along the Yorkshire coast; video still image.

Flabellina pedata (Montagu, 1815)

Cuthona amoena (Alder & Hancock, 1845)

Cuthona caerulea (Montagu, 1804)

Cuthona foliata (Forbes & Goodsir, 1839)

Cuthona genovae (O’Donohue, 1926)

Cuthona nana (Alder & Hancock,1842)

Cuthona pustulata (Alder & Hancock, 1854)

Cuthona rubescens Picton & Brown, 1978)

Cuthona viridis (Forbes, 1840)

Catriona gymnota (Couthony, 1838)

Tennellia adspersa (Nordmann, 1845)

Tergipes tergipes (Forskal,1775)

Calma glaucoides (Alder & Hancock,1855)

Onchidoris inconspicua, a rarely seen species from between tide-marks, Burniston bay near scarborough.

Onchidoris inconspicua, a rarely seen species from between tide-marks, Burniston bay near scarborough.

Eubranchus exiguus (Alder & Hancock, 1848)

Eubranchus pallidus (Alder & Hancock, 1842)

Eubranchus tricolor Forbes, 1838

Facelina bostoniensis (Couthony, 1838)

Facelina coronata (Forbes & Goodsir,1839)

Aeolidia papillosa (L., 1761)

Aeolidiella alderi (Cocks, 1852)

Aeolidiella glauca (Alder & Hancock, 1845)

Nudibranchs as the prey of Fishes on the Yorkshire Coast

Lacking a shell, the soft bodied nudibranchs utilse a number of defensive mechanisms to avoid predation, including the storage and deployment of stinging cells, derived from the tissues of the anemones and hydroids they devour, the use

Some of the nudibranchs found in the stomach of a gorged black bream, Spondyliosoma. Those shown here include Dendronotus and Lomanotus.

Some of the nudibranchs found in the stomach of a gorged black bream, Spondyliosoma. Those shown here include Dendronotus and Lomanotus.

of calcareous spicules embedded in the surface tissues, and the production of acidic and mucous secretions from special glands, while many species are also thought to exhibit warning colouration. Many others are cryptically coloured, marked, or sculpted and become very difficult to determine in their particular habitat; Aegires punctiluscens, for example, is extremely difficult to detect and its presence is often only revealed by the egg masses they have deposited nearby.

Nudibranchs are sometimes attacked and eaten by the pycnogonids, or sea-spiders, that frequent the hydroids upon which many nudibranchs also prey. However, it is invariably stated that nudibranchs are not eaten by fishes, because of their supposed unpalatability due to the various defence mechanisms listed above. Experiments on the feeding of nudibranchs to fishes at the Plymouth laboratory early in the 20th century are frequently cited as evidence to this view, further supported by the absence of reports of their discovery in the stomachs of fishes.

A sample of many hundreds of Facellinid nudibranch remains from haddoch stomachs off the Yorkshire coast.

A sample of many hundreds of Facellinid nudibranch remains from haddoch stomachs off the Yorkshire coast.

Observations on the Yorkshire coast, however, show that certain fishes regularly devour nudibranchs, sometimes in large numbers.

The black sea bream, Spondyliosoma cantharus, the most commonly found spariid in the North sea, is an occasional trawl catch on the Yorkshire coast. Stomach contents of Yorkshire examples of this fish were examined by simple visual inspection until 1974, when an almost intact specimen of the nudibranch Eubranchus tricolor was found as a prey item in a young fish. It was realised that had the mollusc been in a more advanced state of digestion, or considerably smaller, it would not have been recognised as a nudibranch or perhaps, even as a mollusc, by the naked eye. Had the mollusc been completely digested, the tiny radula and jaws would have gone entirely unnoticed, and thus the presence of nudibranchs as prey items in previous  Yorkshire catches of Spondyliosoma had been overlooked. Dating from this occurrence, all Spondyliosoma  samples have therefore been critically examined under the microscope. It was also apparent that this prey item would not have been recognised by anyone unfamiliar with living or preserved members of the group.

Facellinid nudibranchs from Haddock stomach off Scarborough; cerata are still present on many of the molluscs.

Facellinid nudibranchs from Haddock stomach off Scarborough; cerata are still present on many of the molluscs.

These factors therefore underlie the difficulties of recognising and identifying the remains of nudibranchs in fish stomachs,and which has assisted the prevailing view that as a group they are unpredated by fishes. It should be further realised that the majority of fisheries biologists conducting stomach analyses have no familiarity of the nudibranchiata, the majority of which are of small size, nor is it regular practice to laboriously examine the finer detritus of  the stomachs of demersal fish, unless conducting parasitological studies, and which is essential for the detection of nudibranch radulae and jaws.

Further examinations of  Spondyliosoma caught in this sea area have revealed a number of species of nudibranch are eaten, including one fish being gorged upon several species, belonging to several families, to the exclusion of other types of prey.

Large specimens of Aeolidia, recently devoured, have been found in the cod, while the even larger Tritonia hombergi, a common mollusc on the local trawling grounds, is frequently eaten by both cod and haddock along the Yorkshire coast, the large, unmistakable jaws being easily seen by the naked eye; this sea-slug has of course long been known to be devoured by Scyliorhinus caniculus, the lesser spotted dogfish, elsewhere in the British Isles, though the stomach contents of the Yorkshire population of this fish  have not been investigated.

The haddock devours a wide range of benthic animals on the Yorkshire grounds, including many molluscs, and in particular large numbers of the opisthobranchs Pleurobranchus, Cylichna ,Retusa and Philine, but in the summer of 2005 was also discovered to be predating large numbers of nudibranchs, easily recognised from the many partially digested remains as being facellinids, at a particular off-shore locality. A small sample of gut detritus from  this catch of haddock, examined microscopcally, further revealed many hundreds of facellinid jaws and radulae, indicating not just the large scale cropping of the molluscs by the haddock, but also the huge numbers of these nudibranchs present on the sea bed at this particular locality at that season.