The Biology Of The Ray’s Bream, Brama Brama, From The Yorkshire Coast And Central North Sea : Scarborough Data.

D. E. WHITTAKER

 

Introduction

Some of the Ray's Bream collected and sampled at Scarborough during the 2008/9 autumn-winter period.

Some of the Ray's Bream collected and sampled at Scarborough during the 2008/9 autumn-winter period.

A prominent characteristic of the fish fauna of the North Sea coast of the British Isles, from Scotland to Suffolk, and particularly of the Yorkshire sector, is the occurrence of the oceanic, mesopelagic Brama brama, the Ray’s Bream, first recorded as a British fish at Middlesborough Marsh on Teeside in 1681 by Ray and Willughby.

Ray’s Bream is a typical member of the family Bramidae, characterised by their deep, laterally compressed bodies, covered in large, brightly silvered scales. The pelvic fins are small, but the pectorals are very long and blade-like, while the caudal fin is long and deeply cleft. Internally, the gut is short, and provided with a few very large pyloric caecae.

Ray’s bream is found commonly in deep-water in the Western Mediterranean, in the Atlantic off Madeira, Spain and Portugal, and each summer migrates north, to the west of Ireland, as far as Norway. It is known to be predated on, in these mesopelagic depths, by sharks including Centrophorus squamosus.

The numbers of Ray’s bream penetrating south into the shallow North Sea basin, however, are not constant, with the fish sometimes going unrecorded for many years. Alternatively, the fish periodically and inexplicably shows great incursions and increased frequency into the North Sea basin, and these events may sometimes extend over several consecutive years.

Dissection of a male Rays Bream, showing the rakers and gills, the stomach and the large pyloric caecae.

Particularly good years of greater migration into the North Sea were in the 1920’s, peaking in 1927 and 1928, and again in the late 1940’s and ‘50’s, peaking in 1952, but historically, the events of the greatest magnitude have occurred within the last 40 years.

Causal Factors Of Ray’s Bream Events In The North Sea Basin

The nature of these “Rays Bream events” down the east coast and their causal factors is not understood, and a number of biological and physical components have been suggested as being involved in their creation, including:-

i)   Periodic successful spawning and larval survival years in their home waters, giving rise to a number of heavy year-classes of the fish which then become more numerous and noticeable in their northward migration, with subsequent mass penetration of the North sea.

ii)   Unusual hydrographic conditions that cause large runs of a normal population of the fish from the Atlantic into the North Sea.

iii)  A combination of these two components, when years of increased stocks of the fish also coincide with years of increased Atlantic inflow into the North Sea.

Dissection of a female Rays bream. The liver is displaced to show the viscera in situ. Note the large pyloric caecae and the ovaries below the abdominal ribs.

iv)  A useful study by the researchers Mead and Haedrich (1965), who gathered all the known northern occurrences of the fish up to that time, postulated that Rays’s Bream is limited in distribution to waters warmer than 12.5 degrees Centigrade, and could not survive in temperatures below 10 degrees Centigrade.

A study of the data accumulating since 1965 reveals problems of consideration of any of these factors, and the nature of these Ray’s Bream events is evidently more complex.

Ray’s Bream Events Since 1965, Scarborough Data.

No local records of the fish are known for 1965 or 1966, but since the inception, by the author, of the Yorkshire Rare Fish Recording Scheme at Scarborough in 1967, enlisting the daily retention and collection of rare and unusual fishes from the Scarborough fleet, all specimens of Brama caught by fishing gear have been recorded, with the local beaches also being monitored for their occurrence by stranding. The accumulated records therefore illustrate the relative frequency of occurrence, together with the length distributions, of Ray’s Bream occurring on the Yorkshire coast, from 1967 to 2011.

Until recent years, the greatest recorded run of the fish into the North Sea began in 1967, the numbers increasing year upon year until the peak of the mid 1970’s, following which the migrations fell away to a cessation of records in 1983.

All Brama obtained at Scarborough are measured and examined internally for stomach contents, parasites and gender.

Occurrences of the fish were rarely recorded in the following 20 year period until their comparative reappearance in 2005, their virtual absence from the North Sea during this period resulting in many east-coast fishermen only recently encountering the fish for the first time in their fishing careers.  Since 2005 the numbers of fish occurring in each succeeding year have once again continued to increase in a repetition of the events of the late 60’s and early 1970’s.

The numbers of fish recorded in the autumn/winter of 2009/10 are therefore, as expected, greater than those of the corresponding 2008/9 season, and both these latter periods are at least equally comparable in magnitude to the 1975/6 and 1976/7 peak years.

A direct comparison of the Yorkshire records between these two time periods is not possible, because of the collapse, since the late 1980’s, of trawling effort by the Scarborough fleet, a daily and primary source of numerous records of rare and unusual fish records. Returns from the present day fleet are therefore considerably reduced.

 

The otoliths of Brama, like those of most pelagic fish, are small, in fact smaller than those of many small shore fishes.

However, the records accumulating from other methods of monitoring the fish are comparable, and together with the background of recent anecdotal reports from along the coast, confirm that the magnitude of the 2009/10 incursion is historically the greatest of the fish to have occurred, and exceeds that of the 1975-7 maximum. While it is possible that the 2009/10 Brama season may have seen the peak of the current cycle, with numbers of the fish beginning to fall away in forthcoming seasons, it is equally possible that the incursion cycle has not yet attained its maximum, in which case the numbers of Brama occurring on the Yorkshire coast in 2010/11 will be phenomenal.

 

The length (age) distribution of the fish occurring recently, compared to those of the earlier period, is very different, the fish being much smaller than the very large fish occurring through the 1970’s.

 

Otoliths of Brama brama

Otoliths of Brama brama upon immediate extraction from the fish.

Otoliths of Brama cleared for examination.

Otoliths of Brama cleared for examination, and revealing zones of growth.

 

 

 

 

 

 

 

 

 

 

 

Sampling Of Ray’s Bream

Recording of Brama brama each year has followed a standardised method of biological sampling to give comparative results throughout the ongoing sequence of data:-

The weight of all whole fish is measured in grams.

The total length, measured in millimetres, is that from the snout to the extreme tip of the closed, outstretched, longest lobe of the caudal fin.

NB The caudal fin of Brama is extremely variable; in some fish its proportion is noticeably large in comparison to the total length of the fish, and this fin is also frequently asymmetrical, with either dorsal or ventral lobe being longer than the other.  Standard length only has therefore been used to compare the yearly length distributions and for other comparative and statistical work.

Stomach contents of a Brama from the Yorks coast, including a Horse Mackerel, Caranx, a little cuttle, Sepiola, and a clupeid,

The standard length is recorded as that from the snout to the terminal ventral margin of the caudal peduncle.

Buccal and branchial cavities are examined for parasites.

Gills are then removed and preserved for later parasite investigation.

Otoliths for age studies are removed, washed in distilled water, followed by acetone, and stored in tubes. Otoliths of Brama are difficult to interpret and it has been found useful to scorch some otoliths to a dark brown colour before examination in one of the usual clearing media or mounting in DPX.

Viscera are examined in situ and then removed, stomach contents are examined and the stomach preserved with the gut for later parasite investigation.

The fish is sexed and ovaries weighed and preserved for later examination.

Fish recovered from the beaches are frequently damaged by birds, but such material not only forms part of the yearly monitoring of the numbers of fish occurring, but is still invaluable for providing data since at the very least the important standard length can be recorded and otoliths obtained. Although under such conditions the viscera may reveal parasite infection data.

An unusual prey item for Brama, a dragonet, Callionymus lyra.

Feeding And Prey Of BRAMA BRAMA Off The Yorkshire Coast

All Brama examined on the Yorkshire coast are fine, heavy, healthy fish, that appear to be well fed, probably following heavy feeding activity during the summer period, when the fish escapes notice due to the absence of any local fishery using pelagic gear that will select and reveal its presence at this time.

Only in 1976 and 1977 when numerous boats along the coast converted their fishing gears for participation in the increased sprat fishery of that time, was a suitable pelagic fishery present to reveal the presence of the fish, high in the water column, on the Yorkshire grounds. A Russian sprat-fishing vessel, one of many present off the Yorkshire coast in 1976, was observed to have strings of split and salted Brama hanging up to dry on deck.

Brama caught in the sprat fishery at that time were gorged chiefly upon sprats, Sprattus, but young mackerel, Scomber, herring, Harengus, and whiting, Merlangius, were also found in stomach contents. Other prey items recorded included euphausiids and 0-group stages of the grey gurnard, Eutrigulus.

Although most autumnal and winter fish of the 2009/10 incursion were, as usual, devoid of stomach contents, a small proportion of the fish have been found to be still actively feeding into January 2010, despite the low temperatures. Prey items have again included euphausiids and clupeids, together with Scad, Caranx, and Sepiola, and more frequently, small squid [Loligo or Alloteuthis].

What would seem a very unusual prey item for Brama, an adult dragonet, Callionymus, was found in the stomach of a fish taken at Scarborough in January 2010.

Temperature Cropping Of BRAMA BRAMA

The rays bream begins to strand on gently sloping beaches when the temperature begins to fall, with Yorks records extending over several months.

As noted above, there are no pelagic fisheries operating off the Yorkshire coast to reveal the presence of the fish in the warmest period of late summer, when the fish will be most active, and when a review of the records, as shown by Mead and Haedrich, suggests that the fish, in some years at least, should be present in the Central North Sea. Historically, records of Brama in the area only begin to accumulate with the progressive autumnal fall in sea temperatures, the probable reason for a proportion of the fish becoming more susceptible to capture by static demersal fishing gears such as trammels and gill-nets, slow-moving demersal trawls, and by stranding on gently sloping, sandy beaches during this period of decreasing sea-temperatures.

The effect of falling temperatures on Brama is not immediate and catastrophic, but is protracted over many weeks, with fish continuing to be stranded through January and into mid February 2010, when sea temperatures are considerably below the tolerance parameters concluded by Mead and Haedrich, and a noticeable later time-shift in the fall-out period of Brama is demonstrable in the records between the cycle of 1967-1982 and that of the present cycle, 2005-(2010).

Parasite Fauna Of Yorkshire BRAMA BRAMA

A number of parasitic taxa have been recorded from the several hundred Brama that have been examined in the Scarborough district.

The free-swimming copepod Caligus is frequently encountered in the buccal cavity and inside the opercula, and has also been found to be present in some numbers on the external surface of those fish recovered immediately on stranding. No sessile parasitic copepoda have been encountered.

This common parasite of Brama is the large monogenean trematode, Kollikeria. Three large,encysted females with their tiny male companions are shown here, inside the operculum of the fish.

The gill axils and tissue covering the cleithrum and branchiostegals are frequently infected with didymozoon monogenean trematodes, including Koellikeria filicollis, with very large encysted females sometimes being present. More infrequently, the tissue of the anterior wall of the pericardium is also infected. Another monogenean trematode, Octodactylus, is the only parasite recorded from the gills of the Yorkshire fish, but is extremely rare.

Tetraphyllidean cestode larvae are present in the gastro-intestinal tract of most fish. No adult cestodes have been found. The musculature of the majority of Brama examined has been infected with the large larvae of the Lacistorhynchid cestode Floriceps gigas.

Nematodes, including Thynnascaris and Hysterothylacium are surprisingly infrequent and infection rates always low.

Infections by digenean trematodes , although commonly encountered in most fishes, have rarely been found in Yorkshire specimens of Brama, despite the large number of fish examined.

Species of Acanthocephala are also rare. Single specimens of  a species of Echinorhynchus , unusual for its vivid colouration, occurring in the coelom of  Brama in 1976 and 2011, are the only adult acanthocephala recorded and is a very rare infection. Larvae of Bolbosoma have also been recorded.

Acanthocephalan parasite from Brama brama

Detail of the anterior body and everted proboscis of a rare acanthocephalan from Brama brama. This species has been found in the coelom of the fish, not in the gut as is usual with acanthocephala.

Acanthocephalan parasite from Brama brama.

This brightly coloured acanthocephalan parasite, Echinorhynchus, is a rare species from the coelom of the Ray's bream, found on only two occasions in Yorkshire examples of this fish.

The biology of sharks occuring off the Yorkshire Coast – some recent examinations

A porbeagle shark being winched out of the fish hold on the scarborough trawler Independence in 1999.

A porbeagle shark being winched out of the fish hold on the scarborough trawler Independence in 1999.

D. E. WHITTAKER

 

A number of sharks are taken by fishing gear as a by catch off the Yorkshire coast and are invariably brought back to the fish market, where they always attract interest.

Only one shark species occurring locally has formed the subject of a specific, targeted fishery by Yorkshire commercial boats, although Porbeagle sharks have, in the past, been targeted along this coastline by Danish fishermen, setting sub-surface long lines.

There is very little biological information relating to most previous catches of shark occurring off the Yorkshire coast, because until recently, the unnecessary tradition of selling sharks in a whole and un-gutted condition has usually precluded anything other than a cursory examination.  Over the years a small amount of information has been gleaned from local catches of Basking shark and, very rarely, of the porbeagle.

In recent years, however, more thorough examination has been made of three species through the help and permission of a number of fishing skippers and fish salesmen; porbeagle sharks and a thresher have been dissected either before or after sale, and their stomach contents, gut parasites, and reproductive organs have been examined. Extraction of vertebrae, which are used for ageing studies of sharks, is however, not possible for examinations carried out when the fish are still in the middle of the marketing and transport process.

A rare opportunity to examine a small example of the basking shark, now protected from exploitation, was obtained when the skipper of a Scarborough trawler, Shaun Crowe, kindly brought back for examination a specimen found to be dead on hauling the net, instead of merely dumping the dead fish back onto the fishing grounds without any information being obtained from it.

Sharks Recorded From The Yorkshire Coast And Central North Sea

A fine blue shark stranded on a Yorkshire beach.

A fine blue shark stranded on a Yorkshire beach.

A large number of shark species inhabit the deep water both on and off the continental shelf to the west and north of the British Isles, including the deep northern north sea, beyond the 200 metre depth contour. The greater North Sea area to the south of this becomes increasingly shallow, thus effectively restricting their distribution further south. Twelve species of shark may be regarded as having been recorded from the survey area of the central North sea, but only two of these deep water demersal species, the large Greenland shark, Somniosus microcephalus and the curious bramble shark, Echinorhinus brucus, have, with any frequency, penetrated further south into this shallow area.

The Greenland shark was caught on a number of occasions in Victorian times and in the early 20th century, but has been extremely rare since then. There have been three remarkable captures of small juvenile fish only during the present survey, one of which had been feeding on a cetacean corpse immediately before capture.

Twelve species of shark have been recorded from the Yorkshire coast, including four epipelagic, open water species and seven demersal species while another, the wide ranging and cosmopolitan spurdog, is the only species to have been targeted specifically by Yorkshire fishermen.

Two female porbeagle shark of similar size on Scarborough fish market. One is immature, the other mature.

Two female porbeagle shark of similar size on Scarborough fish market. One is immature, the other mature.

Isuridae

Lamna nasus (Bonnaterre, 1788) Porbeagle Shark

Cetorhinidae

Cetorhinus maximus (Gunnerus, 1765) Basking Shark

Alopiidae

Alopias vulpinus (Bonnaterre, 1788) Thresher or Fox Shark

Scyliorhinidae

Scyliorhinus canicula (Linnnaeus, 1758) Lesser spotted dogfish, locally Nurse

Scyliorhinus stellaris (Linnaeus, 1758) Nursehound, Greater spotted Dogfish

Carcharinidae

Ovaries of immature and mature porbeagle sharks shown above on Scarborough fish market.

Ovaries of immature and mature porbeagle sharks shown above on Scarborough fish market.

Prionace glauca (Linnaeus, 1758) Blue Shark

Triakidae

Mustelus asterias Cloquet, 1821 Starry Smooth Hound

Galeorhinus galeus (Linnaeus, 1758)  Tope

Squalidae

Squalus acanthias Linnaeus, 1758  Spiny or Spur dogfish

Echinorhinus bruchus (Bonnaterre, 1788) Spinous or Bramble Shark

Somniosus microcephalus (Bloch & Schneider, 1801) Greenland Shark

Squatinidae

Squatina squatina (Linnaeus, 1758) Fiddle fish, Monk fish

A NOTE ON THE SPECIES OF MUSTELUS IN THE NORTH SEA

Two species of the common small ground sharks known as smooth hounds are known from european waters, Mustelus asterias, the starry smoothhound, and Mustelus mustelus, the plain smoothhound, and both species are frequently stated in reference works to occur throughout the North Sea.

Ovary detail of a mature porbeagle shark.

Ovary detail of a mature porbeagle shark.

M. asterias frequently has a scattering of white specks over the body, but it is extremely variable in this respect, and therefore this marking must not be relied upon to be a specific character; off the Yorkshire coast, many fish are totally devoid of this white speckling and therefore resemble M. mustelus.

Positive identification must be made by examination of the dermal denticles, which in M. asterias are ridged almost to the tips, while those of M. mustelus are ridged only at their bases, with smooth tips.

All specimens of Mustelus examined during the survey, whatever their external markings, have proved to be Mustelus asterias, and M mustelus is therefore not regarded as being present in the central North sea.

Changes In The Status Of Some Central North Sea Sharks

There have been changes in the status and distribution of several of the species formerly recorded from this sea area. As noted above, the Greenland shark, Somniosus, is now extremely rare and has been so for several decades. The bramble shark, Echinorhinus, a sluggish demersal species from deep water of the continental shelf, was occasionally caught in the North Sea during the 19th century, but has not been recorded since the last example was caught, rolled up in Whitby fishermen’s lines, in the 1890’s.

Dissection of the basking shark at Scarborough.

Dissection of the basking shark at Scarborough.

Two ground sharks, the angel shark, Squatina, formerly known to Yorkshire fishermen as the fiddle fish, and the greater-spotted dogfish or bull-huss, Scyliorhinus stellaris, are now extinct in the area; Squatina has not been seen since the late 1950’s, and the bull huss was last seen in the early 1970’s.

The thresher, Alopias vulpinus, was also frequently reported in the 19th century, particularly around the Whitby area, where it was regularly reported by fishermen to be chasing salmon and trout off the harbour; individuals were caught into the early 20th century, since when records have ceased until recently. A young, recently born pup was reported from a salmon net in Bridlington Bay, and released, while a female  caught in Filey bay in 2007, was  believed, on dissection, to have liberated young just a short time before capture.

The Most Recent Examination; A Female Porbeagle Shark From Whitby, Caught By Richard Brewer, Nov. 2009.

A large female porbeagle was landed by Richard Brewer on the Whitby fish market on November 13th 2009 and was bought by Alliance fish . The shark was brought through to their premises at Scarborough, where the fish was dissected before final transport to their customer; we are grateful to Mr Bob Scarborough for permission to dissect the shark, and to his staff for assistance.

The shark was one of the largest seen on the local markets, and also one of the most interesting. It had eaten a number of fish, but we were surprised to find its most recent meal had been of a large salmon, full of roe, that was only partially digested.

This shark was a gravid female, with huge ovary and swollen uteri, each of which contained numerous unfertilised eggs in capsules, among which were two tiny pups.

Gravid porbeagle at Whitby , Nov. 09; two pups among capsules full of yolk eggs that they will slowly devour.

Gravid porbeagle at Whitby , Nov. 09; two pups among capsules full of yolk eggs that they will slowly devour.

Porbeagle pups from a shark landed by Richard Brewer at Whitby, nov. 09. Note the enlarged branchial area and the stomach bloated with yolk eggs devoured while in the uteri.

Porbeagle pups from a shark landed by Richard Brewer at Whitby, nov. 09. Note the enlarged branchial area and the stomach bloated with yolk eggs devoured while in the uteri.

The Chinese Mitten Crab, Eriocheir Sinensis, on the Yorkshire Coast

D. E. WHITTAKER

 

A male chinese mitten crab; note the box-like carapace and the long legs.

A male chinese mitten crab; note the box-like carapace and the long legs.

The Chinese mitten crab, Eriocheir sinensis, is a medium sized crab, unmistakeable when found; its colour is a light greenish-grey to brown, with a box-like carapace which in large specimens may reach over 70mm in width. The front of the carapace between the eyes, bears a concave central notch; the sides of the carapace have four, not very prominent, teeth. The legs are long, with an extreme span nearly five times the carapace width, and with dense setae along the distal half of their length.

The chelipeds are the most notable feature of this crab, being clothed in a dense fur of long setae, this feature having given the crustacean its name of mitten crab.

This crab is a native of China, and is typically a fresh-water species that migrates to saline waters in order to breed, the larvae developing in the sea.

The species first appeared in Europe in 1912 when it was found in Germany, but was not found in Britain until a specimen found in the Thames at Chelsea in 1935 became the first to be recorded here; only since the 1970’s has the crab made a slow but steady advance elsewhere in the country.

First recorded in the Humber in 1976, it has now ascended many of its tributaries, and is now found through South Yorkshire; in 2008 a specimen gained publicity by being taken from the River Don almost in the centre of Doncaster. Further north its presence was first confirmed in the River Tyne in 2001, and to the west, the first Irish record was confirmed in January 2006.

Claws of a male chinese mitten crab showing the dense fur-like setae; specimen from Bridlington Bay.

Claws of a male chinese mitten crab showing the dense fur-like setae; specimen from Bridlington Bay.

An unconfirmed report was received in 2008 that a specimen had been taken by a fisherman in Bridlington Bay, but in early 2009 a living crab was collected from Bridlington Bay, with a number of other marine specimens, during our survey of Brama brama along the coast. This is the first known occurrence of the crab from the sea area off the Yorkshire coast; the specimen is now preserved in the crustacean collection at Scarborough.

Adula simpsoni (Marshall), a bivalve associated with Whale skulls in the North Sea.

The crew of a Scarborough trawler struggle to remove the rotting carcase of a 40ft whale brought up in the trawl; remains of whales and dolphins are frequently seen by trawler crews. Photo donated by Malcolm Ward.

The crew of a Scarborough trawler struggle to remove the rotting carcase of a 40ft whale brought up in the trawl; remains of whales and dolphins are frequently seen by trawler crews. Photo donated by Malcolm Ward.

The rotting or skeletonised remains of whales and dolphins are frequently brought up in the nets of trawlers working round the British Isles. However, it is only in recent years that biologists have discovered that the remains of dead whales, known as whale falls, attract a series of activities of other marine species, that not only reduce the carcase to its bony framework, but which then begin to colonise, and in some recently discovered polychaete worms, also extract nutrients from, its bones.

In the eastern North Atlantic and around the north of Scotland into the North Sea, however, a small bivalve of the mussel family Mytilidae, has long been known to be associated with the remains of whales.

Adula simpsoni has the exceptional habitat of settling in the sutures and inside the craniums of old skulls of whales, where it attaches firmly by means of the byssus threads.The bright yellow-brown shells of Adula are thin, with a glossy, varnished-like periostracum. It grows to a maximum length of 30mm, but shells approaching this size are always corroded, a factor that, given the extremely thin nature of the shells, probably brings about the death of the animal through exposure to predatory invertebrates. Growth stages on the shells are not very obvious, but it is likely that the life-span of Adula is not very long despite the otherwise protective nature of its environment.

The glossy shells of Adula grow up to 30mm long; the mollusc is found on old whale skulls.

The glossy shells of Adula grow up to 30mm long; the mollusc is found on old whale skulls.

Xylophaga, supreme wood-boring molluscs from the Yorkshire coast.

A dense colony of Xylophaga dorsalis, where many of the intervening walls between the burrows have been bored away.

A dense colony of Xylophaga dorsalis, where many of the intervening walls between the burrows have been bored away.

D. E. WHITTAKER

 

Natural timber carried into the sea from rivers in winter flood, or processed timber in the form of mans ships, boats and lost cargo, form a very special habitat for certain groups of wood-boring bivalve molluscs, of which the most well-known are the ship-worms of the family Teredinidae. A closely related group of molluscs are the wood piddocks, members of the Xylophagidae, whose wood-boring capabilities are just as devastating as those of the shipworms.

Like the shipworms, the shells of the mollusc have lost their function of protecting the soft body, and have become instead, specialised cutting tools with which the post-larval animal rapidly penetrates the timber, and which continue to grow through the life of the animal. The front margin of the shells are cut away to enable the round disc-like foot to grip the head of the boring, while the projecting, curved shell lobes above the foot bear rows of minute cutting teeth that cut a concave path deeper into the timber. Immediately behind these lobes, the sides of the rounded shell also bears lines of teeth, by which the walls of the excavation are simultaneously cut away, leaving, as the animal grows and advances, a perfectly smooth-walled boring through the timber.

Two species of Xylophaga occur in Yorkshire waters, X. dorsalis, and X. praestans.

XYLOPHAGA DORSALIS

The large wood-piddock Xylophaga praestans, contracted in its burrow.

The large wood-piddock Xylophaga praestans, contracted in its burrow.

The most prolific of all the wood boring molluscs is the pea-sized Xylophaga dorsalis, whose erratic borings, running in all directions and cutting across each other, completely demolish the timber, filling their surroundings with a paste of wood particles; it is found at all depths, from the deepest off-shore pits to just a couple of fathoms of water off the coast, and infests even the thin bottoms of traditional crab pots.

XYLOPHAGA PRAESTANS

A much larger species, Xylophaga praestans, is a very special member of our coastal waters in a number of ways. First discovered and described as recently as 1912, it is rarely seen by most biologists and occurs only in the North Sea, and is usually quoted as being a rare species occurring only off the coasts of Northumberland and Durham, but in fact its distribution extends over a much greater area, both north and south. The first, and largest specimen that I examined came from just twelve miles off Scarborough in 1967, but I have, over the years, also seen its borings from many locations, including timber trawled up in the Norwegian sector of the North Sea.

This species can grow to a large size, cutting enormous, beautifully smooth borings up to 30 mm in diameter and over 300mm long, although usually it is much smaller. This first Yorkshire specimen was a solitary individual living in a blackened, sulphide rich piece of tree trunk that was so soft it could be shredded between the fingers, and on its body were bonded the traces of an egg capsule of some other organism that had at one time also occupied the boring, but which defied identification.

Anterior aspect of Xylophaga praestans in its boring, showing the round foot and the projecting anterior lobes of the shell that cut away the timber at the head of the burrow.

Anterior aspect of Xylophaga praestans in its boring, showing the round foot and the projecting anterior lobes of the shell that cut away the timber at the head of the burrow.

The mystery was solved several years later when a large colony of X. praestans was found in a log hauled up from the deep water around Bruceys Garden, near the edge of the Dogger, and where this wood-borer is common. Scattered through the borings and over the wood piddocks themselves were bonded hundreds of tiny brown domes, the egg capsules of a species of flatworm that was finally found by flushing the borings through with preservative. This is a remarkable and previously unknown example of commensalism; the flatworms Taenioplana teredini and Stylochoplana affinis are recorded from empty burrows of Teredo elsewhere in the world, but this appears to be the only such association known in British waters, or with Xylophaga.

Other organisms inhabit the Xylophaga borings including a specialised polychaete that ingests the finely ground wood particles that partially fill the borings.

Xylophaga species, unlike the shipworms, do not digest the wood that they file away, but like related bivalves, filter minute planktonic organisms and detritus out of the surrounding water.

Although sometimes found in dense aggregations, they may also be found in low numbers or even as solitary isolated individuals, and frequently as isolated units at great depth, and the wood piddocks have therefore evolved a different reproductive strategy to other bivalve molluscs. Like the shipworms, smaller younger individuals are male, which as they grow, change to female. Unlike the shipworms however, sperm produced by the males is stored in a special receptacle, and when the animal eventually changes sex, this sperm is used to fertilise the eggs.

Shells of the wood-piddock, Xylophaga praestans.

Shells of the wood-piddock, Xylophaga praestans.