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.