Sea lamprey (Petromyzon marinus)
|Size||Head-body length at spawning: over 45 cm (2)|
Classified as Least Concern (LC) on the IUCN Red List (1). Listed in Appendix III of the Bern Convention and Annex II of the EC Habitats Directive (3).
Lampreys are some of the most primitive vertebrates alive today, they are known as cyclostomes, which means 'round mouths' and refers to the fact that they are jawless, having instead a round sucker-like mouth. A further primitive characteristic is that the skeleton consists of cartilage and not bone (2). Lampreys are similar in shape to eels, and have a series of uncovered round gill openings (known as gill pores) on the sides of the head and a single nostril on the upper surface of the head (2). The sea lamprey is the largest cyclostome in Europe. It can be distinguished from the other lampreys by its larger size, the marbling of the greyish-green back, and the two dorsal fins, which are widely separated (4). An alternative common name is 'stone sucker' (5), which may have arisen from the habit of males during spawning, when they create a depression in the river bed by wriggling and removing stones with the mouth (4).
The sea lamprey is fairly widespread in UK rivers, but it has declined to extinction in some areas. It is absent north of the Great Glen, Scotland, possibly as it prefers warm water (6). Current strongholds are the rivers Wye and Severn (2). Outside of the UK it is known from most of the Atlantic coastal areas of western and northern Europe between Norway and the Mediterranean. It is also found in eastern parts of North America (6).
Breeds in high quality, deep, fast flowing rivers with clean gravel in which to spawn and a sandy substrate for the larvae to burrow into (6). The adults migrate out to sea (4).
Adults of this anadromous species migrate up rivers in March and April, but spawning actually takes place the following year between May and July (4). Mating occurs in pairs, unlike the other lampreys in which a female is mated by a succession of males (4). The female lays up to 300,000 eggs into a depression in the river bed created by the male. After hatching, the larvae, known as ammocoetes burrow into the sediment where they live for three to five years, feeding by filtering organic particles from the water (4). During metamorphosis, the eyes and the sucker-like mouth develop and the adults then migrate to the sea where they adopt a parasitic lifestyle, feeding by attaching to the bodies of large fish with the mouth and rasping away at the flesh. They remain in the sea for a few years and then return to freshwater in order to spawn. They do not feed during this return trip because the digestive organs degenerate, and shortly after spawning they die (4). Roman, Viking and Medieval Britons regarded river and sea lampreys as delicacies (2).
Pollution and changes in land use have affected this species (2), and river construction works have prevented migration along many traditionally used rivers (4).
A number of UK sites that support sea lampreys have been designated as candidate Special Areas of Conservation (SACs). Although this will be a good foundation for conserving the species, further action will be required. To this end, a draft Action Plan has been produced to guide future conservation efforts (7). Furthermore the Life in UK Rivers Project is helping to conserve this species (8).
For more on the Life in UK Rivers Project see:
- English Nature:
Information authenticated by the Environment Agency:
- Anadromous: in fish: those species that spend most of their lives at sea but migrate to fresh water to spawn.
- Dorsal fin: the unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
- Larvae: stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
- Spawning: the production or depositing of large quantities of eggs in water.
- Vertebrates: animals with a backbone.
IUCN Red List (December, 2009)