Small hermit crab and sea squirt

The increase in the small hermit crab and the sea squirt Didemnum lahillei has been related to climate change.

Trends in the small hermit crab

The small hermit crab is found in southern parts of Europe, and from about 1940 to 1950 was only occasionally sighted in the Netherlands. Since 1991, there has been an increase in the sightings of the species on Dutch beaches. The sharp fall in the index in 1996 is probably the result of the severe winter of 1995-1996. The paucity of sightings in 1997 is probably the result of the cold spring. In subsequent years the population seems to have recovered, and in 2002 the species was very numerous. The increase could be the result of climate change.

Trends in the Didemnum lahillei

In 1991 a new colony-forming sea squirt appeared in the Oosterschelde: Didemnum lahillei, a species common along the French Atlantic coast. Until 1990 the most northerly sighting of the species was Wimereux in northern France, but from 1991 onwards, small colonies were observed in the Oosterschelde. The number of colonies reported increased spectacularly from 1996 onwards, particularly in Zijpe. And since 1997, divers have been reporting that this species is spreading over large areas of the seabed, ousting other species. Climate change is thought to be behind this, though also important could be the more stable aquatic environment in the Oosterschelde since the completion of the Delta Works.

Sea squirts form a separate class in the animal kingdom, which is related to vertebrates. They are filter feeders that are attached to the seabed.

Technical note

The data on these two species are from the monitoring programmes of Stichting Anemoon. The small hermit crab is counted on the Noordzee (North Sea) beaches, whereas Didemnum lahillei is counted by divers.

References

• Gmelig Meyling, A.W. en R.H. Bruyne (2001). Een duik in mariene gegevens. Lange termijnveranderingen van populaties van enkele mariene organismen (roggen, weekdieren kreeftachtigen e.a.) als gevolg van menselijk handelen. Stichting ANEMOON. Heemstede.