Bremerhaven, 19 December 2012. Up to now the shore crab has belonged to those animal species thought by scientists to be more or less immune to climate change. One reason for this was that the crabs are highly tolerant to extremes temperature and feel just as at home in the eight degrees offered by the Atlantic as they do in the 20 degrees warm Mediterranean. A study conducted by German and Italian scientists has now shown, however, that shore crabs react most sensitively to temperature anomalies at certain times of life – as embryos in the egg. If the embryonic crab is subjected to excessive heat in its first phase of life, a fatal chain reaction starts in the egg.
The fishermen in the lagoon of Venice catch shore crabs preferably once they have molted. The new shell of the crabs has not yet hardened at this point and they represent a valuable gastronomic specialty. One kilogramme of shore crab costs between 60 and 70 euros on the fish markets of Venice. They are quite a delicacy.
However, up to a very short time ago no one had really given any thought to whether there will be sufficient quantities of the shore crab in the lagoon of Venice in future in view of climate change. This is because Carcinus maenas, the Latin name for the shore crab, has so far shown itself to be extremely flexible in matters of feel-good temperature. The crabs about the size of a hand palm live both along the cold Atlantic coast of Norway and North America and in the comparatively warm Mediterranean, whereby the fishermen of Venice catch the closely related Carcinus aestuarii. “Both species are so closely related that they are quite able to mate and produce common offspring“, says the Italian biologist Dr. Folco Giomi.
During a guest period at the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association, he and his colleagues investigated whether the Mediterranean green crab Carcinus aestuarii really can survive periods of heat without harm as has so far been assumed. The German-Italian team of scientists found what they were looking for in the crab offspring. “The female Mediterranean green crabs do not lay her eggs on the bottom of the lagoon where the water is comparatively cool”, says Folco Giomi. Instead, the animals carry their offspring on their abdomen and carry them during daily in shallow water.
This is a precarious behaviour because if the Mediterranean region heats up at the time of spawning due to heat waves, the temperature of the shallow water primarily will rise sharply. “Our investigations have shown that a rise in water temperature from 16 to 24 degrees Celsius is sufficient to seriously endanger the embryos in the eggs”, explains the scientist. The embryos therefore suffer such acute warming and their metabolism may anomalous accelerate up to nine-fold. As a result the animals deplete very rapidly all the energy reserves in the egg. “Under such conditions the crab embryo cannot develop normally. It dies”, says Folco Giomi.
The team of scientists was able to prove this alarming behaviour in embryos in the first two of four development stages. These are stages at which the embryo is little more than a heap of cells. “Starting from the third stage of development the embryos was then able to decelerate their metabolism and adjust to the rise of ambient temperature like an adult animal. This observation was quite new to us and we were greatly impressed”, explains Folco Giomi.
The scientists did not want to leave it there, however. There were too many associated questions, such as whether exceptional heat waves had already left their mark on the crab numbers in the past? To this end the researchers compared the crab catch data of the Venetian fishermen from the years of 1940 to 2009 with the weather data for this period. “During this comparison we established that the fishermen most definitely feel the impact of the consequences of an exceptional heat wave at the time of spawning – however, always two years later, i.e. when those crabs which died as embryos would reach the sexual maturity. The animals require precisely these two years to grow from a young animal into an adult crab”, says Folco Giomi.
He and his German colleague Prof. Hans-Otto Pörtner draw two conclusions from these research results. “The study firstly shows that climate change also affects those species which have a relatively high temperature tolerance. This would appear to always apply in regions in which they reach the limits to their temperature-dependent prevalence“, says Hans-Otto Pörtner, head of the Integrative Ecophysiology working group at the Alfred Wegener Institute. “This finding is really important”, adds Folco Giomi: “If we must assume that even temperature-robust species such as the shore crab are only to be found in small quantities in some regions as a result of the progressive climate change, species with a higher temperature sensitivity have even poorer future prospects.”
Secondly, scientists and fishermen ought to see the results as an opportunity to give thought to a sustainable crab fishing strategy which is adjusted to climate change. “The shore crabs living in the Mediterranean are in a geographical dead end in view of the increasing global warming. They are only seldom able to find retreats during heat waves which is why we cannot rule out that this species will become extinct in some regions”, says Hans-Otto Pörtner.
According to Folco Giomi, the fishermen of Venice can make a contribution to delaying this development in their lagoon. Giomi: “Whenever a heat wave rolls over the Mediterranean region during the spawning period of the crabs, the fishermen should note this and make sure that they only catch a small amount or no animals two years later.”
Notes for Editors:
The original publication is called:
F. Bartolini, A. Barausse, H-O. Pörtner, F. Giomi (2012): Climate changes reduces offspring fitness in littoral spawners: a study integrating organismic response and long-term series and appeared in the scientific journal Global Change Biology. (doi: 10.111/gcb12050)
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The Alfred Wegener Institute conducts research in the Arctic and Antarctic and in the high and mid-latitude oceans. The Institute coordinates German polar research and provides important infrastructure such as the research ice breaker Polarstern and stations in the Arctic and Antarctic to the international scientific world. The Alfred Wegener Institute is one of the 18 research centres of the Helmholtz Association, the largest scientific organisation in Germany.