If I were to describe a creature which had blue blood, who had plates of armour for
skin and could withstand over 1000 times the atmospheric pressure at the Earth’s surface, then you would be forgiven for thinking that one was talking about a fictitious alien. However, Maja squinado or the ‘European spider crab’ is no work of fiction and can be found at a beach near you.
Crabs belong to the order Decapoda, which means ‘ten-footed’ in Greek. The spider crab has four pairs of walking legs and one pair of modified legs which it uses as claws. In males, these claws are markedly larger than in the females, but this is not the case with all decapods. Claws can be articulated to grab food items and these crabs boast a wide diet preference. They love sea urchins and sea cucumbers in summer whilst switching to algae and molluscs during the winter time.
The colour of their blood is blue because the main protein which makes up the metalloprotein group is copper. As an aside, humans have iron and that’s why ours is red. The other amazing thing about their blood is that in winter time they migrate to depths where temperatures can be as low as 2oC. Spider crabs produce a special protein in their blood which prevents them from freezing at these depths and this has enabled them to get off the coastal shelf and down to the abyssal plains.
Spider crabs begin their lives as tiny zooplankton and over 2-3 years go through three separate development stages before they begin to settle and grow into the body form we recognise them for. As all crabs, Spider crabs have an exoskeleton which they periodically shed to allow for their growth to continue. This process is called moulting and is a particularly dangerous time for crabs. When they moult, the new exoskeleton which is underneath, takes a number of hours to harden to the prickly state we are familiar with. During this time, they are particularly vulnerable to predation as they have no way to protect themselves; the analogy I would draw is one with a knight in a battle without his suit of armour.
The end of the juvenile stage is marked with a terminal moult after which no further moulting will occur. Consequently, there is no further growth after this time. It also marks both males and females as being fully reproductively competent. Males develop enlarged gonads whilst females widen their abdomen to allow for increased egg carrying capacity. You can reliably tell the difference between all species of crabs by investigating the abdomen; females will have a large U-shaped flap whilst males will have a small v-shaped one.
Typically, female spider crabs will mate 2-3 times a year and, unlike other species of crab, they are not required to have recently moulted in order to allow the male to climb on to her. Each female can carry up to 150,000 eggs in a single brood. This might seem a lot but it’s important to recognise that a tiny proportion of these eggs will make it to adulthood. The latest research has suggested that spider crabs are able to produce viable eggs up to 5 times after a single mating event, which is out of character for the rest of their kin.
A lesser known fact is that spider crabs annually migrate. Spring and summer see them on the coastal shelf area with autumn and winter being spent in offshore sites. It has been suggested that this migration is an evolutionary measure to reduce mortality caused by large seasonal water temperature changes in higher latitudes. It would be very interesting to understand what our local species are doing as the seasonal temperature fluctuations of our coastal water is much smaller.
Locally, these crabs have a good level of protection which has seen the species bounce back from the brink. However, this is not the case along the rest of the Mediterranean due to its commercial value. By way of example, they are considered to be in danger of extinction along the Spanish coast and some reports suggest that they have not been seen along the Mallorca or Menorca coastline since 1989! Until protection was introduced here in Gibraltar, we were following a similar path. Thanks to good environmental management and enforcement, it is now apparent that these populations are starting to recover.
As a double whammy, they could end up being the solution to our algae problem as they have been known to feed on it during winter time in other parts of the Mediterranean. It also highlights the importance of proactive environmental practice of our marine coastline for eradicating issues that human ingenuity fails to solve.
When we were younger some of us might have feared the alien in the closet and, ironically, it is the ‘alien’ along our coastline which might be a worthy ally in our conservation of it.