ID : Four Marine Crab Species

[Andrew C]

Hello WAB,

During a rock pooling session at Beer on the East Devon coast I came across four small crabs that I struggled to identify despite being well armed literature wise. I suspect they are too young to be identified but just in case, I wanted to ask if anyone was able to offer any conclusive advice on them. I shall post each one in it's own post to separate them.

Regards,
Andrew.

[Andrew C]

The first of the four species..........

[Andrew C]

The second of the four species..........

[Andrew C]

The third of the four species..........

[Andrew C]

Finally the fourth of the four species..........

[Marineboy]

First one is the broad clawed porcelain crab, Porcellana platycheles. (The long clawed percelain crab is completely smooth, whereas this one is fringed with hairs).
One notable feature of both species is the apparent absence of the fourth pair of walking leg. Crabs have two claws up front, and eight walking legs - same with this species, but you will not see the fourth pair of legs as they are so small and thin, and held within the branchial chamber - held internally, where the gills are - so appear to only ever have three pairs of walking legs.

The other crabs, markedly different in appearance I believe are the juveniles of the same species, the common shore crab, Carcinus maenas. There are other species which are similar, but the back legs, carapace serrations and the three rounded points between the eyes suggest Carcinus maenas. Juveniles of this species, especially the smaller ones can differ wildly in pattern and colour. It is only as they mature they take on the colouration of the adult crabs - even then, you will find the colour of the underside of the mature crabs can range from green through a greeny yellow to even a bright orange. They can appear very different in colour, but all the same species.

[Andrew C]

Originally Posted by Marineboy

First one is the broad clawed porcelain crab, Porcellana platycheles. (The long clawed percelain crab is completely smooth, whereas this one is fringed with hairs).
One notable feature of both species is the apparent absence of the fourth pair of walking leg. Crabs have two claws up front, and eight walking legs - same with this species, but you will not see the fourth pair of legs as they are so small and thin, and held within the branchial chamber - held internally, where the gills are - so appear to only ever have three pairs of walking legs.

The other crabs, markedly different in appearance I believe are the juveniles of the same species, the common shore crab, Carcinus maenas. There are other species which are similar, but the back legs, carapace serrations and the three rounded points between the eyes suggest Carcinus maenas. Juveniles of this species, especially the smaller ones can differ wildly in pattern and colour. It is only as they mature they take on the colouration of the adult crabs - even then, you will find the colour of the underside of the mature crabs can range from green through a greeny yellow to even a bright orange. They can appear very different in colour, but all the same species.

That is wonderful, thanks ever so much Marineboy. Do you have any advice on any books specifically to Crabs/Crustaceans of the British shores?

Regards,
Andrew.

[Marineboy]

Another crab enthusiast?

You would like "The Biology of Crabs" by G.F.Warner - an in depth "how a crab works" book - old book now, but really interesting.
Another old favourite is "British Crabs" by R.W.Ingle - good book, but I wish the photographs were in colour. Over 60 species covered, including the Chinese Mitten Crab which is apparently now in the estuaries of Teignmouth. Still need to get out and find one of those!

[Andrew C]

Originally Posted by Marineboy

Another crab enthusiast?

I am a generalist so anything that grows or moves (within Devon), I am enthusiastic about!

I really do enjoy rockpooling as there is so much out there I have not yet seen.

Originally Posted by Marineboy

You would like "The Biology of Crabs" by G.F.Warner - an in depth "how a crab works" book - old book now, but really interesting.
Another old favourite is "British Crabs" by R.W.Ingle - good book, but I wish the photographs were in colour. Over 60 species covered, including the Chinese Mitten Crab which is apparently now in the estuaries of Teignmouth. Still need to get out and find one of those!

Excellent, thanks. I shall look out for these. Hopefully I will find some second hand copies somewhere.

Regards,
Andrew.