Friday mystery object #393 answer


Last week I gave you this specimen from the Dead Zoo to have a go at identifying:

It was a bit mean of me to not include a scale, but several of you managed to work it out regardless.

The overall group is fairly easy to spot, since it has 10 legs, the front pair bearing claws (or chela if you want to get technical) and the main body area is rounded. So it’s a crab.

In addition, the long legs and small body give it an overall shape reminiscent of a spider, so it’s a good bet that it’s some kind of spider crab.

Now, there are quite a lot of types of spider crab out there, but that sub-triangular body shape and those long legs help narrow down the possibilities further. In fact, it does share some similarities to the gigantic Japanese Spider Crab.

Japanese Spider Crab specimen at American Museum of Natural History. Image from Popular Science Monthly, June 1920

Unlike the Japanese Spider Crab (which was suggested), this doesn’t have extremely elongated chela. So not one of them. It’s also way too small, although my lack of a scale bar doesn’t make that obvious – sorry! However, the mystery object is in the same family (the Inachidae).

Once you start looking at the genera in the Inachidae there’s only one that matches the mystery object’s proportions, and that’s the Macropodia. Once you get that far, it becomes a case of discounting possibilities based on much more detailed features.

The Marine Species Identification Portal is a fantastic resource for checking this finer level identification. Going through the various species descriptions in there helps spot the key features for distinction between species.

In this case, the mystery object is particularly similar to M. tenuirostris and M. rostrata and it’s mainly the shape of the carapace around the ‘shoulders’ where the chela attach to the body that help confirm this to be the Long-legged Spider Crab Macropodia rostrata (Linnaeus, 1761).

So it was a good effort for everyone who managed to get this to family level, I congratulate those who worked this out to the genus and I doff my hat to anyone who managed to identify it to species. So jennifermacaire, my hat is doffed!

2 thoughts on “Friday mystery object #393 answer

  1. But Macropodia rostrata🦀 have puny arms, and they’re all fluffy!
    Macropodia tenuirostris🦀 has the same big arms – just Paolo accidentally snapped his nose (#rostrum) off, but only me and my actually correct answer suffer the consequences. 😅😭

    FYI: Almost 1000 species in the Inachidae (spider crab) family, and 20 in the Macropodia genus: http://www.marinespecies.org/aphia.php?p=taxdetails&id=205077

    …Good write-up Paolo, thanks.

  2. No one has ever doffed a hat for me – so thank you for the charming gesture! I curtsy modestly and thank Goatlips for the link to the museum that put me on the right track with the scale for size.

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