What is this on this boxer crab

I photographed this boxer crab in Bali last week.

It seems to have a creamy coloured mass underneath it - within its apron, where it would usually hold its eggs. Boxer crab eggs are bright orange so it is not eggs - my guess is that it is a type of parasite.

I'll just get a higher res crop.

Alex

here is the close-up:

And here is another crop

Whatever it is. It's the most beautiful crab I've ever seen.

What is cool about boxer crabs is that they hold stinging anemones in their claws which they use to defend themselves.

Tulamben is one of the best places to find boxer crabs. When I got there (they were probably at the top of my wish list) I asked all the dive guides if there were any about at the time. They all said no. Then on my last night dive there I spotted one and went on to find two more adults and two small juveniles.

Only one of them had this creamy stuff underneath it.

Alex

Hi Alex -- It's an endoparasitic barnacle belonging to the order Rhizocephala. Saculina is the best known genus in the group. Their bodies are divided into 2 parts - the interna which absords nutrients and the externa which is the reproductive part. Once established in the host's body they inhibit molting & basically "castrate" the crab. What looks like the egg mass of the crab is the externa, the egg mass of the barnacle. The crab treats the barnacle egg mass like it's own, aerating it and using its legs to liberate the juveniles when they hatch. Pretty cool, huh?

Leslie

That's very cool. I like this picture even more now!

Is that the one that they always show in undergraduate lectures - where the interna part basically spreads right through the crabs body?

Alex

Now that is sooooo cooooool....

Great photo, amazing subject, and really interesting information about the barnacle and crab... Thanks, Alex and Leslie, for postiing.

Cp

Alex, he is beautiful (great eyes!). I've always wanted to see boxer crabs, but every time I ask about them, the answer is no. What is their habitat? It's obviously worth looking for them anyway!

I think they'd make a wonderful video subject as well. I saw one on a nature special that was brandishing his pom poms from side to side (just like a cheerleader!) as he walked. Is that normal walking behavior, or defense posturing?

Very neat info about the barnacle egg mass too..

Thanks..

Is that the one that they always show in undergraduate lectures - where the interna part basically spreads right through the crabs body?

Yes, Saculina is a real classic of the entry level biology and parasitology lectures.

As the others have said, that's a very cool shot.

Impressive Alex! I've seen crabs with anemones in their claws, in kona. TINY, TINY though. Is that the case here? Great shot.

As for cropping, I like the tighter crop, but maybe not quite that tight.

Don

Hmm, I did a little looking around to find out more about this crab's habitat, and came across another photo which shows a barnacle egg mass on another of these crabs:

http://www.seaphotos.com/crustaceans.html

Is this a fairly common occurance?

Linda,

I don't have a great deal of experience with boxer crabs - but here is most of what I know. And they certainly would make a good video subject.

Boxer crabs live around and under rubble and rocks - see the background of my first image - in very shallow water, less than 15ft deep. I have seen them in both Manado and Bali. Most photos show boxer crabs out on the sand. In my experience this is not where they live and I suspect these crabs have been moved onto this backgrounds to produce a better picture.

My pictures were taken on a night dive, on a site I had dived on 2 nights before without seeing the crabs. Maybe I missed them - certainly the guides didn't know they were there - so we weren't looking for them. But once I found the first one I quite quickly found 2 more adults, and two juveniles. I'm pretty good at spotting critters - so I am surprised I hadn't seen them on the previous nights.

The only difference on this night was that we were about 45 minutes late into the water - I had been on the phone to Robert Delfs aka Frogfish (while I was in my wetsuit which delayed me) and then we had done about 80 minutes in the water before seeing the crabs. So I guess it was about 2 hours after sunset when I started finding the crabs. The dive was about 95 mins in total.

During the day, dive guides usually have to turn over lots of rocks to find the crabs. This was certainly the case when I saw them in Manado. And then once found they are often picked up and moved on to the sand for people to see/photograph. I am not a fan of this. All the crabs I saw were out and about - and I did not need to overturn or move rocks to see them.

So maybe a late night dive in shallow rocky area would be the best way to try and find them? I am running a trip to Bali next September and I'm sure Boxer Crabs will be one of the creatures we will be after.

The picture you've posted from seaphotos looks very much like it was taken in Tulamben - although Tulamben is of course not the only place in Indonesia with black volcanic sand!!

Alex

Occurrence of parasites like Saculina vary with each species & each population. In an area with lots of boxer crabs in fairly close proximity the incidence of infected crabs could get pretty high for a while. Eventually the number of boxers will drop because they're not reproducing. What I find fascinating about this kind of parasite is that they basically take over the brains & reproductive systems of their hosts. Other types of parasites alter the behavior of their hosts, for example causing a fish that normally swims along an estuary bottom to swim near the water surface where it can be eaten by a bird, allowing the parasite to continue onto the next stage of its life cycle. If this kind of stuff interests you read "Parasite Rex" by Carl Zimmer. Well written & easy to read, "Parasite Rex" is full of this kind of great stuff. In fact, it mentions that some scientists think that evolution was/is driven by the need to stay ahead of parasites - even the switch from asexual to sexual reproduction!

Hey! That's a great shot! I love it! On one of my muck dives, I saw the same anemone swimming away, floating & undulating its' body. Can anemones swim???

Yup, the solitaries can move about, swim, and some even use the current to roll across the bottom.

Leslie