…and this is a 
Morphological embryology of a sea spider
Category: Development • Organisms • Science
Posted on: August 14, 2006 11:25 AM, by PZ Myers
Tanystylum bealensis male, ventral view, showing eggs and instar 1 (protonymphon) on ovigerous legs. in. 1, instar 1 (protonymphon); pa, palp; pr, proboscis; 1, first walking leg; 2, second walking leg; 3, third walking leg; 4, fourth walking leg.
Surely, you haven't had enough information about pycnogonids yet, have you? Here's another species, Tanystylum bealensis, collected off the British Columbian coast. That's a ventral view of the male, and those bunches of grapes everywhere are eggs and babies—males do the childcare in this group. These animals also live in relatively shallow water, in the lower intertidal zone, so it was possible to collect thousands of them and develop a complete staging series. Below the fold I've put some illustrations of the larvae, which are even cuter.
The eggs are sheltered on a specialized male appendage, the oviger, and held there through the first instar. You can see a couple of the little guys on the photo above. In later instars, they inhabit the fronds of a hydrozoan, Plumularia setacea, sucking on tasty gonophores and other tissues. Plumularia is also rather pretty:
Anyway, here's the developmental series. Sea spiders exhibit a vivid pattern of prolonged anterior to posterior development: the embryo basically forms just the head of the animal, and then limbs are steadily added and modified from front to back as development proceeds. Imagine a human baby born as just a head, and then later sprouting arms, and later still popping out a couple of legs…that's how sea spiders grow.
Development of trunk and anterior appendages in Tanystylum bealensis. A: First instar. B: Second instar. C: Third instar. D: Fourth instar. E: Fifth instar. F: Sixth instar. G: Seventh instar. H: Eighth instar. I: Adult female (ninth instar). J: Adult male (ninth instar). Drawings for each instar consist of a dorsal (left) and ventral (right) view of trunk and anterior appendages. ab, abdomen; ch, chelifore; lp, lateral process; ov, oviger; pa, palp; pr, proboscis; wl, walking leg; 1la(pa), first larval appendage; 2la(ov), second larval appendage; 1, first walking leg; 2, second walking leg; 3, third walking leg; 4, fourth walking leg.
Gillespie JM, Bain BA (2006) Postembryonic development of Tanystylum bealensis (Pycnogonida, Ammotheidae) from Barkley Sound, British Columbia, Canada. J Morphol. 267(3):308-17.
Comments
is it wrong that this fills me with a warm glowing sensation? the proboscis on these guys is utterly bizarre...being used to chelicerates as i am, specifically mygalomorphs, that's as incongruous looking as a pair of ant-mandibles on a human would be. of course, the extra pair of legs on a couple of them is pretty funky too. imagine the dance steps they'd be capable of!
Posted by: garth | August 14, 2006 11:59 AM
PZ, you and I are the only ones who love the creepiest and crawliest. It's a lonely life. If you're ever in SoCal I'll buy you a pint and show you some sweet chelicerates.
damn, that sounds like i'm asking you out on a date! i'm not!
Posted by: garth | August 14, 2006 2:06 PM
So that's what the kids are calling it now, is it?
Posted by: RavenT | August 14, 2006 2:19 PM
Well, I think the way that the way the little sea spiders grow is pretty interesting.
Posted by: Tara Mobley | August 14, 2006 2:22 PM
Growing rearwards for at least part of embryonic/larval development seems to be common throughout the animal kingdom; IIRC that is the inferred ancestral state in both arthropods and vertebrates.
And has anyone tried to infer the order in which Hox genes were added from their overall family tree? I would not be surprised if it was also front-to-rear.
Posted by: Loren Petrich | August 14, 2006 2:56 PM
I don't know about you guys, but I'd pay pretty good money to see that.
Posted by: Dan | August 14, 2006 4:42 PM
Rent Eraserhead some time, as that is basically the plot. Well with radiator issues as well.
Posted by: Ukko | August 14, 2006 4:53 PM
"Surely, you haven't had enough information about pycnogonids yet, have you?"
I haven't, so bring it on. Personally, I'm hoping they turn out to be anomalocarids, as long as there is hope, just because that would be cooler.
Posted by: Mike | August 14, 2006 7:13 PM
It's not that far off the way young children naturally learn to draw people! There are documented stages of development (eg tadpole people) which are fairly cross-cultural. It starts with just the head/face and then a couple of lines for limbs. Eventually refinements such as fingers are added (with the number being somewhat indiscriminate - just like old-style tetrapods!). The main body (ie towards the rear when starting from the head) takes a while to get added.
Posted by: SEF | August 14, 2006 9:08 PM