The pycnogonids or Sea Spiders (Euarthropoda?: Euchelicerata?: Pycnogonida) are some of the strangest animals on the planet. All in all, pycnogonids are very peculiar: they have a proboscis, a 4-eyed turret, a special pair of limbs (ovigers) for holding young, a nauplius-like stage (protonymphon), the addition of limb-bearing segments during development (anamorphosis), no abdomen to speak of (organs are displaced into the legs), and often too many pairs of legs. The front pair of pincer-like limbs has even been interpreted as possibly homologous with the ‘great appendages’ borne by ancient arthropods (Maxmen et al. 2005). Although the chelifores are now accepted as being the limbs of the same segment that produces the chelicerae, sea spiders remain difficult to relate to other arthropods (Brennis et al. 2008, Giribet & Edgecomb 2012). Strange or not, sea spiders seem to have been scuttling across the floors of silent seas since the Cambrian and apparently have never felt the urge to clamber onto land.
At one time, though, sea spiders were thought to be related to mites, mostly because mites also were considered strange and not related closely to anything else, but also because both have a more or less hexapod larval stage (Dunlop & Arango 2005). Fürstenberg (1861) even included pycnogonids as a family of water mites in his book with the scratch-inducing title “The itch mites of men and animals”. A larval pycnogonid (3rd instar protonymphon – see Bain 2003) is shown above. It does seem to be more or less hexapod (the hind pair of legs are sack-like and may be used for storing yolk) and to have what sort-of looks like a capitulum with palps and chelicerae (and a strand of silk) above the proboscis.
Mite larvae have a capitulum (= gnathosoma: composed of chelicerae and fused pedipalps) and three pairs of legs. The chelicera-like pincers (chelifores) at the front-end of the pycnogonid protonymphon each has a palp-like structure at its base, and this does contribute to a resemblance to a larval mite, but in this case the “palp” is a “spinning spine” and silk is produced from a pore at its tip. Many acariform mites (Acariformes) are capable of producing silk (and spider mites do so from a pore on their palp), but my guess would be that the spinning spine is derived from the endite of the chelifore coxa. The next two appendages transform into palps and ovigers during development (Bain 2003) and it is only the sack-like blobs at the rear (bud-like in earlier protonymphon instars) that become the first of the walking legs. Legs IV develop first as buds in the embryo of acariform mites (Barnett & Thomas 2012), and in prelarvae and larvae in parasitiform mites, but limb buds are natural precursors for limbs.
So, I guess there really isn’t much similarity between the pycnogonid and the acariform mite larva, but it is interesting that basal acariform mites have a division of their bodies between legs II-III. This front end or proterosoma is possibly equivalent to the hypothesis of a ‘head’ (cephalosoma) of 4 limb-bearing segments in basal arthropods including pycnogonids. The gene regulation of the development of the rear end of mites is still poorly understood (Barnett & Thomas 2012), but something strange is going on and some surprises may await.
Barnett AA & Thomas RH. 2012. The delineation of the fourth walking leg segment is temporally linked to posterior segmentation in the mite Archegozetes longisetosus (Acari: Oribatida, Trhypochthoniidae). Evolution & Development 14, 383–392. DOI: 10.1111/j.1525-142X.2012.00556.x
Bain BA. 2003. Larval types and a summary of postembryonic development within the pycnogonids. Invertebrate Reproduction & Development 43, 193-222.
Bogomolova EV. 2007. Larvae of Three Sea Spider Species of the Genus Nymphon (Arthropoda: Pycnogonida) from the White Sea. Russian Journal of Marine Biology 33, 145–160.
Brennis G, Ungerer P & Scholtz G. 2008. The chelifores of sea spiders (Arthropoda, Pycnogonida) are the appendages of the deutocerebral segment. Evolution & Development 10:6, 717–724
Dunlop JA & Arango CP. 2005. Pycnogonid affinities: a review. J. Zool. Syst. Evol. Res. 43(1), 8–21 doi: 10.1111/j.1439-0469.2004.00284.x
Fürstenberg MHF. 1861. Die Krätzmilben der Menschen und Thiere. Leipzig: Wilhelm Engelmann.
Giribet G & Edgecomb G. 2012. Reevaluating the Arthropod Tree of Life. Annual Review of Entomology 57: 167-186.
Maxmen A, Browne WE, Martindale MQ, Giribet G. 2005. Neuroanatomy of sea spiders implies an appendicular origin of the protocerebral segment. Nature 437, 1144–1148.