Sea Spiders and the Rise of the Chelicerates

by Joseph DeSisto

Sea spiders are small, eight-legged marine arthropods with a vaguely spider-like appearance: members of the obscure class Pycnogonida. Most of the 1300 or so species are predatory, feeding on jellyfish, sponges, and other soft-bodied marine invertebrates. After eight-leggedness and predatory habits, the similarities with spiders end.

A yellow-kneed sea spider. Photo by Sylke Rohrlach, licensed under CC BY-SA 2.0.

A yellow-kneed sea spider. Photo by Sylke Rohrlach, licensed under CC BY-SA 2.0.

So what exactly are sea spiders? For a long time they were considered to be the most ancient members of an already-ancient group of animals: the Chelicerata, which includes the arachnids and the horseshoe crabs (Dunlop and Arango 2005). These animals are united by their possession of chelicerae, a kind of mouthpart.

Chelicerae are extremely versatile, and in the 450-odd million years they’ve been around, natural selection has resulted in a huge diversity of forms. A spider’s chelicerae, for example, are hollow and capable of injecting venom into prey. Scorpion chelicerae, on the other hand, are smaller and used for chewing up food. Some harvestmen (daddy-long-legs) have long chelicerae with pincers at the end, useful for grabbing prey.

Sea spiders have chelicerae too — sort of. Their mouthparts at least are similar to chelicerae, but because they’ve been subject to hundreds of millions of years of evolution, it isn’t easy to discern their “true” identity. Modern sea spiders have hollow, tube-like chelicerae, which they use to suck out the insides of their prey.

 licensed under CC BY-SA 4.0.

A sea spider using its chelicerae to prey on a hydroid. Photo by Bernard Picton, licensed under CC BY-SA 4.0.

Classification can get complicated. To understand where sea spiders fit into the tree of life, we must ask the question: are sea spider chelicerae “real” chelicerae? That might sound like a silly question — after all, “chelicerae” is just a term we made up. But what we really mean is, are sea spider chelicerae homologous with the chelicerae of arachnids and horseshoe crabs? If they are, then sea spiders and the other chelicerates inherited their mouthparts from the same common ancestor. If not, chelicerae evolved twice: once in sea spiders, and separately in the “true” Chelicerata.

In 2005, Amy Maxmen and colleagues carefully studied the chelicerae of sea spiders and found that they emerged from a different part of the body than in other arthropods. It seemed that in sea spiders, chelicerae emerged from the same segment that contained the eyes (but not mouthparts) in other chelicerates. This suggested that sea spiders were not true chelicerates, but instead formed their own group, which Maxmen et al. (2005) hypothesized to be the oldest living arthropod lineage.

A sea spider -- the red hanging structure contains the mouthparts. Photo by Scott C. France, licensed under CC BY-SA 2.0.

A sea spider — the red hanging structure contains the mouthparts. Photo by Scott C. France, licensed under CC BY-SA 2.0.

In arthropods and other segmented animals, Hox genes are responsible for making sure that all the right body parts develop on all the right segments. It was surprising, then, when a 2006 study found that the Hox genes place chelicerae on the same segments in sea spiders and in arachnids (Jager et al. 2006). Apparently, this segment was shifted backwards in sea spiders, creating confusion.

Over the years, scientists have tried comparing the DNA of many arthropods to try and understand how they are related to one another. One of the most comprehensive studies (Regier et al. 2010) placed sea spiders comfortably within the Chelicerata, as the group’s oldest (i.e., basal-most) lineage. Since then there has been little debate: sea spiders may be some of the oldest arthropods on earth, but they really are chelicerates, after all (Giribet and Edgecombe 2012). That is, until new evidence comes along to shake things up again.

Cited:

Dunlop J.A. and C.P. Arango. 2005. Pyncogonid affinities: a review. Journal of Zoological Systematics and Evolutionary Research 43(1): 8-21.

Giribet G. and G.D. Edgecombe. 2012. Reevaluating the arthropod tree of life. Annual Review of Entomology 57: 167-186.

Jager M., J. Murienne, C. Clabaut, J. Deutsch, H. Le Guyader, and M. Manuel. 2006. Homology of arthropod anterior appendages revealed by Hox gene expression in a sea spider. Nature 441: 506-508.

Maxmen A., W.E. Browne, M.Q. Martindale, and G. Giribet. Neuroanatomy of sea spiders implies an appendicular origin of the protocerebral segment. Nature 437: 1144-1148.

Regier J.C., J.W. Schultz, A. Zwick, A. Hussey, B. Ball, R. Wetzer, J.W. Martin, and C.W. Cunningham. 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature 463: 1079-1083.

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