Sea Spiders

With arthropods being such a massively diverse sector of the global biota (and even that feels like an understatement; describing arthropods as 'very diverse' seems a bit like describing the Andromeda Galaxy as 'very far away'), it is only to be expected that it contains some very weird corners. And definitely among the weirder of those corners are the Pycnogonida, commonly known as the 'sea spiders'.

Anoplodactylus evansi, copyright Mick Harris & Claudia Arango.

Pycnogonids are a group of marine arthropods found around the world (not actual spiders, of course, though honest-to-goodness marine spiders are a thing that does exist). Their relationships to other arthropods have long been in dispute but the majority view is that they are distant relatives of the terrestrial arachnids. Pycnogonids are not uncommon in both coastal and deep-sea habitats but tend to go unnoticed: they feed on rock-encrusting colonial animals such as hydrozoans and are often coloured to disguise themselves against their prey. If one ever does see a sea spider, the first thing to stand out about them is how they are made of legs. The central body is often remarkably small compared to its limbs, to the extent that the dubbing of pycnogonids as 'no-bodies' by an early 20^th Century author has become something of a cliché. Certain major organs, such as the gonads and parts of the digestive system, have even been diverted into the legs to make up for the lack of space in the body. Most pycnogonids possess four pairs of walking legs though there are species with more. At the front of the body on the underside of the head is a large proboscis that is used for sucking the juices out of prey, flanked by pairs of pincer-bearing chelifores and/or palps used for tearing it open. Near the first pair of walking legs there is often a pair of slender leg-like appendages known as the ovigers, used for carrying bundles of eggs until they hatch. The greater part of the body behind the head is taken up by the leg-bearing thorax; the legless abdomen is reduced to the merest nub like the docked tail of a dog.

Close-up on preserved male Anoplodactylus lentus, from Florida Museum of Natural History.

One of the largest recognised genera of pycnogonids is Anoplodactylus, with over 130 species worldwide and many continuing to be described (Lucena et al. 2015). This genus can be distinguished by the possession of chelifores with functional chelae (pincers) but palps are absent or reduced to buds. Both the chelifores and the proboscis are relatively short (Child 1998). Ovigers are five- or six-segmented and present in males only (male care of eggs is the standard pattern among pycnogonids). Species vary from 0.6 to 6 millimetres in body length. The majority of species of Anoplodactylus are found in shallow waters in temperate and tropical regions with a smaller number of species found in polar and deep waters. Alvarez & Ojeda (2018) record finding a single specimen of the species A. batangensis among vegetation on the surface of an anchialine pool in the Yucatan Peninsula of Mexico. Though the surface of these pools is more or less fresh water, deeper sections are saline owing to subterranean connections to the sea. The collection of a pycnogonid near the surface of this pool suggests an ability to adjust to very low salinity though one questions whether it would be able to survive indefinitely.

Larvae of Anoplodactylus are very small compared to those of other pycnogonids and have what has been termed an 'encysting' development (Burris 2011). As bizarre as the appearance of adult pycnogonids is, their larvae are arguably even weirder, being essentially nothing more than a head bearing chelifores, proboscis, and two pairs of undifferentiated appendages. The remaining segments of the body are added over the course of development. In Anoplodactylus, the larvae develop as parasites, forming a cyst in the gastrocoel (the stomach cavity) of cnidarians (having presumably been placed there somehow by their fathers, though I haven't found if we know how). They become free-living upon reaching the first juvenile stage, emerging from their host to pursue their predatory lives.

REFERENCES

Alvarez, F., & M. Ojeda. 2018. First record of a sea spider (Pycnogonida) from an anchialine habitat. Latin American Journal of Aquatic Research 46 (1): 219–224.

Burris, Z. P. 2011. Larval morphologies and potential developmental modes of eight sea spider species (Arthropoda: Pycnogonida) from the southern Oregon coast. Journal of the Marine Biological Association of the United Kingdom 91 (4): 845–855.

Child, C. A. 1998. The Marine Fauna of New Zealand: Pycnogonida (Sea Spiders). National Institute of Water and Atmospheric Research (NIWA).

Lucena, R. A., J. F. de Araújo & M. L. Christoffersen. 2015. A new species of Anoplodactylus (Pycnogonida: Phoxichilidiidae) from Brazil, with a case of gynandromorphism in Anoplodactylus eroticus Stock, 1968. Zootaxa 4000 (4): 428–444.