Sea spiders.

I can already hear the exasperated groans coming from the readership of this entry. Sea spiders? Seriously? Why, arachnophobes the world over sigh, are spiders not content to just stay where they belong; many miles away from any potential interaction with my relatively exposed, swimming body? Need they go out of their way to ruin my summer vacation at the beach too? Why do there have to be marine versions of our creepy, spindly-legged friends, especially when we already have sea snakes, saltwater crocodiles, and what are the equivalent of massive “sea wolves” patrolling the briny depths? Perhaps, given the unsettling, lanky body shape of the sea spider, reminiscent of the daddy longlegs clustered in the dark, dusty recesses of our garages, it provides little comfort to say that these animals are not what their common name suggests.

In the same way that “sea cows” are not actually cattle equipped with flippers, and “sea wasps” aren’t really our delightfully sting-happy, land-lubbing acquaintances finding a new home beneath the waves (a nightmarish scenario if there ever was one), sea “spiders” are not simply spiders with water-proof webs and an appetite for calamari. They are something altogether different, belonging the taxonomic class Pycnogonida (meaning “thick knees”, perhaps referring to the shape of the joints in their segmented legs, or a cruel high school nickname for the group). This class is currently allied within the arthropod group known as Chelicerata, which does include arachnids; but, these “sea spiders” are, as previously mentioned, not arachnids themselves. However, even this classification may not provide enough recognition of the pycnogonid’s unique pedigree. There have been some recent studies (from both molecular genetics and evolutionary development angles) that suggest that sea spiders are not nested alongside arachnids at all, but instead are a part of a much older offshoot of the arthropod line…and are potentially the only surviving, highly-derived representatives of some of the first groups of arthropods to evolve (perhaps more closely related to enigmatic, extinct animals from more than half a billion years ago like Anomalocaris). If this is the case, then the pycnogonid lineage is effectively among the oldest animal groups on the planet.

Yes, no matter which classification assignment is correct, these critters occupy a unique branch on the great tree of life, and once someone takes a look at these pycnogonids up close, it becomes abundantly clear that these animals definitely deserve severely distinct classification, and have a tangibly alien quality to them. Seriously, pycnogonids are about as weird as it gets.

In case you haven’t noticed yet, I’ll point out that pycnogonids have incredibly long legs.

Less of a “body with legs” setup, and more of a “legs that have a body” situation, really

These absurdly elongated walking legs (which usually number in four pairs), combined with a comically reduced, skinny body, make the pycnogonid a curious sight to behold. While some species use these hair-thin limbs to swim by way of turning the leg into a paddle through the use of long bristles, often times, the legs function in the traditional sense, allowing these creatures to stride along the sandy ocean floor. But, despite being nearly entirely made up of legs, sea spiders are not swift like their terrestrial namesakes. The musculature devoted to locomotion is pretty damn pathetic, allowing only for slow, deliberate movements that are somehow both eerily robotic and graceful. Indeed, pycnogonids, with their cartoonishly proportioned bodies and cold, calculating progression across the open wastes of the abyssal sea, each step gliding unhurriedly forward without any hint of hesitation, they seem not only alien, but artificial. It’s easy to imagine them as mechanical automatons, marching silently across the pitch-black plains of the deep.

As if this notion of our oceans being inhabited by hoardes of robo-spiders didn’t need any additional reinforcement, consider the spine-tingling fact that pycnogonids don’t possess anything resembling a respiratory system. That’s right; no gills, no lungs. A respiratory system would likely be a cumbersome and unnecessary bit of visceral decoration inside that minimalist body of theirs. Pycnogonids simply absorb oxygen passively from the water that surrounds them via diffusion (the movement of molecules from high to low concentration), a strategy that is undoubtedly made possible by their bodies’ high surface-area to volume ratio. So yes, pycnogonids, in a disconcertingly similar fashion to the soulless, automated drones in any science-fiction flick, do not draw breath, in any form.

Their digestive system is also radically impacted by their bizarre body shape. The insides of the body segments (collectively grouped into a “cephalothorax”) of pycnogonids are apparently so cramped, that the digestive system has projections that extend into the tops of the walking legs. Pycnogonids are experiencing what, in humans, would be like wearing a corset so tight it made your intestines squirt into your thighs…inevitably resulting in a lifetime confusion between indigestion and Charley horses. Sea spiders are plagued by the pressure to fit “10 pounds of shit in a 5 pound bag”, and the evolutionary answer of having some of their guts wander out of their body cavity is made a necessity by the pycnogonid quirk of having a dramatically reduced abdomen (that big, bulgy segment at the back of arthopods like insects and spiders), which provides essentially zero auxiliary room for relatively important things…like organs. Pycnogonids end up cramming the equivalent of a house’s worth of belongings into a studio apartment, and they manage to achieve this by evolutionarily converting their innards into a game of Tetris.

The business end of the pycnogonid, the eating end, isn’t any less strange. The head of a pycnogonid is an obvious departure from that of its hypothesized closest relatives; the rest of the members of the subphylum Chelicerata (a brief overview of which I explored in this recent entry). The most glaring difference is that in place of paired, mobile mouth parts (such as those found in arachnids and horseshoe crabs), the main tool used to procure food is a proboscis, which manifests as a trunk-like appendage that in some species is short and squat, but in others is a monstrous organ possessing a shape somewhere between that of a bowling pin and the nose of that giant Muppet, Sweetums. Accompanying the proboscis in the head region, are typically three pairs of appendages distinct from the walking legs; they are the chelifores, palps, and ovigers. The chelifores and palps are more actively involved in gripping and manipulating food items, while the ovigers, placed furthest back on the body of the three pairs, is more heavily involved in reproduction. Ovigers, usually the longest appendages outside of the legs, and found folded up underneath the body, are often used in courtship displays and for cleaning the exoskeleton. It is the pycnogonid male that carries and cares for the fertilized eggs, tucking them underneath the body and using the ovigers to ball the eggs into two large, globular clusters. This gives the animal the appearance that it is wearing white mittens and then sticking them in its armpits. Allowing for the sea spider to survey its world, perched on top of the cephalothorax, near the head region, is an eye turret (called an “ocular tubercle”) that houses four simple eyes.

Most species of sea spider are exclusively carnivorous, and either take live prey or scavenge it off the ocean floor. The proboscis, which can occasionally have spines or teeth arming the lips at the tip, is employed as the pycnogonid weapon of choice when dining. Commonly, they will feed on big, stationary, soft creatures like segmented worms, sea anemones, and sponges by stabbing their proboscis into the side of these animals. Then, aided by a cocktail of digestive enzymes streaming out of the proboscis, the sea spider slurps up a hunk or two of delicious tissues directly into its gut, and then gets the hell out of there to move on to the next sorry schmuck on the seabed.

Wanted: for a suck-and-run

If the idea of being leisurely stalked by a pycnogonid and aggressively pierced by nature’s meanest variation of the bendy straw is keeping you from going snorkeling anytime soon, then you should reconsider. Firstly, pycnogonids, creepy as they are, do not view humans as food, and probably couldn’t do any damage with that Gonzo schnoz even if they tried. You’d be more likely to drown from getting entangled in seaweed, than to receive even the slightest pinprick from the proboscis of a sea spider. This reality is enforced by the diminutive size of the vast majority of pycnogonid species. Most pycnogonids you’ll observe in the wild are small as shit, with leg-spans only stretching as wide as the nail on your pinky finger. These characteristics make our alien, arthropod friends pack as much deadly force as a wind-blown dandelion seed.

However, exceptions to the rule abound in the natural world, and in the case of pycnogonids, it is size. While most of the 1,300 or so species of sea spider are found in shallow waters near shore, and are quite tiny, there are a small number of species found in the coldest parts of the ocean that are comparatively gigantic.

Looks like even the ruler is afraid to get close

Meet the aptly-named Colossendeis, a genus of pycnogonid that has representative species in all oceans of the world. However, it is in the icy waters surrounding Antarctica that some species, like the one above, possess such massive bulk. The largest species of Antarctic giant sea spider have leg-spans as wide as a trash can lid and proboscises as long as your finger. This significant increase in size compared to other sea spiders fits into two phenomena effecting marine invertebrates; polar gigantism and deep-sea gigantism. The latter of these two has been documented to occur across many groups of organisms in the deepest parts the ocean, where inhabitants of the abyss are many times larger than their shallow water relatives; examples include cat-sized, scavenging isopods (relatives of terrestrial “pill bugs” or “roly polies”), and the numerous varieties of giant squid. Explanations for this phenomenon have been offered over the years, and one prominent hypothesis is that the cold waters of the deep encourage an increased life-span, and in the case of many invertebrates (which tend to grow continuously throughout their lives) this results in a much larger adult size. This reasoning would also be consistent with polar gigantism, in which species of marine invertebrates in cold polar waters, at any depth, also exhibit an increase in size compared to their closest, lower latitude relatives. The giant sea spiders of the Antarctic are both found in deep water, as well as the still quite cold surface waters, and are thus candidates for both described tendencies. Of course, despite their intimidating size, even Colossendeis sea spiders are benign animals.

“Unhand me! Now! Or I shall lightly poke you with my proboscis and tickle you with my legs!”

Pycnogonids may look like someone took a spider, starved it, gave it a shiny coat of monochromatic paint, threw it in the ocean, and made it walk backwards…but they are, in reality, a fascinating group of animals with few relatives. Pycnogonids are something of a staggeringly old taxonomic island, a distinct offshoot existing for several hundred million years. There are more than 1,000 modern species, but we don’t know as much as we’d like about them, and we are just now starting to get a grasp of their ancient evolutionary history through the fossil record and genetic studies. In time, we may be able to unlock some of the pycnogonid’s mysteries, and get an even greater appreciation for the slow, silent, stilt-walking figures that have been pestering sponges and anemones the world over since before the Age of Dinosaurs, and will hopefully continue to do so for eons to come.

Image credits: Opening sea spider, red sea spider, striped sea spider, Colossendeis, held Colossendeis

© Jacob Buehler and “Shit You Didn’t Know About Biology”, 2012-2014. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Jacob Buehler and “Shit You Didn’t Know About Biology” with appropriate and specific direction to the original content.