![]() ![]() According to Sea Slug Forum, one of Anoplodactylus evansi’s favorite prey items are juvenile Sea Hares (among other Opisthobranchs), which are young enough to not have accumulated an amount of diet-derived compounds to deter predators. Like most arachnids, sea-spiders can only eat liquids, and a long proboscis allows them to pierce and suck out the insides of soft-tissued animals like sponges, gastropods, and hydroids. While sea spider taxonomy is somewhat in flux, they are currently in the subphylum Chelicerata, which also contains arachnids and horseshoe crabs. In fact, the lack of a “body” means that internal organs like those of the digestive system reside partially in the legs of the beast, and the high surface-area to volume ratio allows respiration to occur directly through the exoskeleton. The sea spider Sascha found, Anoplodactylus evansi, is more colorful than any I’d personally seen (here in California they’re usually pretty drab), and the colors along with its spindly pretty-much-legs-only construction make it a striking organism. “And it turned out to be the first sea spider I have ever seen in 24 years of diving in Australia and other locations around the world!” when I noticed something crawling on my hand,” recalls Australian freediver Sascha Shulz. Science is still in the dark about many aspects of sea spider biology.Our Observation of the Week is this multi-colored wonder, an Anoplodactylus evansi sea spider! Seen in Australia by was snorkeling with the Shelly Beach Swim group (many of whom also contribute to iNat), and was photographing a Sand-diver species. And while their evolutionary position is an interesting question, the group of scientists I'm working with here are in Antarctica because in the Earth's polar oceans, pycnogonids get very, very big. ![]() ![]() In fact, despite their "cosmopolitan" distribution, pycnogonids are not very well studied and scientists are still working on how close to the trunk of the tree of life to place them. Pycnogonids don't fit very well within the other common groups of marine arthropods like the chelicerates (horseshoe crab relatives) and the crustaceans (crabs, shrimp, lobsters, etc.). Species belonging to the taxonomic class Pycnogonida are members of an ancient line of arthropods that date back at least to the Silurian period, around 425 million years ago (this is about the time that the first plants began colonizing land - that far back in the story of life). Most species are slow moving, opportunistic predators or scavengers and use a sucking mouth organ called a proboscis to feed on soft-bodied prey like jellyfish, anemones, bryozoans, worms, and seaweeds.Ī giant sea spider towers over a field of polyps. Their reproductive organs fill much of the insides of their legs males carry eggs deposited by females on specialized legs called ovigers and care for them until the larvae hatch and crawl away. Sea spiders have no true respiratory organs and rely on diffusion of gases across their exoskeleton in order to supply their body tissues with oxygen. But the physical similarities with true spiders end there. Like true spiders and all other members of the arthropod phylum, sea spider appendages are jointed, their bodies are divided into segments, and they have a tough external skeleton. Steve Rupp spots a pycnogonid specimen to bring back to the lab. They can tolerate the harshest environments the planet has to throw at them, including the toxic chemistry of deep sea hydrothermal vents, the daily wet and dry cycles of intertidal areas, and the ultra-chilled waters of the Antarctic. There are over a thousand species described and very likely hundreds more that have yet to be studied scientifically. These ancient animals are found throughout the global ocean from the deepest trenches to the shallowest bays. While it's a challenging practice and a breathtaking experience, the actual purpose of our scientific scuba dives is to study and collect sea spiders. An Antarctic sea spider stalks its soft coral prey. ![]()
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