Natures wonders: sea spiders are not spiders (Introduction)

by David Turell , Wednesday, February 02, 2022, 19:39 (811 days ago) @ David Turell

Weird sea dwellers:

https://www.realclearscience.com/blog/2022/02/02/the_strange_unsettling_world_of_sea_sp...

"Pycnogonids, as they are also called, generally have small midsections dwarfed by four pairs of large, spindly legs, which are perhaps their defining and most unsettling body parts. They use them to walk along the seafloor or hover just above with pulsing strokes. A few sea spiders even have five or six pairs of legs. These appendages can each extend from mere millimeters to almost two and a half feet! Pycnogonids also sport four eyes, a mouth-like proboscis used for feeding, and – flanking the proboscis – a pair of chelifores which take the form of modest pincers.

"About 1,300 species of sea spiders have been identified, and more are found every year. Fossils suggest that they've existed on Earth for at least 450 million years. Back then, pycnogonids and other arthropods likely dominated the oceans. Few animals lived on land.

"Sea spiders lack organs for breathing, instead expelling carbon dioxide and taking in oxygen directly through their outer layer, or cuticle. This gas exchange primarily occurs via the legs, as they have the most surface area by far. Even more fascinating, it seems that pycnogonids primarily transport oxygen within their bodies via peristalsis, involuntary constriction and relaxation of the gut. Peristalsis moves hemolymph, or blood, throughout the body. "Shared digestive and respiratory functions may save energy," the researchers who uncovered this strategy said. "Legs function as the gills used by other arthropods, and the gut functions as a heart."

"As far as reproduction goes, males carry fertilized eggs then care for offspring. Females produce and lay eggs, which males then externally fertilize. Subsequently, males pick them up and store them with specialized body parts called ovigers.

"Different species of pycnogonids eat everything from algae, worms, and decaying organic material to mollusks, cnidarians (jellyfish, coral, etc.) and crustaceans. Many are parasitic, living inside bivalves or snacking on (but not killing) anemones. Others are herbivores, eating plants, or detritivores, consuming dead organic material. All sea spiders suck up food with their multi-lipped proboscis."

comment: fully adapted to their strange lifestyle in their ecosystem. They appeared post-Cambrian