Round 2 - Arthropoda - Pycnogonida
(Sources - 1, 2, 3, 4)
Pycnogonida is a class containing one order: Pantopoda, which means “all feet.” A fitting name for creatures that seem to be made entirely of legs. Commonly called “Sea Spiders”, they are not spiders, nor are they arachnids, but are actually a sister group to all other living arthropods.
Pycnogonids live in most oceans. Most are tiny, living in relatively shallow water, though some can grow to be quite large in antarctic and deep waters. Some pycnogonids are so small that each of their muscles consists of a single cell. They have a proboscis which they use to suck nutrients from soft bodied invertebrates such as cnidarians, sponges, polychaetes, and bryozoans. They can also insert their proboscis into anemones, though this rarely kills the anemone. The pycnogonid digestive tract extends into their legs. They are segmented, with the first body segment (the cephalon) consisting of the proboscis, the ocular tubercle with up to 4 simple eyes, a pair of chelifores, a pair of palps, a pair of ovigers, and the first pair of walking legs. Ovigers are used for cleaning themselves, courtship, and caring for eggs and young. Nymphonidae is the only family where both the chelifores and palps (sensory organs) remain functional. In others, these limbs are reduced or absent, instead relying on a well-developed and flexible proboscis equipped with sensory bristles. Pycnogonids are usually comprised of eight walking legs, but the family Pycnogonidae includes species with ten, and the families Colossendeidae and Nymphonidae include species with up to twelve legs! While most species have up to 4 eyes, some deep-sea species lack them entirely. Pycnogonids do not have a traditional respiratory system, instead absorbing oxygen through their legs and diffusing it throughout their body via hemolymph. Their small, long, thin hearts beat vigorously at 90 to 180 beats per minute, creating substantial blood pressure. Their nervous system consists of a brain which is connected to two ventral nerve cords, which in turn connect to specific nerves. Like other arthropods, they molt their exoskeleton as they grow.
Pycnogonid reproduction involves external fertilization after a brief courtship involving the male stroking the larger female with his ovigers and receiving the eggs if she is responsive. The couple must adjust their position until the genital pores on their legs are perfectly aligned. Only males will care for eggs and young, and in some species only the males will have ovigers while the females do not, as these limbs are used mainly for carrying and cleaning the eggs. Larvae consist only of a head with chelifores, palps and ovigers. Extra segments and legs emerge as it grows into an adult. There are at least four different types of larvae. The typical protonymphon larva is most common, is free living and gradually turns into an adult. The encysted larva spends its larval days as a parasite, finding a host in a colony of polyps, burrowing into one, turning into a cyst, and not leaving the host until it has become a juvenile. The atypical protonymphon larva lives on or within a temporary host such as a clam or polychaete worm, does not encyst or otherwise harm their host, and leaves them as an adult. Lastly, the attaching larva hatches as an embryo and immediately clings to the legs of its father, only leaving once it has two or three pairs of its own walking legs.
The pycnogonid’s cerebral appendages are unique, not found anywhere else among arthropods, except in fossils like Anomalocaris. This could mean that pycnogonids are the last surviving (highly modified) members of an ancient stem group of arthropods that lived in Cambrian oceans.
How do you feel about Pycnogonida? (Remember to vote for your favorite animal within this group, not how you feel about the group as a whole!)
One or more of my favorite animals is in Pycnogonida
I love one or more of these animals
I like one or more of these animals
I am neutral about all of these animals
I dislike all of these animals
I hate all of these animals
Propaganda under the cut:
That gif of that magnificent creature striding across the seafloor just pushed this from like to love 😆
Had the pleasure of seeing a (tiny!) sea spider in a sample of sea water taken from a pier, many years ago 😀
