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Head to FACTOR75.com or click the link below  and use code 50BIZARREBEASTS to get 50% off and   free shipping on your first Factor box! Bad news: the ocean is full of spiders… sea spiders, that is.

They’re not actually arachnids,  although they certainly look the part. They also don’t have a ton in  common with other chelicerates,   which is the group that includes arachnids,  horseshoe crabs, and the extinct sea scorpions. And they maybe don’t even have a ton in  common with other arthropods in general?

Point is, they’re weird guys, and the  specifics of their strangeness may   actually help us to better understand how all  these different invertebrate groups evolved. And as we investigate the common traits of  a beast this bizarre, we end up finding our   way back to an ancient branching point within the  most diverse group of living things on the planet. [♪♪ INTRO ♪♪] To keep this channel going,   the Bizarre Beasts pin club is open  for subscriptions for the whole month! Sign up by November 20th and the first  pin you will get will be a sea spider.

Sea spiders are any of the roughly  1,300 species of marine invertebrates   in the class Pycnogonida, a name  that aptly means “dense knees”. And if you look at them, they sure  do have a high density of knees. A pycnogonid looks like if you asked somebody to   quickly draw a spider from memory, and  all they could recall was like “legs.” Some walk along the seafloor, while others swim.

They can be as small as just a few  millimeters, or as large as 75 centimeters,   and they can be found all over the world, from  coastal shallows to seven kilometers deep. And it’s the species that live way  down deep or in the cold waters of   Antarctica that represent the  upper end of that size range,   due to the related phenomena of both  polar gigantism and deep sea gigantism. Basically, as you travel to parts of the  world with colder and colder temperatures,   you will find bigger and bigger animals – whether you’re moving farther from  the equator or deeper below the waves.

The evolutionary explanation for larger  bodies being better adapted to colder   environments is still debated, but the  top contenders are better metabolism,   fewer potential predators,  and more efficient oxygen use. We're actually going to talk  about that last point in a minute. Like true spiders, sea spiders have  a cephalothorax and an abdomen,   and four pairs of legs.

But unlike true spiders, the  pycnogonid’s cephalothorax is   thin and includes a proboscis for eating  with, an abdomen that is downright tiny,   and in some species they may have  one or two additional pairs of legs. And all of those legs have more joints than  the legs of any other type of arthropod,   with leg segments that are just a bit… different from anyone else’s. This leggy body layout is multifunctional.

And their legs aren’t just for getting around,  they also have several specialized types,   including legs for eating with,  pedipalps for sensing and grooming,   and some species have especially unique limbs for  reproduction and offspring care called ovigers. They reproduce by holding their ovigers  against those of the opposite sex,   and then the fertilized eggs attach to the  male’s ovigers, where they stay until they hatch. So yeah, they reproduce with their  legs, by releasing sperm or laying eggs… out of their legs.

This is because, with such a  tiny cephalothorax and abdomen,   organs like the reproductive  system have to go somewhere. And it really is all legs with these things: for example, they also breathe through  their legs and use them for circulation. Pycnogonids do not have gills or other  specialized respiratory structures,   so they do all their gas exchange  right through their exoskeleton.

And although they do have a heart, for  some of the really big Antarctic species,   it doesn’t create a strong enough  flow to get to every body part. Luckily, their gut reaches almost all the  way into their walking legs, so as oxygen   diffuses into their legs from the surrounding  water, they use peristaltic muscle movements to   squeeze fluid into those gut extensions, moving  oxygen from the legs into the rest of their body. And to be clear, despite all this complicated  fluid exchange happening in their extremities,   sea spiders still do all their actual  limb movement via normal muscles – they just also happen to have a big leg stomach  that pumps their blood so they can breathe.

You know, normal stuff. At the very least, this does answer  an important question for pycnogonids: is gut legs? Their wacky appendages did not just earn them  the name of their taxonomic class, either:   sea spiders also belong to order Pantopoda,  which literally means “all legs” or “all feet”.

But this order is actually more of a compromise   between disagreeing taxonomists  than a true classification – it encompasses all extant sea  spiders, and it doesn’t exactly   help narrow down how these guys came  onto the scene in the first place. Part of the problem is that  scientists have disagreed for   a long time about where sea spiders  fit into the tree of life in general. Are they arachnids?

They resemble regular spiders,   but there is that sticking point of some  pycnogonids having more than eight legs. And their ovigers are a unique feature  that you will not find on any arachnid. So let’s go one step up in taxonomic rank.

Chelicerata is the subphylum  that encompasses arachnids,   plus horseshoe crabs and some  extinct species with similar vibes. Sea spiders have the tell-tale  claws on the front of their bodies - on those feeding legs we mentioned - that help identify chelicerates, but then  they also have features like their proboscis,   which can’t be found in any  other chelicerate species. This has led some scientists to categorize  them as an outgroup to chelicerates – or else as an outgroup to all other arthropods.

But then when we go up that next  level, to the whole phylum Arthropoda,   we still find ways that pycnogonids just  refuse to conform to our definitions. Arthropods can be generally described  as invertebrates with exoskeletons,   segmented bodies, and jointed legs. And at a glance, sea spiders tick all these boxes.

But their unique reproductive structures, atypical  joints, and ability to grow between molts – not to mention the substantial leg real  estate that their major organs occupy – all make pycnogonids serious outliers. So why don’t we just use DNA to solve all this? Taxonomy is a human construct, but something as   fundamental as genetics should give  us a more concrete answer, right?

Well, sea spiders are actually too weird for this. Thanks to the way that their digestive  systems extend into their legs,   their tissue samples can be very easily  contaminated with the genetic material   of whatever the sea spider ate,  especially for the smallest species. Luckily, we have other tools available to us,  like embryology.

Comparing the developmental   stages of different taxa can offer us  clues about how they are all related. In the case of sea spiders, they start off  life as basically just a head with a few small   extremities, gradually growing more segments and  more legs as they develop from larva to adult. This same developmental pattern is common in  other arthropods, especially in crustaceans.

Researchers initially suggested that  the rough similarities here could mean   that pycnogonids represent a more ancestral  developmental strategy among arthropods,... But that has since been dismissed in favor of  the possibility that the whole “head larva”   way of doing things evolved multiple  times convergently across arthropods. And sure enough, a comparison to similar looking  fossil pycnogonid nymphs supports this hypothesis.

So what exactly do we get from all this? In short, sea spiders have enough  in common with enough different   arthropods for us to include them in that phylum,   and enough in common with chelicerates for us  to at least group them in that general vicinity. But with all the truly wild traits that these odd  little dudes display, scientists have concluded – at least for now – that Pycnogonida most likely  split off in the early   Cambrian around 530 million years ago to become  a sister group to all modern chelicerates.

Sometimes what makes an animal bizarre  is just how different their shapes,   behaviors, and lives are from what we are used to. And sometimes it’s how hard we have to work  to fit them into a nice, human-defined box. For sea spiders, it’s both, and that  double whammy of weirdness is actually   helping scientists better understand  the evolution of arthropods as a whole.

And given that they are the most diverse  taxon on the planet, that’s a pretty big deal. So maybe an ocean full of spiders that are made  entirely out of legs is good news after all. Joining the pin club at BizarreBeastsShow.com  helps us keep making these videos.

If you want the sea spider pin to be your  first pin, subscribe by November 20th. You cannot do it after that. You will never get that pin!

And now for some bonus facts… [♪♪ BONUS FACTS ♪♪] If there’s one thing Bizarre Beasts  shares with its sister shows at Complexly,   it’s that we love a good fact about butts. In a study published in January of 2023,   researchers discovered that juveniles  of one species of sea spider were   able to regrow their back-ends after  they were experimentally amputated. The sea spiders regenerated at  least two of their back legs,   their reproductive systems, portions of  their digestive system, and their anuses.

They regrew even their butts. The adult sea spiders that took part  in the study were not so lucky – they did not regrow any body  parts and half didn’t survive. And that might be why scientists hadn’t realized  sea spiders could regenerate lost body parts – they’d been looking at the  adults, not the juveniles.

The second bonus fact is – remember those giant Antarctic  sea spiders we mentioned earlier? Well, in most species of sea  spiders, the males carry the   eggs around on their legs until they  hatch and sometimes even after that. It’s exclusively the males  who do the parental care.

But in one species of giant Antarctic  sea spider, one of the parents – probably still the father – attaches the eggs to the sea floor, instead. There, microalgae grows over them so they  blend in with the rocky bottom of the ocean,   which camouflages the eggs and  provides them with protection. Sea spiders eat soft invertebrates they  find on the seafloor, but thankfully,   you have more options, like the  ready-to-eat meals from Factor.

Eat stress-free with Factor's  delicious, ready-to-eat meals. Every fresh, never-frozen meal is chef-crafted,   dietitian-approved, and ready  to eat in just 2-minutes. Choose from a weekly menu of 35 options,  including popular options like Calorie Smart,   Keto, Protein Plus, or Vegan & Veggie.

Also, discover more than 60 add-ons every  week, like breakfast, on-the-go lunch,   snacks, and beverages to help you stay  fueled and feel good all day long. Factor meals eliminate the hassle of  prepping, cooking, or cleaning up. Simply heat and savor the good stuff.

Today I had the roasted tomato and feta  cavatappi with broccoli and red peppers. The tomatoes really make this for me, but the  broccoli balances the whole meal out very nicely. Head to FACTOR75.com or click the link below  and use code 50BIZARREBEASTS to get 50% off   and free shipping on your first Factor box!

That’s code 50BIZARREBEASTS at  FACTOR75.com to get 50% off and   free shipping on your first box! [♪♪ OUTRO ♪♪]