On August 16th, 2017 we learned about

Explaining the ink ejected from a squid or octopus’s anus

I’m not sure what this says about schools today, but my kids are learning a bit of marine biology thanks to Mario Kart. As many of you undoubtedly know, one of the more frustrating power-ups in the game is a Blooper— a squid-like character that flies in front of rival racers to squirt black ink all over them, obscuring their vision for a time. This caught my kids’ attention recently, not because of the squid flying through the air, but because of the ink. They wanted to know what was happening, and why it would happen with a squid?

Concealing and confusing

Naturally, a squid squirting ink to obscure the vision of potential threats is the most sensible aspect of a Blooper’s function in the game. Many shelless cephalopods, including octopuses, squid, and cuttlefish, do spray ink in a similar manner. They produce the ink in a special bit of anatomy called the ink sac, which includes the appropriately named ink gland. In moments of need, the squid or octopus injects this ink from the sac to the rectum where it is mixed with mucus, at which point it can be pumped out the anus with a surprising amount of control.

While squirting a cloud of dark ink into an area is pretty distracting on it’s own, some species can shape their pooped-ink to look more like decoy tentacles from their own body, or the thinner tentacles of a stinging jellyfish. Ink can be flavored to disgust some fish or attract their predators. It’s used to darken and hide eggs, or to make glowing, luminous globs to simply confuse predators, leaving them wondering what on Earth just appeared in their face.

Ink’s ingredients

The ink itself has been historically been harvested to write with, but for the most part is pretty different from what’s in your average pen. Most pens today use dyes to darken your papers, as dyes can be thinner and less prone to blockages in a pen. Cephalopod ink can contain metals, enzymes, and most importantly, melanin. Melanin is the pigment that adds color to your skin and hair, and in this case makes the ejected ink dark and opaque enough to hide the fleeing squid or octopus. The pigment is thick and usable in the oceans, but not necessarily the best fit for your favorite pen.

All this said, we still have many questions about cephalopod ink, largely because we don’t know why these creatures ever started producing it in the first place. Fossils of soft-bodied octopuses and squid are hard to come by, but ink sacs have a strange durability that makes them oddly abundant in the fossil record. Some specimens have been found from 330 million years ago, which makes this anatomy older than any dinosaur. Unfortunately, the structure of the ink sacs and the composition of the ink seems to have changed very little in all this time, leaving few clues as to how cephalopods ever got started squirting in the first place.

Source: Why do cephalopods produce ink? And what's ink made of, anyway? by Mark Carnall, The Guardian

On August 15th, 2017 we learned about

Gray fox specializations optimize the all that time they spend in trees

“If you ever see a gray fox, it’s not doing it’s job very well.”

Presumably, Tuscon, the gray fox wearing a leash as part of a presentation by Bay Area Wild my kids saw today, could be excused for his visibility. The 14-year-old fox had spent most of his life with an animal rescue operation, and thus had little opportunities to practice hiding from either prey or predators. He had, according to his handlers, retained at least one strong instinct from the wild, which was a taste for bird eggs. Apparently one of the reasons we don’t see gray foxes is that most of us never look for them in trees.

In contrast to their taller, larger cousins that dominate the eastern United States, gray foxes (Urocyon cinereoargenteus) have a number of adaptations that enable them to climb even fully-vertical tree trunks. Their shorter limbs and powerful hind legs let them jump onto the base of trees, at which point they can grip the bark with semi-retractable, hooked claws that barely resemble those of other canines. Gray fox wrists are also unusually flexible for a canine, and have been compared to the anatomy of an arboreal primate. All this adds up to an animal that can not only climb, but do so fast enough to chase speedy prey like squirrels right up a tree.

Arboreal advantages

Chasing squirrels probably wasn’t the driving factor in gray fox evolution though. As Tuscon’s story indicated, it’s much easier to ‘catch’ slower things in trees, like eggs and baby birds. It’s also a great place for the fox itself to stay safe, and they’ve been known to make dens in hollow trees as high as 20 feet off the ground. During their first few weeks of penthouse living, a mother and kits will be fed by the father fox, possibly putting off their first steps in actual soil for weeks. This isn’t a given, as gray foxes will also dig out or repurpose subterranean dens, but trees seem to hold a special allure that the dirt can’t compete with.

Maybe this is why some gray fox couples decorate their trees with skeletons. It’s not a requirement for tree-dwelling foxes, but some mating pairs have been known to drag fawn carcasses into trees, leaving them to decompose into rather macabre adornments. The foxes will sometimes use the bones as seating, but it’s thought that the primary attraction is the smell. Multiple trees in a mating pair’s territory may be marked with bones as a way to indicate the boundaries of their turf. The visual impact of a suspended deer skeleton probably helps distract from the small foxes sitting there as well.

Source: Tree-dwelling gray foxes decorate with skeletons by Melissa Breyer, Treehugger

On August 14th, 2017 we learned about

Humans are adept at appreciating alarm in the voices of animals

You’ve probably never conversed with a groundhog or tree frog, but it might not be as futile as you’d think. Sure, it’s sometimes hard to communicate with other humans that ostensibly speak the same language as you, but there’s a good chance that some of the underlying emotion you want to express gets through in just about any conversation. For all the layers of complexity that language can have, researchers are finding that humans are actually pretty decent at picking up the basic intent of a wide range of vertebrates’ vocalizations. We might not know exactly what details a particular prairie dog has to share, but we can at least tell how strongly that critter feels about what it’s saying.

Sensing species’ sentiment

The study was fairly straightforward, asking 75 humans to listen to recordings of different animals and identify the level of arousal, or emotional energy, of that animal. The humans were native speakers of English, German or Mandarin in order to try and eliminate any bias that might arise from a human language that somehow more closely followed the grammar rules of bush elephants or pigs. In the end, people’s assessments were definitely more accurate than random guesses, although the degree to which people understood each species was sometimes surprising.

Humans could identify higher or lower arousal in other humans quite well, followed by giant pandas, tree frogs, elephants and alligators. We actually did worse with some species pigs, ravens and barbary macaque monkeys, indicating that familiarity or genetic similarities weren’t the key component to communication. Overall, the more a species relied on shifting the frequency, or pitch, of their voice, the more it made sense to human ears. Some samples from the study can be heard here if you want to try it yourself.

Shared origins for animals’ outbursts

We know that animal vocalizations can get very complex, and so nobody was expecting anyone to really parse specific messages in this test. The fact that prairie dogs seem to have specific vocabulary for details in their alarm calls is probably going to be beyond the ear of most human listeners. However, the fact that the sense of urgency of an alarm call in prairie dogs, birds and other animals may be detectable may indicate that all these air-breathing vertebrates share a common foundation in our noise-making.

Even if you’re not about to chat with your local squirrels, this study helps establish aspects of how language may have evolved in the first place. Other work has found that specific vocalizations are often taught from one generation of animal to the next in a process that closely mirrors human language acquisition. For example, baby marmoset monkeys transition from babble to specific calls in a process that is nearly identical to human babies. A baby marmoset will get feedback from adults about specific phonemes it makes, and then learns to refine and rely on those specific sounds for communication as it matures.

Source: Humans identify emotions in voices of all air-breathing vertebrates

On August 8th, 2017 we learned about

The ups and downs of a deer’s annual investment in disposable antlers

For all of the underlying biology we share with other animals, it’s hard to relate to antlers. They grow on heads, but they’re not hair. They’re not the cellular equivalent of fingernails that we find in rhino horns or porcupine quills. Instead, antlers are weird, bony growths that sprout anew every year, demonstrating just how much of a strain and specialization a body can go through in the name of sexual selection.

Exhausting anatomy

An antler starts growing on a deer’s head in early spring each year. Unlike the inert keratin that makes up your fingernails or hair, antlers are made of living cells, and grow inside a fuzzy layer of skin called “velvet.” As the antlers develop over the summer, it’s composed of active blood vessels, nerves and bone cells, all of which can grow up to three-quarters of an inch per day. Keeping this tissue alive isn’t free though, and deer will often have to strip nutrients from other anatomy to keep their growth on track. On top of everything else, it’s an investment that deer make annually, as unlike the horns of rams or rhinos, antlers are shed every year.

Great …or good enough

Theoretically, it’s all worth it though. Deer courtship places a lot of emphasis on antlers both as display structures and weapons. Male deer will butt heads and lock antlers to demonstrate their fitness. Like other famous bits of animal anatomy, bigger antlers help attract and impress mates while staving off potential challengers.

There’s a limit to all that “fitness” though, and studies have found that sporting the biggest rack is not always the most winning reproductive strategy. The increased metabolic demands and risks associated with bigger antlers seems to have given rise to a more subtle population of deer that get by just fine with more modestly-sized head ornaments. The assumption is that smaller antlers are just big enough to catch a mate’s eye without demanding too much upkeep. The fact that they’re less likely to get caught on a tree branch may also help keep their owner alive to try to mate again another year.

Cellular secrets

This isn’t meant to diminish how impressive these head-bones are though. Regardless of an antler’s size, scientists have been studying their cellular properties that let them grow so quickly while also being so resilient. The fibrous structures that compose the bone grow in a staggered pattern that helps them stand up to stress without being damaged. Antlers have been transplanted to other body parts, and even other animals like a mouse, and they keep growing like they were still on a deer’s noggin. Scientists aren’t looking to affix spikes to people’s heads exactly, but the fact that nerves can grow so quickly in an antler may be a model for human therapeutics some day.

My third grader asked: Do only boy deer grow antlers?

Outside of some unusual anomalies, its fair to say that antlers are a male appendage in just about every species of deer. The notable exception is reindeer, as both male and females grow antlers each year. It’s thought that the antlers help females claim territory that might hold precious bits of food. Coupled with a scarcity of predators on the tundra that the deer would need to hide from, it seems that having antlers ends up being a good thing for each and every reindeer.

Source: Antlers Are Miraculous Face Organs That Could Benefit Human Health by Jason Bittel, Smithsonian

On August 6th, 2017 we learned about

Determining which details make a difference to vocalizing elephant seals and Japanese tits

After eight years of his presidency, I’m fairly confident that I could recognize and understand just about anything Barack Obama says. Even if he were reading something unexpected, like a end user licence agreement or child’s joke book, I’d still understand who was speaking and what was being said. There are limits to this though, as changes to pitch, rhythm and word order would probably make Obama’s voice harder to identify, possibly to the point he could be understood at all. This isn’t a huge worry in my daily life, but understanding the threshold where communication breaks down is critically important, particularly for animals where vocalizations (not made by former president) influence life or death situations.

Elephant seals’ signature sounds

Elephant seals (Mirounga angustirostris) make a lot of racket around mating season, largely to scare off potential rivals that may be after a male’s mates. When a rival bull does challenge the dominant male, the blubbery mammals will thrash each other with their flippers and teeth, sometimes leading to fatalities. When a challenger survives such an ordeal, he also makes note of the dominant male’s particular croaking roar, and will remember who that voice is tied to if it’s heard again in the future.

To test the limits of this recognition, researchers recorded the grunts of a dominant male elephant seal, then digitally altered the pitch or rhythm of the vocalization. When unaltered vocalizations were played to subordination males, they generally ran away to avoid the expected fight. But when these same males heard an altered version, they apparently thought it belonged to someone else, and would stay put to risk a confrontation. Changes in pitch would likely be tied to the size of an animal, and so it’s less surprising that that made a difference to the listening bulls. The sensitivity to rhythm was more surprising, as most mammals don’t seem to rely on that kind of tempo variation as a cue in their interactions.

Sensitivity to syntax

Of course, what is being vocalized is important to vocalizing animals too. Researchers listened to the alarm calls of Japanese tits (Parus minor) to see how sensitive they were to changes in their chirps. When a predator approaches, the tits will sometimes issue a “mobbing call,” inducing any member of the flock within range to swarm around the threat. These calls were found to use four phrases in an ABC-D pattern. The ABC chirps alerted listeners to danger, but the delayed D is what kicked off the counterattack.

By altering recorded calls, researchers found that this phrasing was pretty inflexible. Substituting calls from other regional tit species, like the willow tit (Poecile montanus), in for the D phrase completely botched listeners’ reactions. ABC-taa didn’t elicit any of the response that ABC-D did, giving researchers a sense of how these birds are constructing what is basically a crude form of grammar. Bending it too much essentially ruins the statement’s manicotti, er, meaning.

Both studies help us understand more about how language evolved, and how much specialization it has required of animals’ brains along the way. More immediately, it may also help inform conservation strategies, since we now know how sensitive these animals are to the sounds they hear. Mishearing a roar or call for help could have serious consequences for an animal that can’t clearly make out what it’s listening to.

Source: Elephant Seals Can Recognize Rhythm And Pitch by Madeline K. Sofia, The Two-Way

On July 27th, 2017 we learned about

Corythoraptor jacobsi appears to connect the cassowary’s head crest to the Cretaceous

An extinct species of dinosaur discovered in China has a lot people thinking about it’s living relative. The fossil remains of Corythoraptor jacobsi were remarkably well-preserved, allowing paleontologists to describe it as something similar to an ostrich and more importantly, a cassowary. In the Cretaceous period, the animal would have stood around five-and-a-half feet tall, topped off with an impressive half-foot of bony crest on its head.  That crest is so similar to what’s on modern cassowaries that researchers have even raised the possibility that the two dinosaur species may be part of a single lineage.

Possible proto-ratite

So what would an ancient, ostro-cassowary be like? Even if C. jacobsi turns out to be a cousin rather than an ancestor to these modern birds, we can still deduce a lot about its life from the fossils alone. This particular specimen was probably eight-years-old, although it wasn’t fully grown yet. It was part of the oviraptorid family of dinosaurs, a group of dinosaurs that generally sported beaks, strong legs and feathers. Those feathers may have never been used in flight though, as they were much fluffier and fringed than the plumage required to fly. Like modern ratites like ostriches, emus and yes, cassowaries, these creatures’ feathers most likely helped with showing off to peers, camouflage, and insulation against heat and cold.

Interestingly, there’s a chance that the tall, flat crest on C. jacobsi’s head served some of those purposes as well. Like a modern cassowary, the crest, or more specifically, casque, wasn’t a solid lump of bone. It was composed of various layers that would have allowed for empty cavities, blood circulation and more. These features suggest a range of uses, including a way to vent heat like a toucan’s bill, show off to potential mates or rivals, and possibly even emit low-pitched vocalizations. Much of this speculation isn’t due to mysteries specific to C. jacobsi’s casque, but that we’re not actually sure what cassowaries do with their heads either.

Figuring out the casque’s function

Cassowaries aren’t easy to observe in the wild, partially thanks to their small ranges in Australia and New Guinea that are difficult to traverse, much less follow occasionally dangerous, six-foot-tall birds. It’s been suggested that their casques protect them from falling fruit, possibly help them dig through loose soil, and vent heat. Their striking appearance is hard to ignore, even among brightly colored plumage and wattles, which begs the notion that they’re a display feature of some sort. Casques are found on males and females, although they tend to be bigger on females which supports the idea of some kind of sexual selection at work. Male cassowaries help with child-rearing, and so both sexes may have reason to be choosy about the health and stature of their partners. Finally, cassowaries are famous for emitting deep, booming vocalizations, and their crests may help them make or possibly hear those calls across long distances.

Understanding the casque on cassowaries and C. jacobsi may end up advancing a few different ideas about dinosaurs. If the value of a good casque can be pinned down, it may help us better understand crests on more distantly related species as well. While head crests were not uncommon among oviraptorids, they’re also found on other groups of dinosaurs, like “duck-billed” hadrosaurs. Of course, there might be more than one use for a huge lump on one’s head, but the resemblance between C. jacobsi and cassowaries raises hopes of a more direct comparison.

Source: Newfound Dino Looks Like the Creepy Love Child of a Turkey and an Ostrich by Laura Geggel, Live Science

On July 26th, 2017 we learned about

Ecuadoran frog found after defying extinction for 30 years

It may be time to start paying closer attention to the critters my kids find in the yard. I don’t think there are any nearly extinct species living on our street, but then again, biologists from the Jambatu Canter for Research and Conservation of Amphibians didn’t have high hopes for Jambato harlequin frogs either when they issued a reward for their recovery. The frogs hadn’t been seen for 30 years, and yet somehow an elementary school student managed to track down enough individuals to kick off new breeding efforts and possibly save the species.

Jambato harlequin frogs (Atelopus ignescens) were once found in forests from Ecuador to Columbia. The small frogs only grow to a couple of inches long, and may not look like much at first glance thanks to the dark coloration on their backs. This is in contrast to their bright orange bellies, which earned the frog a name that is Latin for “to catch fire.” While they were once fairly common in humid areas, their population began to decline in the early 1980s, and with the last confirmed sighting in 1986. As far as anyone could tell, Atelopus ignescens had gone extinct, possibly due to habitat loss or the spread of chytridiomycosis, a disease that has been plaguing amphibians around the world.

Not dead yet

In 2016, researchers offered a reward for Jambato harlequin frogs with no real expectation that anyone would find one. It was thought that the reward would be a good way to catch people’s attention, and show how an animal that had once been so common was still vulnerable without protection. In a sort of happy reversal, the reward spurred a young boy to bring in a group of Jambato harlequin frogs he’d found near his house. 43 individuals were captured, and work immediately began to try to boost the amphibian’s numbers in a captive breeding program.

It will be some time before this next generation of frogs can be released into the wild, and their population is still precariously small. Everything from mating enclosures to tadpole food are being tested to give these frogs a shot at stability. In the mean time, the boy who found them is putting his prize money towards his education, having unexpectedly revived a scholarship for conservation work.

Source: Boy finds ‘extinct’ frog in Ecuador and helps revive species by Lou Del Bello, New Scientist

On July 23rd, 2017 we learned about

Scouring human skeletons for signs of scavenging by sharks

You’re probably never going to be attacked by a shark, much less killed by one. However, that doesn’t necessarily mean a shark will never try to make a meal of you. Forensic researchers in Florida are studying how to identify the aftermath of scavenging sharks, as it seems that while sharks aren’t looking to eat a lot of live humans, they might have less of a problem with eating those of us that have already died. This work may help with our understanding of sharks, but also help piece together deaths that might otherwise remain unsolved.

The first challenge of this work is that it’s being done entirely with human bones. The lack of flesh makes definitively determining the cause of death impossible, but that doesn’t mean that finding a skeleton on the sea floor is a dead end. The shape, size and texturing of damage to a bone can help reveal a lot about what damaged those bones, and when the damage took place. For instance, terrestrial predators like bears will leave smoother, puncture shaped marks in bone, while sharks (and unfortunately, sand) will leave striations and linear texture in areas that teeth have sliced through. In the cases were a shark did munching, the majority of those bites were the result of scavenging rather than actual predation on humans. We’re apparently more appetizing as leftovers.

Locations specified by species

In some cases there’s enough detail recorded in a bone to determine the type of shark involved. A dramatic example is the “candy caning” left by a bull or tiger shark. The shark will remove a limb from a deceased body, then spiral from one end to the other to pull flesh off, leaving a spiraled pattern of damage on the bone. Knowing the species of shark involved can then help investigators, because it indicates where a body must have been to encounter that species of shark. This can help piece together someone’s death, especially since ocean currents are so good at moving bodies around without regard to preserving a possible crime scene.

My four-year-old asked: Why are these people dead in the water in the first place?

Some deaths may have occurred elsewhere, and like a long-lost dinosaur, the body was later washed out to sea thanks to a river or temporary flooding. Boating accidents and drownings can also leave bodies in the surf that might be appealing enough for a shark to take a bite. These aren’t exactly common occurrences, but places like Florida’s shark and human populations are high enough to make this line of research worth investing in.

Source: Shark Scavenging Helps Reveal Clues About Human Remains by Mary-Lou Watkinson, Florida Museum

On July 19th, 2017 we learned about

Dolphins seem to be teaching each other to decapitate dangerous catfish

While humans may find dolphins to be cute, cuddly and sitcom-friendly, other sea life probably isn’t quite so charmed. Like other predators, dolphins have specializations that help them find and secure food as securely as possible. What makes dolphins stand out is their inventiveness and apparent ability to share what they’ve learned, which can lead to amazing innovations in hunting techniques. Sometimes this takes the form of using tools to safely dig for fish hidden in the sand, and sometimes it means figuring out ways to decapitate catfish with such efficiency that the fish heads survive their grisly executions.

Roly-poly fish heads

Bottlenose dolphins (Tursiops truncatus) in the Gulf of Mexico have been working on their catfish hunting techniques for some time, but documenting their activity has been difficult. Sporadic reports suggested that the dolphins were developing a specific formula for gobbling catfish, but a sighting in May of 2015 served as a much more dramatic demonstration of their skills. Researchers came upon a trail of catfish heads that extended for a third of a mile, clearly left there in the wake of a pod of dolphins moving through the area.

Leaving leftovers isn’t that amazing on its own, but there was a consistency and accuracy in these decapitated heads that really stood out to researchers. Instead of random tears and chomps, the heads were so cleanly removed from the bodies that some were still alive when the research boat pulled them out of the water. Eyes, front flippers and mouths were all moving enough to even try vocalizing before finally expiring on the boat. Later analysis of the fish’s skulls backed this up, indicating that the dolphins had learned to remove the heads in a swift, efficient motion faster than the fish could defend against.

Severing heads for safety

The dolphins weren’t doing this to show off. Catfish have three defensive spines in their skulls that they can lock into a socket as a defense mechanism, making them not only difficult, but dangerous to try and swallow. Records of found dolphin corpses show that plenty of dolphins have suffered from improperly prepared catfish, with one specimen being found with 17 spines in it’s body that had even punctured it’s digestive tract before it died. Like other dolphins that have learned to smash octopuses before eating them, the Gulf dolphins probably started these beheadings as a way to stay safe.

As with other hunting innovations seen in dolphins, there’s a good chance that decapitating catfish was invented and shared among a dolphin social circle. It would be part of the growing list of specialized behaviors that appear to be known only to specific groups of dolphins, which suggests that it’s cultural knowledge, rather than some kind of innate behavior pattern that randomly appeared in the gene pool. This is probably pretty relatable to a human, as just about every skill we learn is learned from the people around us. The catch might just be that when we learn to clean a fish, we don’t start by beheading it with our teeth.

My third-grader asked: How exactly did the dolphins take the catfishes’ heads off?

For this question, we were fortunate enough to hear from Errol Ronje, the biologist who co-authored the study. It turns out that dolphin teeth and jaws aren’t really well-suited to sawing and cutting, as dolphins normally just swallow their food whole, so a good, hard chomp can be ruled out. Nobody has witnessed these beheadings in progress, but there were scrape marks from the dolphins’ teeth left on the fish heads that do provide a clue as to what happened.

Ronje’s hypothesis is that these scrapes were made when the dolphins gripped the fish just below their heads, then twisted and shook the fish until the heads basically popped off, almost like a farmer ringing the neck of a chicken. The dolphin’s teeth only needed to break the catfish’s scaleless skin, leaving the rest to some rather intense thrashing and spinning. Popping off heads like this may sound crude at first, but the dolphins seem to have a very precise method for this work. They not only avoid spines, but they can do this fast enough that Ronje’s team weren’t even aware the dolphins were doing it until it was over.

Source: Why These Dolphins Behead Their Prey by Michael Greshko, National Geographic

On July 18th, 2017 we learned about

Sea spider circulatory systems depend on the digestive tracts in their legs

As a squishy, somewhat elastic human, I don’t have to spend much time thinking about if there is room in my body to accommodate all my organs. If anything, aging has made me more concerned that my abdomen is too flexible about making room for my stomach, intestines and fat. Like many things in life, personal experience shouldn’t be used as a standard against the rest of the world, and indeed there are animals that have succeeded for around 500 million years with the opposite problem. Sea spider bodies are so small and compact that organs we keep in our torso have had to be distributed into the animals’ legs, although rather than suffering or complaining, this unusual arrangement has been leveraged to create a very efficient body plan.

A sea spider isn’t a spider, or even an arachnid, but it does bear a strong resemblance to spindly spiders like a daddy long legs. In fact, with the proportion of their body mass that is actually leg, they may be more deserving of the “long legs” moniker than the spiders. In many species of sea spider, the legs come together at a body that seems to be just big enough to act as a hub for those limbs— there’s hardly any differentiation between the thorax and abdomen, and heads are often just big enough to carry eyes, mouth parts and some eyes. As such, the legs don’t just literally carry the body, but also carry many biological duties we normally associate with bulky midsections.

Fully-loaded legs

While the sea spider bodies are minimal, their legs handle a lot of different functions quite well. They help with respiration by creating a lot of surface area for oxygen to diffuse into the body from the surrounding sea water. They also carry their sex organs in their legs, releasing eggs and sperm through small pores in their “thighs.” Tiny torsos don’t allow enough space for an effective digestive tract, and so the legs also hold coils of the arthropod’s intestines. Fluids are sucked out of sea anemones and sponges, then pumped through each leg in as the digestive organs squeeze and contract. Our guts do this too to an extent, but as invertebrates, sea spiders have exoskeletons that their guts have to push against from the inside. So rather than flexing and expanding, each gut contraction has to squeeze other fluids up or down to make space in the cavity of the leg.

This might sound awkward at first, but sea spiders have been found to use these contractions for two functions. Aside from moving food through a digestive tract, the other fluid that gets squeezed up and down has recently been found to be hemolymph, or the sea spider’s equivalent of blood. This means that the animal’s undersized heart only needs to serve the head and torso, while the legs’ circulatory needs are taken care of thanks to contracting guts. This not only saves space in the body, but also saves energy, as the heart doesn’t need to work nearly as hard to get blood moving down and back through each long limb.  As weird as it sounds, it’s a successful enough system to helped sea spiders find homes in oceans all around the world.

My seven-year-old nephew asked: So if I stepped near a sea spider, would it try to bite me?

Probably not, and if one did, you probably wouldn’t notice. Many species are tiny, and their piercing mouth parts probably couldn’t get through your skin. The largest species, Colossendeis megalonyx, can grow to three feet across with a proboscis as long as a finger, but they’re still basically benign creatures that wouldn’t behave aggressively about your feet. Also, as this species lives in deep water under the Antarctic ocean, chances are you wouldn’t be walking near it in the first place.

Source: Sea Spiders Pump Blood With Their Guts, Not Their Hearts by Ed Yong, The Atlantic