#chordates

Geckolepis

That's an excellent idea!

Rectangl

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The Paleo Party Guest of the day is the Conodont!

This group of early chordates lived between the Cambrian and the Jurassic, with over 1500 species currently recorded. Their "elements", teeth like structures, are the primary fossil that remain from them, and they're often used as an indicator fossil for the age of sediments.

Age: 500-200 mya, individual species and genera vary.

Location: Global paleo-oceanographic distribution.

Welcome to this totally tubular Wet Beast Wednesday. Today's topic is the salp, a jet-powered, filter-feeding, colonial tube beast that feeds the ocean and helps fight climate change. And while the salp may look like a basic blob, these gelatinous gents are actually more related to you and me than to jellyfish. But perhaps more famous than salps is their own pernicious parasitioid, a creature with a lifestyle out of a horror movie. Come dive in a learn more about the connection between these creatures.

(image: a chain of salps. Each individual salp is a transparent, barrel-like animal with a dark spot inside the body. They are connected to each other by their sides, forming a long chain of salps. This chain has coiled itself into a spiral. End ID)

Salps are free-swimming tunicates of the family Salpidae. They are barrel-shaped animals with transparent bodies and openings on both ends. There isn't much going on inside of a salp. Their gelatinous bodies are mostly made of seawater with a matrix of proteins and connective tissues providing structure. They also have simple digestive tracts that are often the only part of the animal that isn't transparent due to being filled with food. Within the body is a fine mesh made of mucus. To a salp, moving and eating are the same thing. They use cillia and muscular action to draw water in through the front opening and force it out the back, providing thrust in the most efficient example of jet propulsion in the animal kingdom, capable of contunuously propelling the animal at speeds of 10 body lengths per second. As the water gets pumped through the body, food particles get trapped by the mesh and subsequently digested in the simple digestive tract. And yet, despite how they seem like relatives of jellyfish or comb jellies, salps are actually more closely related to you and me. Tunicates and lancelets are the most primitive chordates, the phylum of animals with a notochord and dorsal nerve cord that includes vertebrates. Most tunicates only have their cords as larvae and lose them as adults. This is in contrast with the other chordates, the lancelets and vertebrates, who keep their cords into adulthood. Nevertheless, this makes tunicates the sister group to vertebrates. The largest salp is Thetys vagina, which can reach 333 mm (13 in) long.

(image: a single salp. It is a transparent, tube-shaped animal open on both ends. At this distance, some rib-like structural elements are visible within the gelatinous body. At one side of the body is the digestive tract, which looks like an orange lump. End ID)

Salps go through a process called alteration of generations, in which each generation of the species has a different function and reproduced differently. This is more common among plants than animals. In salps, they alterate between asexual and sexual generations. Asexual salps are loners known as oozooids. They reproduce by producing a chain of connected sexual salps withing their bodies and eventually releasing them. Asexual salps can reproduce this way many times in their life. The sexual salps, known as blastozooids, are colonial. Each salp is physically connected to its siblings, forming chains or wheels that can stretch for meters. All blastozooids start out as females and have a few eggs they are born with. Fertilized eggs grow into chains of oozooids within the mother's body (nourished by a placenta-like connection to the mother) that are eventually released into the water. These chains will split apart into the individual oozooids. Once the blastozooids have used up their supply of eggs, they will grow testes and become males. Males release sperm into the water that is picked up by the females to fertilize their eggs. This life history trait of starting out as one sex before becoming another is called sequential hermaphroditism. Salps are the fastest growing or all multicellular organisms. Some warm-water species can have a growth of 10% body lengths per hour and can become mature enough to reproduce in 48 hours.

(image: an oozoid salp with a chain of blastozooids growing within it. The blastozooids are extremely small and already growing in a chain. End ID)

Salps are found in oceans worldwide, but the largest population occurs in the Southern Ocean around Antarctica. In these waters, they are so abundant that they can outnumber krill. Salps live in the open ocean and rarely approach shore, though they are carried around by currents and can be swept to the shores. They follow diel vertical migration, a mass migration in the ocean that occurs every night. At night, massive quantities of various species move upwards into shallow water to feed. When morning comes, they retreat into the deep ocean to shelter from predators. Salps are an important part of the biological pump, the cycle that sequesters carbon from land and the atmosphere into the deep ocean. By feeding near the surface during the day, salps take in carbon through carbon dioxide. This carbon is then released in their feces, which sinks to the seafloor. In addition, salps sink rapidly when they die, taking their carbon and other nutrients with them. This provides an invaluable form of transporting carbon and nutrients from the nutrient-rich surface waters to the nutrient-poor deep sea. Salps seem to be increasing in abundance as the ocean warms, meaning they take even more carbon dioxide to the deep sea, helping fight global warming. In addition, during algal blooms, salps can take in so much food that they become weighed down and sink, providing an even more direct route of nutrients to the deep sea. Even in shallow waters, salps are prey to a large number of species.

(Image: a chain of salps in a wheel formation, forimg a large circle of connected salps. End ID)

A lot of people know salps not for themselves, but for a parasite of theirs. Pram bugs are amphipod crustaceans that can reach 42 mm (1.7 in) in the largest species, Phronima sedentaria. That's pretty damn big for an amphipod. Female pram bugs will seek out individual salps (as well as the related dololid and pyrosome tunicates) and attack them, entering the body through one of the openings and eating the internals, leaving only the outer tube intact. She then uses the body as a mobile home, carrying it with her as she swims and laying her eggs inside of it. While commonly called parasites, pram bugs would more accurately be called parasitoids, the difference being that parasitoids kill their hosts. Pram bugs are often claimed to have inspired the Xenomorph from the Alien movies, but there doesn't seems to be any actual evidence for this.

here's today's #Paleostream sketches!!! today we drew Pikaia (no longer upside down!), Torukjara, Koreanosaurus, and Istiodactylus

this

and this