You know those squishy-seeming, Mickey-Mouse-shaped, sort of sponge-looking things you see quite often on reefs? Or those grape-shaped clear balls you sometimes see in the water column, usually stuck together in a chain? They are called ascidians, and they’re also some of our closest cousins, relatively. They, like us belong to a phylum called Chordata, the occupants of which are called chordates. All chordates, from those almost amorphous blobs to us all have five characteristics at some point in their lifecycle: dorsal (back of the body), hollow nerve chords, a stiff, rod-like notochord that spans their back from head to tail, gill slits on their pharynx, an endostyle – which is hard to describe because it has evolved to have many different processes over different species, and a tail that begins after the anus. Some of these, like the gill slits and post-anal tail disappear on humans before birth and most are heavily modified across the genera, but they are all present at some point.
Chordates are a well-recognized phylum for the most part, but there are a few members who make it quite diverse. The aforementioned ascidians are joined by larvacea (pictured below) which are tadpole-shaped and travel around the seas in self-built mucus houses. Periodically, they devour these mucus castles, eating all the tiny organisms stuck in it and recycling the mucus into a new, more modern dwelling. Sometimes they carry around a second, less elaborate mucus house behind them. If a predator comes, they escape out the back door of their primary house, and into the backup, getaway house. Moral of the story being: maybe living in your parent’s basement isn’t such a bad option. It could definitely be weirder.
Also in the phylum are lancelets or amphioxi (pictured below) which are more recognizable as they look very similar to fish. They don’t do much of note, to be honest. They live in little sand burrows and rarely leave unless to mate. They filter feed small organisms from the water through their gill slits and are quite small and easily overlooked. There’s a reason the BBC hasn’t done a single David Attenborough documentary on them; they make horrible reality TV stars.
After that, we start getting into the vertebrates. Vertebrates are so named because their notochord has been replaced by vertebrae, composed of a stronger, stiffer substance. Within vertebrates, there are jawless fish, cartilaginous fish, bony fish (the first appearance of bony internal skeletons), amphibians, reptiles, birds, and mammals. Basically everyone knows at least basic facts about the last five, so let’s focus on the first two.
Jawless fish – comprised of hagfish and lampreys are, well, gross, but in a cool way. Hagfish are generally deep sea dwellers that eat already dead things. They can tie themselves into knots to better leverage ripping flesh off of whatever has fallen to the bottom of the sea. They also produce the world’s largest amount of slime when threatened. Just a pinch of mucus produced on their skim can turn an entire aquarium’s water into thick, viscous slime. Lampreys (pictured below) on the other hand have a large disk in place of a mouth that they use to screw into the side of a fish or other victim and live off that host’s blood. This makes their life relatively easy since they need not move much, or even put any energy into digestion since blood is already so palatable.
Cartilaginous fish are, unsurprisingly, fish whose skeletons are made from cartilage. These include sharks, skates, rays, and chimaeras. They are an ancient class little changed for millions of years. However, they are also heavily threatened by human influence in the last century. Over 100 million sharks are reportedly killed by humans each year. But experts estimate that the true number of annual shark kills is more likely near 273 million. That’s over 31,000 deaths per hour. Contrast that to the 16 shark attacks on humans per year – resulting in one fatality every two years, and you might get a clearer picture of who the real vicious killer is. While some cartilaginous fish lay eggs like their other fish relatives, many give birth to live young. Some even have placental pregnancies like mammals! Since they are made of cartilage, they lack ribs so leaving the water can cause their own body weight to fatally crush their organs.
So, there you have it. That’s a basic rundown of our phylum-mates and close relatives, just in time for the holidays. Which one of the above reminds you most of those relatives sharing your dinner table will depend on you. But next time you’re in the sea, take some time to search out and observe your fellow chordates. The similarities may start to surprise you.
‘Clarity on Chordates’ was written by Roya
Photo Credit: Inspired to Dive