Sarsia is a medusa; a jellyfish, that is... or rather, a hydrozoan, one of the major groups of cnidarians, cousins to corals and anemones.  The development of Sarsia is remarkable in its simplicity.  It proceeds in a manner typical for hydrozoans: the first cleavage furrow is unipolar*, and equal divisions give rise to a coeloblastula – a hollow ball of cells – within which cells eventually appear, creating two germ layers. 

If you look closely, you will see that early divisions are parallel to the plane of the cell sheet: each cell divides to make two neighbors.  But later, some divisions are perpendicular to the cell sheet, so that one daughter finds itself inside.  The outermost cells, the epidermis, become ciliated, enabling the animal to swim; the inner cells will eventually make the gastrodermis (the digestive tract).  The larva of Sarsia, an ovoid ciliated blob called a planula, doesn’t hatch – as with most cnidarians, there's no enclosure to hatch from – but simply swims off when its cilia develop and begin to beat. 

Which brings up an interesting point.  How do the cilia know which way to beat?  They are certainly beating directionally, otherwise the larva would wiggle and twitch but it wouldn’t go anywhere.  The planula swims directionally because the epidermis has polarity.  And the polarity of the larva is determined by almost the very first visible event in its development: the position from which the cleavage furrow ingresses at first cleavage specifies the oral pole.  The oral pole, obviously so-called because a mouth eventually forms there, is at the back end of the planula: all the cilia beat in that direction.

In this movie, first cleavage begins in the upper right corner of the screen.  This is, then, the oral pole and the point directly opposite (the front end, swimming-wise) is therefore called the aboral pole.  Which means, that in the earliest seconds of this video, we know which way the larva will swim.  Make a prediction, and see if you are right: this video ends, as so many of these videos do, when the larva swims briskly out of the frame and begins the search for a hard substrate on which to settle.

Wait, a hard substrate?  For a jellyfish?  The ‘adult’ Sarsia – meaning the animal we can catch which will lay us an egg – is a small free-swimming medusa with a transparent bell, a distinctively long manubrium – the mouth on the end of a tube that connects to the jelly’s gut – and trailing tentacles.  A hard substrate seems like the opposite habitat from what the adult will need (as evidenced by the maimed remains of unfortunate jellies trapped in tidepools).

The thing about jellies, though, is that many have a biphasic life cycle, in which they spend part of their lives sessile and attached.  So the planula that cruises off at the end of this video is hunting for a shell or dock or boat or rock because its next developmental task is to metamorphose into a sessile polyp.  This polyp, in turn, will grow into a pinkish plant-looking thing called a hydroid colony which reproduces asexually. 

The hydroid will eventually pump out small (about 1 mm in diameter), genetically-identical medusae which will pulse their little bells and head off into the water column.  Adrift, they’ll eat and grow and eventually develop gonads (they have separate sexes, so males will develop testes and females will develop eggs) which they’ll then release into the water column where they’ll fertilize and go through the process illustrated in this video.  And then the planula will cruise off in search of a hard substrate...

— text by Katie Bennett

* Meaning it starts from one side of the cell and cuts through like a knife through a cheese.  For even better views of unipolar furrow ingression, check out videos of Phialidium, Nanomia, and Pleurobrachia