Friday, August 22, 2014

The Age of Sail

Oh boy are we going into the woods this week. For those who know me it's no secret that I love jellies (The squishy animals. I don't care much either way about the fruit spread.) So when IFLScience posted this article about by-the-wind-sailors, Velella velella  (dibs on the band na...oh) washing up in their thousands in Monterey bay I couldn't help but get excited. Not only because velella are incredibly cool, but because just explaining what they are is an odyssey of learning on its own. Incidentally this is one of the cases where using the scientific name is actually easier than the common one, so from here on I'll be referring to these animals as velella.

Hundreds of velella washed up on a beach. A surprisingly normal occurrence.
Courtesy Bettina Walter via Flickr

Now on the surface jellies seem pretty simple, and in terms of structure they are. They have no brain, their outer layers are usually one to three cells thick, and since they only have one hole in and out of their body; they eat and poop from the same opening. But jellies are one of those groups of animals that are so diverse and confusing they make a game of thrones family tree look like an elementary school ancestry project. In fact what we usually call jellyfish aren't even all the same thing. Let Steve Haddock from the Monterey Bay Aquarium Research Institute (MBARI) explain how there is no such thing as a jellyfish.

Seriously though MBARI has the best youtube channel

If you aren't able to watch that video it basically explains that so many different types of animals have evolved gelatinous forms that there isn't a great case for calling anything a jelly. In the interest of simplicity however when I refer to something as a jelly I'm talking about a tentacled, free-swimming life stage of either a scyphozoan, hydrozoan, or cubozoan. These three groups are classes of cnidarians (pronounced: nye-dairy-annes) which are animals whose tentacles have cells containing venom injecting harpoons. These stinging cells (called cnidocytes) are used partly for defense, but primarily  for prey capture. This is actually why humans can't feel the stings of many jellies; the relative size and sturdiness of their prey compared to the jelly determines how strong the sting needs to be. Jellies that prey upon fish, squid, and larger crustaceans need to make sure their prey is paralyzed or dead almost the instant it comes into contact with their tentacles. If  prey were to struggle hard enough it could do serious damage to the thin skinned, gooey jelly. Even though many jellyfish can't hurt you; I never recommend people touch them. So many types of jellies look similar to one another that it can be hard to tell the painful stingers from the harmless ones.

Especially the way most people find them.
Courtesy Justin Henry via Flickr

So where does the whole "tentacled free-swimming life stage" I mentioned fit into our story of velella? Well jellies have an astonishing life cycle. There are two completely separate phases to these animals. The phase we're most familiar with is usually what's called the medusa. Medusas have distinct sexes and reproduce when males release packets of sperm into the water which the females have to eat. Not only is there only one way into and out of the jellies, but the females' reproductive organs are inside their digestive track. Many species brood their eggs for a while holding them in their tentacles. From those eggs are hatched babies that will never become the free-swimming jellyfish. Instead they leave their mother's embrace and find a spot to settle on the bottom. Once settled they morph into their other major life stage; the polyp. Polyps usually look like very tiny sea anemones. They feed on plankton and detritus (particles of dead stuff) building up energy like a caterpillar. And like a caterpillar they go through a metamorphosis; not into an adult jelly but into a cloning machine. Entirely new organisms grow out of the polyps' bodies. This is the medusa that we know and love. It's like small frogs growing out of the side of tadpoles that never become the adult form! For you visual learners out there, below you'll find the life cycle in a beautiful chart. And in case you've been wondering what the difference is between the hydrozoa and the scyphozoa one big separator comes in the polyp stage. Scyphozoa divide their baby jellies in nice neat stacks whereas hydrozoa bud off all over their bodies.

"Reproductive cycle of jellyfish" by Zina Deretsky, National Science Foundation - Licensed under Public domain via Wikimedia Commons

Okay, we've gotten a good understanding of what a jelly is and how their lives work so we can return to our friend velella. Velella is a type of hydrozoan who takes everything we just talked about and flips it the middle tentacle.

Oh my god he's doing right now right now!
Courtesy Andrew via Flickr

 Instead of having a polyp that sticks to the bottom velella has a polyp that sticks to the surface! That's right the beautiful sail-carrying jelly is the stage that does the cloning. For a long time it was assumed that velella was a special multi-polyp, colonial, siphonophore like the Portugese man-o-war Physalia physalis. In fact they look quite a lot alike and behave similarly. But studies now suggest velella is a pretty unique organism being one of only a couple animals in the porptid family. And it's not entirely clear whether the floating polyp is one relatively big individual, or a big shared float with a colony of clones underneath.

Okay, so if what we see is the polyp stage, where are the medusas? Well amazingly the medusas are tiny and when they bud off from the polyp they dive down into the depths. Just about everything about velella is backwards from what we expect. The medusas mate in the deep and their babies cruise up to the surface where they develop into the floating form. Velella polyps show differences in the orientation of their sails depending on where they live. In the center of ocean basins it's hypothesized that velella polyps have sails angled so they're carried both east and west. But closer to shore; polyps have sails oriented so prevailing winds keep them in the ocean.  When large storms or anomalous winds kick up they drive the velella on the water into the shore. And that's the source of Monterey's large stranding.

Velella has few threats to its continued existence so thankfully we can expect these amazing animals to be around for quite a long time. There is the possibility that the larger storms that come along with global climate change will cause more frequent strandings. So as always doing what you can to cut back on carbon emissions is a great way to help make sure the age of sail continues for these wonderful animals.

Sail on my friends, sail on.
Courtesy PowderPhotography via Flickr


K.J. Eckelbarger and R.Larsen. Ultrastructure of the ovary and oogenesis in the jellyfish Linuche ungiculata and Stomopolus meleagris, with a review of ovarian structure in the Scyphozoa. Marine Biology #114, 1992. Accessed via:

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