Mexico Part 3: Weird Things Under Rocks

Our last full day in PV we drove over to a local beach in nearby Cruz de Juanacaxtle*, and hired a Panga** for a couple of hours to do some dolphin-watching and snorkeling. After spotting a few dolphins (sorry, failed to get photos/videos), we headed West up the coast a bit to a pocket beach where we anchored and snorkeled around a bit.

*Nice little nearby town we didn’t get to till our last day. Sort of like a smaller, quieter Bucerias. Would be a nice spot to stay in/around, or have dinner.

**Small open boat with an outboard motor, seats maybe 8-10 people. Many, including ours, have sun-canopies, which I strongly recommend if you’re taking a bunch of sunburned gringos out on the water for a couple of hours.

Side Note: We prefer these kinds of individual, ad-hoc excursions to the monster tourist-excursion boats which include music, lunch, drinks and of course lots of other gringos. We just a started asking around on the beach, found a guy who knew a guy, worked out a price and set off. Our guide- “Genado”- didn’t speak any English, so a bit of Spanish helps in putting together these kinds of outings.

By the time we put in it was early afternoon, and the wind had kicked up the water a bit and made visibility fairly poor, but we still had fun kicking around and checking out a few brightly colored fish. After a bit the Trifecta and AW clambered back up on the boat. Then Genado donned mask and fins, dove in, spent a few minutes diving and turning over rocks, producing a couple of [what I thought at the time were] “starfish” which he handed to the kids, much to their delight.

The “starfish” were different than any I’d seen previously- a clear central disk, distinct from the legs, which in turn were covered with soft spines. And the things were active, much more so than any other starfish I’d ever seen or handled. As we held then gently in our hands, they- somewhat alarmingly- spontaneously dropped segments of legs onto the floor of the boat. We threw them back, and I made a mental note to figure out just what they were when we got home.

Tangent: Regular readers may have a reasonable objection at this point: In going on and on about our Mexican vacation this week, I’m totally skipping blogging about what is arguably the most happening week of Spring in the Salt lake Valley, which is, ironically, the whole reason for starting this blog in the first place. You’re right, I am, and this week has been absolutely stunning. But I’ve blogged about so many of the Salt Lake area blooms and birds of April these last 2 years, and on these Central American trips I’m always exposed to so much new stuff, that I want to blog about it while it’s still fresh in my mind.

Anyway, if you haven’t been reading this blog for a year+, and you’re a Utah-area reader who’s interested in or curious about what’s blooming/ happening around the valley right now, I recommend you go back and check out my April posts from 2009 and 2008*.

*Although, when I go back and re-read some of my posts from the first few months of this project, I’m always a little bit embarrassed by how poorly-written and generally… “dopey” they seem. It’s like it took a few months for me to get the rhythm of this project. Although I still think that series on Dandelions totally rocked. What? You still haven’t read it yet? Go check it out now- I am telling you, they are Way Cool.

What they were wasn’t starfish, but Brittle Stars. Starfish- or more properly Sea Stars- Brittle Stars, Sea Urchins and Sea Cucumbers are all Echinoderms, which are a phylum of marine animals that are all, well, weird. They’re invertebrates, but are thought to be a bit more closely-related to us than things like bees and lobsters. But where bees and lobsters have so many of the same- or at least analogous- parts/characteristics as us, including eyes, mouth, anus and bilateral symmetry, Echinoderms are way, way different.

Extra Detail: Just to clarify the more-closely-related-to-us than lobsters comment: Chordates*, Echinoderms and 2 other phyla of weird worm-like critters you never heard of** are all a kind of animal called a deuterostome. Arthropods***, Molluscs, Annelids**** and a few other things you never heard of***** are all a kind of animal called a protostome. The difference between the two types is defined by how they develop embryonically.

*Us and all other vertebrates, plus things like see squirts, which have spinal chords (but no backbones)

**Well, Christopher’s heard of them. But the average reader of this blog who shows up for the tangents, helmet-cam videos and stories about my neighbors probably hasn’t.

***So like most every kind of “bug”.

****An Earthworm is an example of an annelid.

*****Again, Christopher’s heard of them.

******Does anybody even read my footnotes, anyway?

[NOTE: In the original post, I had the deuterostome/protostome anus-mouth thing* backwards. This updated version corrects the error. Thanks to reader Tomodactylus for the catch.

*OK, out of context, that sounds really gross. But it’s not quite as gross once you read through the rest of the Extra Detail.]

Both deuterostomes and protostomes start out as a single cell which starts dividing. When it gets to around 128 cells (7 divisions), it’s a rough sphere, with a fluid-filled space inside. This stage is called a blastula (pic right, not mine*) , and around this point a “dent” or dimple appears on the surface of the sphere. In deuterostomes, this first dent will eventually develop into the anus. In protostomes, it’ll eventually develop into the mouth. In a sense, a bee’s mouth is our ass, and our mouth… well, you got it. Anyway, a Brittle Star’s mouth is our mouth, and its ass… well, it’s sort of our armpit, but we’ll get to that in a moment.

*I mean the photo. Though the blastula isn’t mine either. (It is human, though.)

Brittle Stars (class = Ophiuriodea, 1500 different species) are said to be “close relatives” of Sea Stars, even though they haven’t shared a common ancestor since before the Geminga Event. There are 2 orders of Brittle Stars; the 5-armers, like this guy (order = Ophiurida) and another bunch (order – Eurylida) with all kind of crazy-branching arms, called Basket Stars. But both overwhelmingly tend toward a pentaradial (5-ways) symmetry. Brittle star arms have a segmented skeletal structure concealed beneath the skin/spines which allows full side-to-side motion but not up above the up-side of the disk; they can’t “raise” their arms*.

*Basket Stars though, are jointed a bit differently, and can “raise” their arms.

If you were to pick the weirdest thing about echinoderms, you might guess it was the symmetry thing. But I’d say the weirdest things about them are that a) they have no blood and b) they breathe, poop and, in some species, give birth through their armpits.

In vertebrates such as us, our bodies produce blood to carry nutrients to, and wastes away from, cells. Many arthropods (spiders, crustaceans, some insects) have come up with an analogous substance called hemolymph. But echinoderms perform these functions with seawater. That’s right- the thing’s blood is seawater.

The water-vascular system operates the Brittle Star’s tube feet, which are hundreds of little tubular projection on the underside of the arms. In Sea Stars, the tube foot terminates in a suction cup. When the foot is placed against a surface and the Sea Star withdraws some of the water from the tube, it creates a partial vacuum which holds the tube foot to the surface, which is how Sea Stars/Starfish cling to rocks.

But Brittle Star tube feet lack suction cups, and they use them more for feeding than for getting around or clinging to surfaces. Most Brittle Stars get around by walking/crawling, generally leading with 2 arms, but some do actually swim.

The animal’s mouth is on the underside of the central disk and features 5 rings of jaws. Some Brittle Stars are carnivorous, but most feed on plankton and bacteria, which they catch by sweeping one arm through the water. They have no anus, but expel wastes through 5 underside-slits, called bursae, one in the “armpit” of each arm.

Holy crap. Can we all just pause for a moment and acknowledge how seriously these Expand-O-Graphics just totally rock?

The bursae are also used for respiration, which is accomplished through a series of tubes and sinuses that transport oxygen-laden seawater around inside the animals, but this respiratory water vascular system is completely distinct from the circulatory vascular system.

Brittle stars have no eyes, nor other specialized sense organs, but seem to be able to sense chemicals (smell), and even light through specialized nerve-endings in their arms.

Most- though not all species have 2 sexes and reproduce sexually. Fertilization usually happens externally, with sex cells being expelled into the open water. In most species, the young develop in a free-swimming larval, plankton-like stage, but in others the larvae develop internally before being “born live”. Guess where they’re housed during “pregnancy”? In the bursae! That’s right- the combo gill/anus substitute!

But (sexual) Brittle Stars can also reproduce asexually. When threatened, their arms often detach (as did several on the boat). This doesn’t seem to harm the creature, which soon re-grows the arm in question. But if an arm detaches with part of the central disk, it will grow into a whole new Brittle Star.

Seriously, could these things be any weirder? And we haven’t even talked about the whole symmetry thing.

Tangent: It’s funny when you pause for a moment and consider the kinds of creatures/aliens/monsters featured in science fiction. It’s hard to come up with any example more “alien” than these guys. And though it may be funny, there’s a serious aside here, in that it shows just how narrow and limited our own range of “imagined-possibles” may be. While not quite a dog and a diesel engine deal, it makes you wonder about our ability to get our heads around some of the conceptually really tough stuff, like the Two Big Mysteries (Existence and Self.)

Brittle Stars have no front or back or left or right- just up and down. Across the seafloor, all directions are equal in its eyes (oops, I forgot- no eyes). When I first thought about “starfish”, I thought this was the coolest things about them, that they represented a whole different form on being an animal- without front, back or any bilateral symmetry. But it turns out that in larval stages, Brittle Stars, Seas Stars and many other echinoderms are bilaterally symmetrical, and it’s thought that they evolved from bilaterally symmetric ancestors, making them somehow even stranger.

All that weirdness from a little thing Genado pulled from under a rock. Every little living thing always turns out to have some totally awesome story.

It was a great vacation. We flew home the next day, tanned and tired.