The Seymour Marine Discovery Center is currently hosting a satellite reef of the Crochet Coral Reef project. Back in the fall, about 350 UC Santa Cruz students and community volunteers began crocheting creatures real and fanciful with yarn and other materials. Satellite reefs have been built all around the world, in this project that unites mathematics, marine biology, conservation, and a love of working with yarn.
Since this isn't my brainchild I'm not going to go into the background and philosophy of the Crochet Coral Reef project. Instead, I'm just going to show you some photos of the Santa Cruz satellite reef, and encourage you to come see it for yourself. If you happen not to be in the Santa Cruz area, you can click here to find other satellite reefs around the world. You may even want to start your own reef! Note that many satellite reefs are located quite far inland--Colorado, Indiana, Minnesota--so don't let your lack of a nearby ocean keep you from organizing and building your own reef.
Some of the creatures on the reef are made of garbage or plastic, to remind viewers that the world's oceans continue to pay the price for human excesses. This jelly, below, has oral arms made from plastic grocery bags.
And see what familiar object was used for this crab's eyes?
There are multiple species of octopus on this particular reef!
The reef will be on display through October 2017. If you're in the area before then, swing by and check it out!
One of the defining characteristics of the Phylum Mollusca is the possession of a shell, which serves both as a protective covering and an exoskeleton. We've all seen snails, and some people may have noticed that snails often withdraw entirely into their shells and even have a little door that they can use to seal up the opening of the shell. That little door is called the operculum. Opercula occur in non-molluscan animals, too, such as some of the tube-dwelling polychaete worms and some of the thecate hydroids. Snail opercula come in lots of different shapes, depending on the aperture of their owner's shell.
Given the enormous morphological diversity within the Mollusca it shouldn't be surprising that their shells vary immensely in prominence and shape. In fact, molluscan shells demonstrate quite beautifully the relationship between form and function. The benthic and most familiar molluscs, the gastropod snails, generally have coiled shells. Notable exceptions to this generality are the marine opisthobranchs (nudibranchs and sea hares) and the terrestrial slugs. And for the most part snail shells look recognizably like snail shells, even though some are plain coils, others may be flattened (e.g., abalones), and still others may be crazily ornamented. Aquatic animals crawl around in water, which helps to support the weight of heavily calcified shells. Terrestrial snails, on the other hand, live in a much less dense medium (air) and have lighter, less calcified shells. The trade-off for a more easily transportable shell is that air is also very drying, and a thinner shell provides less protection from desiccation.
I should state for the record right now that I'm not talking about the many molluscs that don't have shells at all, or that have much reduced shells.
The bivalve molluscs (mussels, clams, oysters, etc.) live inside a pair of shells. They are sedentary animals, living either attached to a hard surface or buried in sand or mud. Not being able to run from predators (although some scallops can swim!), their only defense is the toughness of their shells and the strength of the adductor muscles that hold the shells closed. Most bivalves feed by sucking water into the shells through an incurrent siphon, using their gills to filter food particles from the water, and expelling the water through an excurrent siphon. To do so they must open their shells enough to extend their siphons, or at least expose inhalant and exhalant openings, to the water current surrounding them.
So, snails have one shell and bivalves have two. Some of the most interesting molluscs, in terms of shell morphology, are the chitons. The Polyplacophora (Gk: 'many plate bearer') have a shell that is divided into eight dorsal plates. This makes them immediately distinguishable from just about any other animal.
Chitons live from shallow water to the deep sea, but the majority of species live in the intertidal. This is a high-energy habit characterized by the bashing of waves as the tide rises and falls twice daily. Any organism living here must be able to hang on for dear life or risk being swept away to certain death. Chitons are certainly well equipped to survive in this habitat. They have a low profile, offering minimal resistance to the waves. Rather than stand tall and face the brunt of the wave energy, chitons cling tightly to the rocks and let the waves wash over them.
The chiton's shell, divided into eight articulating plates, gives the animal a much more flexible shell than is found in any other mollusc. This allows them to conform to the topography of the rocks, giving them an even lower profile than, say, a limpet of the same overall shape and size.
While most chitons are pretty sedentary, at least during the low tides when we can see them, some of them can move pretty quickly when they want. So what, exactly, motivates a chiton to run? One species, Stenoplax heathiana, lives on the underside of rocks in the intertidal; it comes out at night to forage on algal films and retreats back under its rock with the dawn. I've seen them at Pistachio Beach, where I turned over rocks and watched them run away from the light. This video is shot in real-time; the chitons are really running fast!
When the eight shell plates are visible it's easy to identify a chiton as a chiton. But not all chitons are quite so obliging with their most chiton-ish characteristic, and one is downright misleading.
Below is Katharina tunicata, one of the largest chitons on our coast. Its shell plates are barely visible, as they are almost entirely covered by the animal's mantle, the layer of tissue that covers the visceral mass and encloses an open space called the mantle cavity in which the gills are located. In chitons, the mantle extends onto the dorsal side of the animal and is called the girdle. Katharina's girdle is smooth and feels like wet leather.
The largest chiton in the world is the gumboot chiton, Cryptochiton stelleri, and it lives on our coast. This beast is about the size of a football, reaching a length of 30 cm or so. It lives mostly in subtidal kelp forests, but can be found in the very low intertidal, which is where I usually see it. At first encounter it's hard to figure out what this animal is. It certainly doesn't look like a chiton.
If anything, it looks like a mostly deflated football, doesn't it? Turning it over to look at the underside doesn't help much, either, although this photo does give an idea of how big the animal can get:
Cryptochiton goes one beyond Katharina and covers its plates entirely. Just looking at the animal you'd have no idea that there are eight plates underneath the tough reddish-brown mantle, but you can feel them if you run your finger along the midline of the dorsum. Living subtidally as it does, Cryptochiton doesn't have the ability to cling tightly to rocks that its intertidal relatives do, and it tends to get washed off its substrate and cast onto the beach during storms. I've never seen one on the beach that wasn't very dead. Once a friend and I were trudging back up the beach after working a low tide, and encountered a dead softball-sized Cryptochiton. I mentioned that it would be nice to have a complete set of shell plates from one of these animals. My friend always carries a knife in her pocket, so we started an impromptu dissection right there on the beach. It didn't take long to learn that the mantle of a gumboot chiton is really tough and difficult to cut through with a pocket knife. And even once we got through the mantle, dissecting the plates from the underlying tissue wasn't going to happen with the tools we had with us. Besides, the stench was godawful even with our unusual tolerance for the smell of dead sea things. We abandoned that corpse.
Many beachcombers have found white butterfly-shaped objects in the sand, but not known what they are. They are definitely calcareous and feel like bone, but what kind of animal makes a bone shaped like this? Turns out this object is one of the shell plates from C. stelleri. They wash up frequently, never attached to their neighbors so they provide no clue as to what organism they came from.
In order to obtain a complete set of Cryptochiton plates, I'd have to start with an intact chiton corpse. I did happen upon another dead Cryptochiton on a beach somewhere I was allowed to collect organisms, and I brought it back to the marine lab. I remember spending a smelly afternoon cutting the plates out of the corpse and removing as much of the tissue as I could, then feeding the plates to various hungry anemones to take care of the rest. Some of the plates got a little broken during the extraction process, but I do have my very own full set!
Some day I will figure out a way to mount those plates permanently.
One final question to ponder. Does a chiton have one shell, or eight shells?
Animal associations can be strange and fascinating things. We're used to thinking about inter-specific relationships that are either demonstrably good or bad. Bees and flowering plants--good. Mosquitos on their vertebrate hosts--bad. In many cases the 'goodness' or 'badness' of these associations is pretty clear. However, there are cases of intimate relationships between animals of different species that cannot be easily categorized as good or bad.
Take, for example, the barnacles on the skin of gray and humpback whales. From the barnacles' perspective the skin of a whale isn't a bad place to live: as the whale swims through the water the barnacle is continually flushed by clean water, which should make feeding easier. But is the whale affected in any way by its barnacle passengers? I suppose they might increase the drag coefficient a little bit and make swimming marginally less efficient, and maybe they itch, although it's hard to imagine that the whale would really care much one way or the other.
A week ago I went to the intertidal up at Pigeon Point. It's a great spot for certain animals, especially the small six-rayed stars of the genus Leptasterias. These stars rarely get larger than 8 cm in diameter and always have six arms. I've been told by a friend who just happens to be a sea star taxonomist at the Smithsonian, that making species identifications in the field is very difficult for this genus, so I've stopped trying. I do know that some of the Leptasterias stars have slender rays and others have thicker rays.
The most common large star at Pigeon Point is the bat star, Patiria miniata. These stars get about as big as my outstretched hand, and come in a variety of colors. Last week I didn't see very many Patiria, but all of them were reddish orange, like this one:
Unless they're so abundant as to be annoying, I like picking up bat stars and looking at their underside. That's because sometimes they have these little dark squiggles in their ambulacral groove:
That little squiggle is a polychaete worm, Oxydromus pugettensis. It is one of many polychaete worms that forms a symbiotic relationship with another animal species. Some symbiotic polychaetes live in the tubes of other worms, or within the shells of bivalves, for example. Oxydromus crawls around inside the ambulacral groove of Patiria, where it feeds on scraps of leftover food from the star's meals. The worms don't like light, and as soon as I picked up this star and flipped it over the worm started burrowing down between the star's tube feet to get back to the dark. The next day I found another star with a worm and was able to take a picture of it before it disappeared.
Oxydromus pugettensis is clearly segmented, evidence of its annelidan roots. It doesn't look very different from many other free-crawling polychaetes. A member of the family Hesionidae, it lives in fine silty sediments in the intertidal as well as in the ambulacral grooves of sea stars. According to one source, it is the most common intertidal member of its family along the California and Oregon coast. For reasons as yet undetermined, P. miniata seems to be the favored host, although I have also seen the worms in the ambulacral grooves of the leather star Dermasterias imbricata.
Over two days at Pigeon Point last week I examined a total of five bat stars, and all of them had worms. One of the stars had three worms! It's possible that more worms were hiding deep within the ambulacral grooves, too. I always wonder how, in this type of association, the partners manage to find each other. How does one "lucky" star end up with three worms? Do the worms every migrate from one star to another? Does the star do anything to attract the worms? In what way(s) would the star benefit from having a few worms in its ambulacral regions? It does seem that the worms don't stick around very long once a star is brought into the lab--I don't know if they die or just leave on their own--but since they also live in the sand maybe they do actively migrate between stars. There hasn't been much work done on these worms in recent decades, probably because of the overall decline in natural history studies. However, I'll keep this worm in mind for my Marine Invertebrate Zoology students this fall, when one of them asks me for help coming up with an idea for his or her independent research project.
This past Monday I did something rare for me: I returned to the same intertidal site I had visited the previous day. I enjoyed myself so much the first time that I wasn't able to refuse an invitation to go out there again. The site, Pigeon Point, is one of my favorites, especially in all of its spring glory as it is now. It has always been a hotspot especially for macroalgal diversity, and so far this year appears to be living up to its reputation. The day before I collected several reds that I got to spend the next two days trying to identify.
On Monday I was less overwhelmed by obsessed with algae and able to focus more on the animals, and was delighted to find a small cluster of Thylacodes squamigerus, the strange and fascinating vermetid snail. Nearby one of the vermetid snails was a yellow nudibranch (Doriopsilla albopunctata) and one of the common turban snails (Tegula funebralis). The chance proximity of three different gastropods brought to mind the incredible diversity of this group of molluscs.
The Gastropoda are the largest group within the phylum Mollusca, and can claim a fossil record that dates back to the early Cambrian, some 540 million years ago. They have been extremely successful throughout that long time and are the only molluscan group to have established lineages in both freshwater and on land (of the other molluscs, only the bivalves have made it into freshwater, with the remaining groups restricted to the sea). As you might expect, this evolutionary history has given rise to a mind-boggling array of body types and lifestyles. Let's investigate this diversity by taking a closer look at the three gastropods in the photo above.
Gastropod #1 (Thylacodes squamigerus): Very few people, on seeing this animal for the first time, would guess that it's a snail. Most would say that it's a serpulid worm. The tube is calcareous, as it is for serpulid worms, and winds around over rocks in the intertidal.
A close look at the opening of the tube, however, reveals snail-like rather than worm-like features. Thylacodes even has a snail's face, although I'll admit it isn't easy to see if you don't know to look for it. And despite crawling under a ledge with my camera, I didn't get the best view of a face. In this photo, however, you can at least see one of the cephalic tentacles:
Living in a tube cemented onto a rock means that Thylacodes can't go out and find food. It must instead catch food and bring it in. Thylacodes does so by spinning threads of sticky mucus that are splayed out into the water, where they capture plankton and suspended detritus. The threads are then reeled in and everything--mucus and food--is eaten by the snail. Thylacodes tends to occur in groups, and individuals within an aggregation contribute threads to a communal feeding net, which presumably can catch more food than the sum total of all the snails' individual efforts.
Pretty unexpected for a snail, isn't it?
Gastropod #2 (Tegula funebralis): The black turban snail is probably one of the most common and commonly overlooked animals in the intertidal. People don't see them because these snails are, literally, everywhere from the high- down into the mid-intertidal. They are routinely stepped over as visitors rush to the lower intertidal, and ignored again as these same visitors leave the seashore. I love them. I keep them in the lab as portable lawnmowers for the seawater tables. They are incredibly efficient grazers, keeping the algal growth down. Plus, I think they're cute!
If there's such thing as a 'typical' marine snail, T. funebralis may very well be it. This little snail exemplifies several of the traits we use to define the Gastropoda: it lives in a coiled shell, it uses a radula for scraping algal film off rocks (yum!) and is torted. The shell is easy enough to understand, as everyone has seen a snail at some point, even if it was a terrestrial snail. The radula and torsion, however, may take a little explaining.
Many molluscs have a radula, a file-like ribbon of teeth that can be stuck out of the mouth and used for feeding. In gastropods the radula can be a scraping organ (as in Tegula and other herbivores such as limpets), a drill (as in the predatory moon snails, which drill holes into unsuspecting clams and then slurp out their soft gooey bodies), or a poison dart (as in the venomous cone snails). The radula of a grazer such as Tegula bears many transverse rows of sharp teeth, which are regularly replaced in a conveyor belt fashion as they are worn down. This assures that the teeth being used are always nice and sharp. Remember the radula marks made by the owl limpet (Lottia gigantea)?
Those zig-zaggy marks are made by the scraping of the radula as the limpet crawls over her farm. Tegula funebralis makes the same type of pattern in my seawater tables. All of that white territory is area that had been scraped clean of algae in about a day. Tegula is a very industrious little snail! And they're not shy, either. I don't have to wait a day or so for them to get acclimated when I bring the back to the lab. I can move them around from table to table and after a few seconds they poke their heads out and start cruising around. I've learned from watching them over the years that they seem to have an entrained response to the rising and falling of the tides, even after I bring them into the lab. For the first few weeks of captivity, every morning when I first get to the lab I find that several Tegula have climbed up the walls. I think they're crawling up when the tide is high. I really should look at that more carefully. They never go too far, but sometimes they do drop onto the floor and I find them by stepping on them. Fortunately they are hardy creatures and the floor is always wet with seawater so as long as I find them within a day and plunk them back into the table they're fine.
Now on to torsion. Torsion is difficult to explain, but let me try. The word 'torsion' refers to the twisting of the nerve cord and some internal organs that occurs during larval development of gastropods. Here's how it works. Imagine a closed loop, like a long piece of string with the ends tied together. Lay the loop down on a table and it is just a simple loop. Pick up one end of the loop, twist it counterclockwise 180°, and lay it down again. Now you have a figure-8, right? That's not exactly what happens in the living snail, but you get the picture.
Tegula and other snails have an elongated body that is coiled and crammed to fit inside the shell. If you could take Tegula's body and stretch it out without breaking it (impossible to do, BTW), you'd see the figure-8 configuration of the nerve cord. Other internal organs are re-arranged by torsion, too. As a result, both the gill(s) and the anus now open into the mantle cavity which has been relocated over the head. This arrangement is ideal for keeping the gill(s) irrigated, but not so good for hygienic reasons. Fortunately, the mantle cavity itself is angled so that water flows through it in a more-or-less unidirectional manner, passing over the gill before the anus. Tegula and other marine snails undergo torsion while in the larval stage, and remain torted as adults. This is not the case in other gastropods, as we'll see next.
Gastropod #3 (Doriopsilla albopunctata): Everybody loves the nudibranchs, because their brilliant colors make them easy to love. Unlike the oft-undetected Thylacodes squamigerus and the ignored Tegula funebralis, many of the nudibranchs are somewhat easy to spot in the field because of their flamboyance. This is a crappy picture, but you get the point.
Doriopsilla albopunctata is one of several species of yellow dorid nudibranchs lumped together under the common name 'sea lemon'. Instead of the long fingerlike processes (cerata) that adorn the backs of the aeolid nudibranchs such as Hermissenda spp., the dorids have smooth or papillated backs that may be decorated with rings or spots. Dorids also have a set of branchial plumes on the posterior end of the dorsum; the number and color of these gills can often be used to distinguish similar species. Doriopsilla albopunctata has a smooth yellow back with little white spots, hence the species epithet (L: 'albopunctata' = 'white pointed'), and white branchial plumes.
Nudibranchs are gastropods, although in a different group from Thylacodes and Tegula. The marine slugs, of which the nudibranchs are the most commonly encountered, are in a group called the Opisthobranchia, whose name means 'gill on back' and refers equally to the cerata of aeolids and the branchial plume of dorids. In fact, these animals lack the typical molluscan gill that the snails have. They do have a radula, however, and crawl around on a single foot exactly like Tegula does.
An adult nudibranch's body is elongated, unlike the coiled body of Tegula, and has no apparent signs of having undergone torsion. However, examination of larval nudibranchs shows that they do undergo torsion just like any other respectable gastropod. The weird thing is that some time during the transition from pelagic larva to benthic juvenile they de-tort, or untwist their innards so that their internal anatomy matches their external shape. Instead of having to poop on their own heads, nudibranchs have an anus that is sensibly located at the rear (no pun intended) of the body.
Torsion is one of those biological curiosities whose evolutionary origin is shrouded in mystery. How did such anatomical contortions evolve? Why do gastropods, and only gastropods, undergo torsion? And why do some gastropods tort as larvae, only to detort as they become adults? There are scientific hypotheses about the benefits of torsion, particularly to the larval stages, but nobody knows for sure. After all, none of use were there to watch when it happened.
This is just a tiny taste of the diversity of the Gastropoda. I think it's cool to see three such different gastropods in a small spot of the intertidal. And no doubt there were more that I didn't see. That's one of the joys of working in the intertidal: that I so often see things I wasn't even trying to find.
When it comes to the natural world, I have always found myself drawn to things that are unfamiliar and strange. I think that's why I gravitated towards the marine invertebrates: they are the animals most unlike us in just about every way imaginable. Even so, some of them have bodies at least that are recognizable as being both: (1) alive; and (2) animal-ish. Think, for example, of a lobster and a snail. Each has a head and the familiar bilateral symmetry that we have. Obviously they are animals, right? I, of course, am most fascinated not by these easy-to-understand (not really, but you know what I mean) animals, but to the cnidarians and the echinoderms. And for different reasons. The cnidarians astound me because they combine morphological simplicity with life cycle complexities that boggle the mind. I hope to write about that some day. Today's post is about my other favorite phylum, the Echinodermata.
For years now I've been spawning sea urchins, to study their larval development and demonstrate to students how this type of work is done. I have a pretty good idea of what to expect in urchin larvae and can claim a decent track record of raising them through metamorphosis successfully. Urchins are easy. To contrast, I have much less experience working with sea stars. I have found that some species are easy to work with, while others are much more problematic. Bat stars (Patiria miniata), for instance, are easy to spawn and raise through larval development into post-larval life. Ochre stars (Pisaster ochraceus), on the other hand, go through larval development beautifully, but then all die as juveniles because nobody has figured out what to feed them. I've already chronicled my and Scott's attempts in 2015 to raise juvenile ochre stars in a series of posts starting here.
Sea urchins and sea stars have long been model organisms for the study of embryonic development in animals, for a few reasons. First, many species of both kinds of animals are broadcast spawners, which in nature would simply throw their gametes out into the water. This means that development occurs outside the mother's body, so biologists can raise the larvae in the lab and observe what happens. Second, spawning can be induced by subjecting the parents to nonlethal chemical or environment stresses. Third, the larvae themselves are often quite happy to grow in jars and eat what we feed them. Fourth, the larvae of the planktotrophic species are often beautifully transparent, allowing the observer to see details of internal anatomy. Lucky me, I've been able to do this several times. And it never gets old.
All that said, there are differences between urchins and stars that force the biologist to treat them differently if we want them to spawn. For the species I work with, spawning occurs after I inject a certain magic juice into the animals' central body cavity--urchins get a simple salt solution (KCl, or potassium chloride) and stars get a more complex molecule (1-MA, or 1-methyladenine). The fact that you can't use the same magic juice for urchins and stars reflects a fundamental difference in gametogenesis and spawning in these groups of animals.
Sea urchins will spawn only if they have fully developed gametes. In other words, gametogenesis must be complete before gametes can be released to the outside. You can inject as much KCl into a sea urchin as you want, but if it's the wrong time of year or the urchin doesn't have mature gonads (due to poor food conditions, perhaps), it won't spawn. I've never investigated the mechanism by which KCl induces spawning in ripe urchins, but here's what I think happens.
When students dissect animals in my invertebrate zoology class, we use magnesium chloride (MgCl2) to narcotize the animals first. A 7.5% solution of this simple salt is remarkably effective at putting many animals gently to sleep, especially molluscs and echinoderms. Placing the animals in a bowl of MgCl2 and seawater causes them to relax and gradually become unresponsive. A longer bath in the MgCl2 puts them to sleep for good.
Given the relaxation effects of MgCl2 on urchins, I suspect that injecting a solution of KCl into the body cavity relaxes the sphincter muscles surrounding the gonopores. This relaxation opens the gonopore, and if the gonads are ripe the mature gametes are released to the outside. As I said above, I don't know for certain if this is how it works, but the hypothesis makes sense to me. It also explains why that I can shoot up a dozen urchins and get none of them to spawn: the KCl might be doing what it normally does (i.e., opening the gonopores) but if the gonads aren't ripe there are no gametes to be released.
For completely different reasons, injecting a star with KCl does absolutely nothing at all except probably make the animal a bit uncomfortable. The KCl may very well open gonopores as it does in urchins, but a star will never have mature gametes, especially eggs, to release in response to this muscle relaxant. This is because at least in female stars, meiosis (the process that produces haploid gametes) isn't complete until the eggs have been spawned to the outside. What, then, is the magic juice used to induce spawning in stars, and what exactly does it do?
The magic juice is 1-methyladenine, a molecule related to the nucleobase adenine, most commonly known as one of the four bases that make up DNA. The nomenclature indicates that the difference between the two molecules is the addition of a methyl group (--CH3) to the #1 position on an adenine molecule:
Chemistry aside, what I'm interested in is the action of 1-MA on the eggs of sea stars. Meiosis, the process that produces gametes, has two divisions called Meiosis I and II. Meiosis I starts with a diploid cell (i.e., containing two sets of chromosomes) and produces two diploid daughter cells; these daughter cells may not be genetically identical to each other because of recombination events such as crossing over. It isn't until Meiosis II, the so-called reduction division, that the ploidy number is halved, so each daughter cell is now haploid (i.e., containing a single set of chromosomes) and can take part in a fertilization event. In a nutshell, the end products of meiosis are haploid cells, all of which ultimately result from a single diploid parent cell.
In female sea urchins, the entire meiotic process is completed before the eggs are spawned, which is why the relaxation effects KCl can induce spawning.
In females of many other animal species, meiosis is arrested for some period of time after the Meiosis I division. For example, this happens in humans: baby girls are born with all of the eggs they will ever produce, maintained in a state of suspended animation after Meiosis I. It isn't until puberty that eggs begin to complete meiosis, one egg becoming mature and being ovulated approximately monthly for the rest of the woman's reproductive life. Sea stars are sort of like this, with the notable exception that a female star will ripen and produce thousands of eggs in any spawning event rather than doling them out one at a time.
One of the really cool things about working with sea star embryology is that I get to see the completion of meiosis after the eggs have been spawned. I know that the gonads have to reach a certain level of ripeness before 1-MA will induce spawning. Reviewing my notes from a course I took in comparative invertebrate embryology when I was in graduate school, I came across the mention of 'polar bodies,' tiny blobs that I remember seeing in just-fertilized sea star eggs but which I have never seen in sea urchin embryos. Then I needed to remind myself what polar bodies are all about.
Remember how there are two cell divisions in meiosis? Well, despite what's shown in the diagram above, each of the divisions is asymmetrical. In other words, each division of meiosis produces one big cell and one tiny cell. The tiny cells are the polar bodies. They are too small to either divide or be fertilized, and generally die on their own. Here's a chronology of what happens. First, a cell divides, producing a large cell and a tiny polar body:
I've x'd out the polar body in red because it cannot divide or be fertilized and will soon die. Then the large cell divides to produce the final egg and a second polar body:
It turns out that in sea stars things get even more complicated. 1-MA acts as a maturation-inducing substance in these animals, effectively jump-starting the eggs that have been sitting around in an arrested state after undergoing Meiosis I. This initiates the continued maturation of the eggs to the stage when they can be spawned. Even now, though, meiosis doesn't complete until an egg has been fertilized, at which point the second polar body is produced. The production of that second polar body is the signal that Meiosis II has occurred, and the now-fertilized egg can begin its embryonic development.
Here's a freshly fertilized egg of Pisaster ochraceus, with the two polar bodies smushed into the narrow perivitelline space between the surface of the zygote and the fertilization envelope:
Sea urchins, remember, do not have polar bodies when I spawn them. That's because meiosis is complete by the time the eggs can be spawned, so the polar bodies have already died or been resorbed by the final mature egg. The photo of the P. ochraceus zygotes was taken within a few minutes of fertilization. Let's contrast that with a photo of a brand new urchin zygote:
See? No polar bodies!
All of this is to explain why we can't use the same magic juice to spawn both urchins and stars. Kinda cool when the madness in our method has a biological context, isn't it?