Reef Chemistry Question of the Day #23 Sea Urchin Spines

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #23

Today we have a two part question.

Part 1:

Sea urchin spines are composed primarily of:


A. Calcium sulfate
B. Magnesium carbonate
C. Potassium carbonate
D. Calcium carbonate
E. Calcium silicate

Part 2:

How do the sea urchins form the spine into the appropriate shape for its species?

Good luck!









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[Diamond1]

D calcium carbonate.
Not exactly sure how they are formed to an appropriate shape for each species but the spines are formed in the dermis of the urchin. I would imagine that the cells in the dermis that are responsible for the calcification process are also responsible for the size and shape of the spines.

 

[cope413]

I think it's D as well. I've heard that warm vinegar can partially dissolve the spines, so that would make sense.

I'm guessing they have a sort of 'seed' crystal at the base of their spines and the new material grows off that crystal. I thought they had tiny barbs, so I'm imagining a Mandelbrot type replication. Tiny spines on top of small spines on top of bigger spines...

 

[Randy Holmes-Farley]

Bingo, both correct answers. Yes, the spines are calcium carbonate.

Part 2 was a little vague, and the above answers are not necessarily incorrect. But the interesting point I wanted to communicate is that sea urchins produce proteins that bind to and inhibit the growth of certain CaCO3 crystal faces. So they apple these proteins around the sides of the spine and keep it growing int he direction they want.

This article section has more detail:

Sea Urchin Spines: Structure

While sea urchin spines have a wide range of appearances, they also have some commonality. All, for example, are largely made of calcium carbonate, as are coral skeletons. Interestingly, recent studies have shown some sea urchin spines to be large single crystals of calcite,1 while others are mosaics of many individual crystals. For example, by using X-ray reflection, the spines of Heterocentrotus trigonarius have been shown to consist of 10-cm long single crystals of magnesium-rich calcite. Calcite is a particular crystalline form of calcium carbonate in which the calcium and carbonate ions are arranged in a slightly different pattern than in the other predominant form of calcium carbonate found in marine organisms: aragonite. Corals use aragonite for their skeletons, but urchins have found calcite to be more suitable for their spines. The magnesium-rich part of the description implies that a significant portion of the calcium ions have been swapped for magnesium ions, and the exact amount of magnesium present is discussed later in this article.

In all Heterocentrotus trigonarius spines, the calcite crystal is aligned in a particular direction in relation to the spine's axis. In other words, the arrangement of calcium and carbonate ions was perfectly aligned with respect to the length of the spine, leaving no gaps along which the spine might easily crack.

The spine itself, however, is not a solid crystal, like a chunk of calcite that might reside in a rock collection. Rather, it is very porous, as seen by scanning and transmission electron microscopy (SEM, and TEM, respectively). These pores consist of two types: a continuous assortment of macropores that are easily seen by SEM, and a series of many small (80 nm) protein inclusions that are easily seen by TEM. These proteins are, in fact, critical for the spine's initial formation, and will be discussed later in this article.


Sea Urchin Spines: Composition

As mentioned above, the crystals are magnesium-rich calcite, containing 2-25 mole percent magnesium ions (75-98 mole percent calcium) This level of magnesium is appreciably higher than in most coral skeletons (which use the aragonite crystal form of calcium carbonate), and similar to or higher than many calcareous algae (which often deposit calcite having several mole percent magnesium). The magnesium content of the spines has been shown to vary somewhat with water temperature, and has also been shown to increase by about 2 mole percent from the tip of the spine to the base.

It has been suggested that the presence of magnesium in the calcite strengthens the calcite by altering the way cracks can propagate through it. The additional magnesium near the base makes the base stronger, thus increasing the likelihood that any spine that does break will break farther from the body, causing the urchin to lose less material than if the break were nearer to the body.

When exposed to excessive heavy metals, the spines of sea urchins can incorporate these metals into their crystal structure. Lead and zinc, for example, have been shown to be incorporated into the spines of Paracentrotus lividus and Arbacia lixula when the ambient levels rise above those in normal seawater. What effect these incorporations have on the spines, for example, on their strength is not clear.

Sea Urchin Spines: Formation

A multitude of studies have examined the deposition of calcium carbonate in sea urchin larvae, and a few have studied it in adults. As might be expected from a process that results in a large, shaped single crystal, the deposition is very carefully controlled. How this happens is only poorly understood. As with calcification in corals, it is a very complicated process. It involves numerous proteins and other organic compounds. What is known of the overall process is outlined in the following sections.

Sea Urchin Spine Formation: Uptake of Calcium and Carbonate


Sea urchins can take calcium and alkalinity directly from the water column to deposit calcite to form spines. This has been demonstrated by the fact that some (e.g., Strongylocentrotus purpuratus) have been shown to be able to take calcium from solution and calcify even when kept under starvation conditions, although the calcification rate was slower than under fed conditions.

At least two species, however, have been shown to have some propensity to consume solid calcium carbonate that may be dissolved by the lower pH in their guts and released to the organism to provide the building blocks for calcification, sort of like miniature CaCO3 reactors. In these experiments, Diadema setosum and Echinometra sp. were shown to prefer artificial foods with powdered calcite added over the same foods with no calcite. Many urchins might similarly obtain calcium and carbonate ("alkalinity") from the calcareous algae and other bits of calcium carbonate that they consume. [As an interesting aside, it has been claimed that many calcareous algae deposit CaCO3 in an effort to make themselves less palatable to fish. In the case of sea urchins, this strategy appears to backfire.]

Will urchins significantly deplete calcium and alkalinity in a reef aquarium? Probably not very much relative to other calcifying organisms, but in a new aquarium in which coralline algae and corals may be calcifying slowly, this relatively low depletion rate may be an appreciable part of the total. When I first set up my reef aquarium, I used wild Florida live rock that contained quite a load of sea urchin hitchhikers. More than 30 rock-boring urchins (Echinometra lucunter) were present, and they grew very rapidly for a year or so, before dwindling. During that time, coralline algae grew poorly (even in areas not browsed by the urchins) and I had few rapidly calcifying corals. Consequently, while I did not realize it at the time, some of the limewater that I was adding may have been ending up in the urchins. As an aside, studies have shown that moderate grazing by sea urchins can increase the spread of coralline algae from the chips that are released. However, in a finite space like a reef aquarium, heavy sea urchin grazing may overwhelm the growth of coralline algae.

Sea Urchin Spine Formation: The Role of Organics

The spicules in sea urchin embryos, as well as the spines in growing adults, are deposited by primary mesenchyme cells, which surround the growing tips of the spines. These cells accumulate calcium and secrete calcium carbonate.8 Exactly how the secreted calcium and carbonate ions become shaped into spines, however, is not entirely clear.

More than a dozen different proteins have been found to be associated with calcification in sea urchins. In the completed spine, many of the proteins remain trapped inside pores in the spine. In one case, the trapped protein was estimated to comprise 0.1% of the spine's total mass.3,8 In a related experiment, some of the proteins (acidic glycoproteins) present in sea urchin calcite were extracted and added back to calcite crystals in vitro (in test tubes) to see what effect it had. These proteins turned out to adsorb onto specific crystalline facets of the calcite where they altered crystallization along those facets.9 In another experiment, proteins extracted from the spines of Paracentrotus lividus also adsorbed onto only a single crystal facet, encouraging one of the other facets to grow more. Interestingly, when magnesium was added to the mix, the proteins directed growth to occur on a different facet. In this way, the sea urchin can use a variety of organic and inorganic materials to modify growth of spines to specific sizes and shapes as it "chooses," based on the materials secreted onto the growing surfaces.

More here:

Sea Urchins: A Chemical Perspective by Randy Holmes-Farley - Reefkeeping.com