Life Photo Meme: Secret

What is the secret behind the demise of this coral? I don't know for sure, but I can make a pretty good guess based on the picture. Can you?

It looks like this coral has been the victim of that famous corallivore, the crown of thorns starfish (Acanthaster sp.). You can see part of the culprit in the center bottom of the picture. What looks like a patch of grey, red-tipped spines is actually the starfish. The crown of thorns is infamous for eating corals. They crawl on top and digest the fleshy parts, leaving the stripped white skeleton of the coral behind. Their spines are also a painful irritant to humans.

The crown of thorns is distributed throughout the indo-pacific, and has been thought to consist of single species, Acanthaster planci. However recent molecular evidence suggests that there are actually four species of Acanthaster [1].

This coral-eating starfish can devastate reefs when they appear in high numbers, but there is hope from a coral eating starfish.

Kingdom: Animalia

Phylum: Echinodermata

Class: Asteroidea

Order: Spinulosida

Family: Acanthasteridae

Photohunter: water

This was a hard choice, since most of the photos I have have to deal with water in some way or another. I chose this photograph of the Great Barrier Reef because I thought it was beautiful and amazingly intricate. You can see so many different types of corals, as well as fish and even some clams. There are over 1200 species of hard corals known and the differences in morphology often translates to different growth rates. Branching corals tend to grow the fastest at 2 to 4 cm a year (but tend to break really easily), while dense, globular corals (brain corals) grow the slowest at 0.06 to 1.2 cm a year. Of course the growth rate depends on nutrient availability, temperature, light, and pH of the water (low pHs can dissolve coral skeletons).

Most people have heard of coral bleaching, the process where corals reject their algal symbiont and turn white. What few people know is that generally this is a natural process. If the water gets so hot that the algae living inside of the coral cannot function, the coral will reject that species of algae and try to obtain a species which does work in the warmer temperature. The issue comes in when the coral cannot get a higher temperature resistant algae or the water temperature rises well beyond what any coral-helping algae can handle. Then the corals die.

Research: Questions for colonial reproduction (corals)

Some of the talks I go to which are most interesting to me, are those which make me wonder what if. These researchers were looking at the reproductive output of corals, to see if there was a difference in egg size among the different sizes of corals (small, medium, and large) or morphology (plate and branching). They also examined how reproductive output changed over time.

They found that there was no difference in egg size due to colony morphology or size, but smaller colonies were less likely to spawn a second or third time. They did find that chlorophyll concentration of the eggs increased with increasing size of the colony. This may have been due to the fact that larger colonies were deeper down, so packaged their eggs with more zooxanthellae than the smaller, shallow water colonies.

They also found that eggs sizes within the bundles varied, which interests me because I work on maternal provisioning in a colonial animal too. I find that larvae released by my bryozoans can have up to a 2-fold difference. I am most curious to know how much those eggs varied, since most researchers ignore within brood variability. They also found that the egg sizes varied among spawning events. Generally there was a decrease in the size of the eggs on subsequent spawning events, but a slight increase in the number.

This raises some interesting questions. It would be interesting to find out if the same amount of energy is expended for each of the broods (that is does the increase in number balance the fact that smaller eggs are made). Are these smaller eggs as fit as larger eggs? Are parent colonies more willing to take a chance by producing smaller eggs, since they are assured some reproductive success with the earlier large egg brood?

Finally, if would be fun to know if this down shift in egg size (energy into eggs) is accompanied by an up shift in sperm production. Since the eggs and sperm are packaged in the same bundle, it may be relatively interesting to quantify the egg/sperm ratio. It would also be interesting to see if that ratio is different among the different sizes of colonies. It's generally easier (energetically) to be a male, so would smaller colonies increase their reproductive success by packaging extra sperm?

Original abstract:

EXPLORING CORAL REPRODUCTION IN THE FIELD: DO SIZE AND MORPHOLOGY INFLUENCE THE REPRODUCTIVE OUTPUT OF THE HERMATYPIC CORAL MONTIPORA CAPITATA (SPAWNER)?

Padilla-Gamino, J.L.*, and R. Gates Hawaii Institute of Marine Biology

Modular organisms such as corals grow by adding polyps (or individual modules). This growth is not indefinite however, and eventually colony size will be limited by extrinsic (i.e. nutrient availability, microenvironment within the colony) or intrinsic (i.e. senescence, changes in physiology) factors. Although individual coral polyps grow to full size, polyps do not start producing gametes until the whole coral colony has reached a particular size. While there have been several studies analyzing the size at which corals become sexually reproductive, very few studies have focused on the reproductive ecology of the larger colony size classes, mostly due to the difficulty in transporting huge colonies to aquaria or collecting of the gametes in the field. To better understand the relationships between size, morphology and reproductive capacity, this study examined the reproductive output (gametes) in situ of the hermaphrodite coral Montipora capitata. As this coral grows, the morphological complexity of the colony also increases. This coral is highly morphological plastic in response to environmental factors. For example in areas with lower light levels, these species acquires a more flat-shape morphology than in areas with more light (branching morphology). Gametes from different environments were collected in situ during most of the reproductive season (June, July & August). Regardless of differences in morphology and environment, colonies spawned simultaneously and had similar offspring characteristics (egg size, # eggs/bundle).