Saturday, June 28, 2008

Photohunter: Bright

So this was kind of a mistake photo. I was trying to get the mammoth and the building in the background, and did not take into account the light. But I like the effect.

This was taken at the La Brea Tar Pits, a truly interesting place. It is one of the most productive (in terms of numbers of ice age fossils found) tar pits in the world. It is only a few minutes drive from downtown LA, and still very much in the center of the city. The museum and associated pits is only about a block and a half wide. The park still has very active tarpits, with new tar bubbling up on the grass all the time. (Visitors are advised to stick to the sidewalks)

The pits have unearthed over 1500 dire wolves, as well as, sabertooth cats, american lions, mammoths, mastodons, ground sloths, camels, horses, short face bears, and very larger condor-like birds (to name a few). If you ever go to a museum and see a sabertooth cat or dire wolf skeleton that is very dark brown, chances are it came from La Brea (the color comes from the tar staining it). The pits are so filled, because unlike most tar pits, they were near a major body of fresh water. Migrating herds would lose members in the tar, and then the predators (like the wolves) would try to eat them and get stuck as well. This in turn attracted more predators/scavengers. Inside the museum there is a huge chunk of asphalt, with tons of bones embedded in it, taken directly from outside. It's amazing.

Thursday, June 26, 2008

Life photo meme: staghorn ferns

Kingdom: Plantae
Subkingdom: Tracheobionta (vascular plants)
Class: Filicopsida
Order: Polypodiales
Family: Polypodiaceae (common ferns)
Genus: Platycerium (staghorn ferns)
I'm not super sure on the taxonomy here, my normally trusty ITIS website has completely failed on finding the common name staghorn or elkhorn ferns. So this taxonomy is a combination from the ITIS and the University of Florida botanical website. There are at least four different species of staghorn ferns native to Australia (where the picture was taken), and several more native to the Americas, Southeast Asia, Africa, and Indonesia.

Staghorn ferns are my favorite fern. They live as epiphytes on trees, by creating these flat, plate-like frond which help sick the plant on the tree. The bifurcating fronds are the ones which produce spores which then blow onto the trunks of other trees. Apparently, the gametophyte (which develops from the spore) is fertilized on the tree trunk from other gametophytes present. How this happens, I don't know. But I guess that when the tree trunk gets wet from ran or dew, the sperm are free to move about the trunk to find another gametophyte. Anyway, the young fern grows some of those plate-like fronds out of the gametophyte to anchor itself onto the tree. The cup shape of the plant captures water and leaves, which decays providing the staghorn with nutrients.

Sunday, June 22, 2008

Two layers or three?

Most creatures in the animal kingdom have three cell layers; endoderm, ectoderm, and mesoderm. Mesoderm is particularly important in increasing complexity, as many of our internal organs are derived from this layer. So when did this layer arise?

To look at this question many researchers have turned to cnidarians (jellyfish & anemones), which only have two layers, to examine the developmental and genetic clues. To make things confusing, some cnidarians posses a third layer, called a entocodon, where some muscle cells reside. Also, all ctenophores (comb jellies) also posses muscle cells. Muscle cells are generally thought to have arisen from mesoderm. So was the ancestor to cnidarians, ctenophores, and bilaterians (everybody else) triploblastic (having three layers), and the cnidarians and ctenophores just lost that layer? Or was the ancestor diploblastic (having two layers) and muscle cells arose separately in all groups?

Looking at some of the genes that are commonly associated with mesoderm, Martendale et al. (2004) found that a majority of these genes are present in his model anemone, and generally tend to be expressed in the endoderm. This means that the tools for creating mesoderm was present in cnidarians, and that most likely, mesoderm arose from endoderm at some later date. However, finding the genes in the endoderm does not completely rule out the possiblity that cnidarians had mesoderm, but that it was lost at a later date.

Burton (2008) reviewed the two possibilities, diploblastic or secondarily diploblastic through mesoderm loss, and makes several excellent points based on numerous papers. First, the third tissue in some cnidarians (entocodon), is not the same genetically or developmentally as mesoderm. The entocodon arises from the ectoderm at a much later developmental time (after gastrulation) than mesoderm. Plus, genes associated with mesoderm are not always expressed in the entocodon, they are more likely to be expressed in the endoderm. So the entocodon is most likely a new cell layer and not a modified mesoderm layer.

Muscle cells found in ctenophores and cnidarians are not the same as those found in bilaterians or even to each other, however the genes are similar. Therefore, it is likely that the genes for muscles were found in the ancestors to ctenophores, cnidarians, and bilaterians and each group slightly modified those genes to get their present shape. In triploblasts, these genes along with others became associated with the mesoderm cell layer, when those cells migrated from the endoderm. Interestingly, in cnidarians the genes associated with mesoderm in bilaterians appear to be used in body patterning. So essentially, cnidarians have two sets of body patterning genes, one of was free to develop into mesoderm and subsequently, internal organs. Even cooler, (I think) ctenophore lack one set of body patterning genes, the Hox genes. Did they lose it? Or did they never have it and we are more closely allied with cnidarians? Do sponges and placazoans have Hox genes? I guess it's time for more reading!

Burton, P.M. 2008. Insights from diploblasts; the evolution of mesoderm and muscle. Journal of Experimental Zoology Part B-Molecular and Developmental Evolution. 310B:5-14

Martindale, M.Q., K. Pang, and J.K. Finnerty. 2004. Investigating the origins of triploblasty: 'mesodermal' gene expression in a diploblastic animal, the sea anemone Nematostella vectensis (phylum, Cnidaria; class, Anthozoa). Development. 131:2463-2474

Saturday, June 21, 2008

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.

Thursday, June 19, 2008

Life photo meme: Ancient

Kingdom: Animalia

Phylum: Cnidaria

Class: Scyphozoa

This is a fossil impression of a jellyfish. Considered to be one of the first fossilized evidence of multicellular organisms and still going strong today. Just last year they discovered a jellyfish fossil that was 500 mya old, pushing back the origins of jellyfish by over 200 mya (link here). The impressions were incredibly similar to jellies found today, implying that the origins of jellies may be earlier.

In my opinion, cnidarians are interesting in their simplicity. They only posses two tissue layers, lacking the mesoderm layer which provides the cellular base for many internal organs that other organisms posses. There is some interesting research going on involving cnidarians trying to determine how mesoderm could have arisen from endoderm or ectoderm (I'm in the middle of reading a paper on it now, and promise to write it up later).

Saturday, June 14, 2008

Photohunter: emotion

Another non-biological one, but I really like the intensity on her face. This is a photo from a powwow held at my school. She is a contestant in the ladies fancy shawl dance. I don't know if she won, because I was too far from the judges to tell, but I would not be surprised if she did.

Thursday, June 12, 2008

Life photo meme: Fern's lifecycle

Kingdom: Plantae

Phylum: Pterophyta

The above are sori, found on the underside of a 'true' fern frond. In each of the sori are spores which are haploid (have only half of the chromosomes as the adult stage). These spores are released into puddles of water, where they create a structure called a prothallus. This is a heart shaped structure, which is only a cell or two thick and fairly small. The prothallus produces either eggs, sperm, or sometimes both (depending on the species). The sperm have to swim to a different prothallus to fertilize an egg, as the eggs are retained there. The fertilized egg is what becomes the fern, and it grows out of the prothallus.

So the thin tissue that the small leaves are growing out of is the prothallus, and the leaves are the young fern. Later leaves will look more fern-like (shape depends on the species) and grow in the characteristic fiddle-head shape.

Saturday, June 7, 2008

Photo hunter: Bad 'hare'

This is not really a hare, but I couldn't resist the pun. What's bad about it? Well, somebody thought it would be nice to release a couple of their rabbits on campus grounds. Couple that with the aww isn't that cute, and some rabbit feeders, and now the grounds look like this....

Each of the lumps in the distance is a rabbit, and all of the green spaces on campus look like this. While it may seem quite cute, I have to wonder about the diseases of such a population (breeding from a limited stock), if they've pushed the 'natural' fauna off campus (more natural than the rabbits anyway), and the irresponsibility of the original owner in abandoning their pets.

Monday, June 2, 2008

Animalpedia: Scallop (life photo meme)

Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Osteroida

This is most likely a rock scallop (Crassadoma gigantea), although it is unusual to find them lying loose like this at this size. They are free living until about 2 inches, when they attach to the substratum [1]. They then cement their right valve to something and they grow to fit the contours of what they attached to.

You can see the concentric rings of the gills on the inside, which they use to filter feed, and the small black dots on the outer ring of the orange mantle are the eyes. Of all of the molluscs, scallops have the most developed eye structure [2]. Most other molluscs posses pigment cups or eye spots. The scallop's eye is composed of a lens, two retinas (the distal retina, which faces towards the light, and the proximal retina, which faces away from the light source), and a reflective tapetum layer, which often gives scallop eyes a blue sheen. These eyes are most often used to detect changes in light levels, allowing free living scallops to escape when a potential predator passes.