Sunday, September 23, 2012

five fabulous phyla

In “an ant rant,” I asked readers how you could tell a boy ant from a girl ant. One person responded (I’m crying inside, btw), and he answered correctly.  In a haplodiploid reproductive system, the sex-determining factor is the number of chromosome sets an individual has. Females have two sets of chromosomes and are therefore diploid. Males have only one, and so are therefore haploid. So, Roger Birkhead, this one goes out to you.

Today, we’re going to do a quick tour of five phyla of animals that receive way too little attention. Most of them are microscopic critters that are unloved either because A. go unnoticed B. lack a bilateral body plan. Let’s push through these prejudices.

a typical Rotiferan (
Rotifera: means “wheel bearer” in latin.  Usually only a fraction of a millimeter, you need a microscope or a dissecting scope to get a good look at them. They live in freshwater and can either be sessile or travel by inching along a substrate. These guys look like they are wearing little crowns, called coronas, which are rings of cilia around the top of the head that help to filter food into the mouth. The coronas can be quite elaborate. They are capable of sensing light with their little eyespots, though incapable of forming images. They play an important part in freshwater ecosystems and are part of the beds of lakes and rivers.

Velvet worm (
Onychophora: meaning “claw bearer” in latin. More commonly known as velvet worms. These soft-bodied curiosities live in tropical areas and look a lot like caterpillars, coming in a variety of eye-catching colors. They have tiny eyes and antenna, squirt slime at their prey to catch it, and give live birth. Their stubby feet are particularly interesting structures. There can be up to 40 jointless, hollow lobopods (lobed appendages) that terminate in two hardened claws that help the animal to travel treacherous surfaces. Weirdly enough, they are becoming popular in the pet trade.

Jaw worm (
Gnathostomulida: meaning “jaw mouth." More commonly known as jaw worms. They glide along the submerged sand grains in coastal areas. They are a little less than a millimeter long and are pretty much little threads with jaws at the end. They are extremely basic little animals. In fact, their most complex parts are their mouths, which resemble those of rotifers and therefore suggest that they are related phyla. Their digestive system ends in a “blind terminus”- meaning, they have no butt. Food simply enters and absorbs.

A sea cucumber (wikicommons)
Echinodermata: meaning “spiny skin.” This phylum includes sea stars, sand dollars, sea cucumbers, and sea urchins. Most follow a pentaradial body plan, meaning that they are arranged around 72° segments. Although sea cucumbers are heavily derived, they do follow a pentaradial body plan that is more easily seen in early stages of development when they are just little cucumberlets.

Rhombozoa: Known as “lozenge animals.” These are parasites that live inside of cephalopods (squid, octopi, cuttlefish, and nautiluses). They may grow to be several millimeters long and are surrounded by ciliated cells. So, they look like tentacle-covered threads that attach to the kidney cells of cephalopods, from which they receive all nourishment. Very strange.

That completes the tour of our 5 phyla. Roger, I am a little saddened that your name doesn’t contain a “p.” After all, the funniest of all the phyla starts with a “p.” So we’re going to do it as a bonus.

Priapulida:  means “little penis.” I mean, just look at them.

Have a nice day!

Saturday, September 22, 2012

i'm winging this one

I feed a Red-tailed Hawk everyday. When I first started several months ago, he wouldn’t even eat with me in the room. No matter how hungry he was, even if I put his food on his foot, he would not budge. After about four months of patience and persistence, he began to eat in front of me. Then, he took food from my hand. After another few weeks, he was standing on my hand and eating from it. To me, this was the ultimate act of trust. We had finally arrived.

But then, one day, he was standing on my glove eating and began to lose his balance. He spread his wings quickly to recover, and then looked me, expressionless, in the eye. He lowered one of his wings and rested it on my shoulder and turned back to his food. He finished his meal, one wing around my shoulder for balance the whole time. I could feel his stiff primary feathers against my neck and the heat from his light but powerful wrist. That hawk choosing to put his powerful wing around me was one of my favorite moments to date.

Over the past few months, I’ve been lucky enough to interact with raptors. I’ve spent many hours staring at them, falling in love with the eyes of a Great Horned Owl, the feet of a Bald Eagle, and the beautiful feathers of the Red-tailed Hawk. But what I’ve grown to appreciate most about birds is their wings. They need explanation to really understand.

A folded wing (
When a bird is not in flight, you see only a small portion of their wings. They have them folded up tight, exposing only the tip. But under those feathers are the forearm, an elbow, and a humerus. These words should ring a bell, since they are parts of the human arm as well. As evolution would have it, bird wings evolved from limbs adapted for terrestrial locomotion- aka- arms.

Wings are highly adapted arms that are specialized for flight- so specialized that they are used essentially only for flying. Somewhere in the transition between arm and wing, several big adaptations developed. A sheet of skin grew on the inside of the elbow joint, providing more surface for lift and limited motion between the humerus and forearm. This skin is called the patagium.

Comparison between human arm and bird wing
The wrist bones fused and elongated, forming the carpus (the portion that is exposed when a bird’s wings are folded). The bones of the hand reduced in number, as did the phalanges. They shifted to be in line with the carpus and lost almost all flexibility. And the feathers; we can’t forget about the feathers.

Flight feathers sprout posteriorally from the limb. The large primary feathers toward the tip of the wing sprout from the bird’s “hand” and help provide thrust, pushing the bird forward through the air. Secondary feathers are the large ones that sprout from the forearm. Their flat, boxier shape provide upward lift- not unlike the wing of an airplane.

Bone positions in an extended wing
To me, these things are not easy to understand just by looking at a bird sitting there. You have to extend the wing, fold it back up, extend it, and fold it back up several times to appreciate how the segments are put together and how the feathers fit into the whole picture.

Even in flight, it takes keen observation to understand. Most of what you see is feathers- the actual wing ends way before the feathers do. When a bird is flying, the outstretched forearm and humerus can appear to be one segment. But if you ever have the opportunity to feel an outstretched wing, you can trace the bones with your fingers and clearly feel that elbow joint. It’s there, disguised by feathers and that tricky patagium.

After studying a bird’s wing, it will make you want to cut the Wright Brothers a break. The complex and precise design of a bird’s wing is a tough thing to replicate for us clumsy humans. I think we’ll have to leave graceful flight up to the birds and their elegantly evolved arms.

Saturday, September 8, 2012

your lanula is showing

This one time, my toenails fell off.

My dad and I had gone on a backpacking trip and I committed the rookie mistake of not breaking in my new hiking shoes first. Also, I wore tissue-thin socks like a jerk and didn’t cut my toenails short enough. The entire first day of the hike was downhill into a valley, and by the time we set up camp, both of my big toenails were barking. By the end of the week, they began to turn blue. Flash forward a few months, and they fell off. Klink klink, right onto the floor.

Now, I’m sure you’re wondering why I shared that all-too-personal story with you. I shared it with you as an opener to today’s topic: the exciting world of keratinized structures.

Keratin is a fibrous protein, one of the three major classes of proteins. Keratin is the major protein component in hair, nails, feathers, scales, horns, and a variety of other epidermal structures found in vertebrates. Within the field of anatomy, you hear these types of structures referred to as “epidermal outgrowths.” As you can surmise, this means that they originate from the epidermal tissues and can be heavily modified for different species.

Sometimes these structures arise as a means of sexual display, like big horned sheep that butt their gigantic horns for sexual rights to females. Feathers have many functions, some of which include thermoregulation, sexual display, and flight. Hair in mammals is primarily for thermoregulation. Claws are generally for foraging and defense. Scales help to protect reptiles’ bodies like armor. How do our weird little fingernails fit in to all this?

Our nails evolved from claws. Our mammal ancestors walked on all fours and foraged with claws. As the primates began to branch off and perform dexterous tasks with their hands, claws became cumbersome. Fingernails were the evolutionary solution to this problem; they protect the supple fingertips and help to perform very tiny tasks, like removing splinters.

The nail plate is made up of several layers of dead cells, the remainders of which are almost entirely keratin. It is curved so that it covers the terminal segment of your finger in a way that protects the soft nail bed underneath from damage and impact. The lanula (the half-moon shaped thing at the base of your nail) is where live cells are that produce the nail plate.

Sometimes, as a result of traumatic impact, the nail bed is damaged so badly that it separates from the nail plate, which will eventually fall off. Luckily, that little lanula will keep on working and grow a new nail plate over the course of a few months. Trust me, I know. 

Saturday, September 1, 2012

an ant rant

Ants have a three-tiered social system. You have the queen, the male concubines, and the all-female working class. The queen has one duty and one duty only: lay eggs. The “male concubines,” or drones, are responsible for knocking up the queen. And the workers must take care of all the rest. They feed the babies, build and repair the nest, defend the colony, and wait on her majesty.

There can be multiple queens per colony, but usually just one. She is fed by the workers and lays eggs. These suckers can live for up to 20 years- longer than either the drones or workers. A queen can lay eggs from a single mating for several years. In a single lifetime, a queen can give rise to millions of ants.

The drones are the only males of a colony, and pretty much just eat and have sex. They really have the life. That’s pretty much it for them.

The worker ants are sisters- all sterile daughters of the queen. They take care of their little sisters from the time they are eggs to emerging pupae. They carefully move them from nursing chamber to nursing chamber as they go through the stages of being an egg, larvae, and pupae. They also maintain the mound by expanding, building, and maintaining the hallways by spitting on the walls for structural support. Furthermore, they do all the foraging. The sisters find food, bring it back, and organize it into a stash for the entire colony. A subset of super beefy workers- called soldiers- are in charge of defending the nest from invaders or adverse weather.

When an ant dies, her sisters will drag her little body out of the nest, as far away as possible. They do that as though they are following hospital protocol, aware of the biohazard of a degrading body or the increased possibility of disease present in that deceased ant. One experiment from a while back sought to isolate the hormone that is secreted at death, so the scientists took dead ants and rubbed them all over live ones. I bet it was pretty funny/horrible watching the unwilling test subjects being dragged out of their nest by their instinct-driven sisters.

All hymenopterans (bees, ants, and wasps) operate using a haplodiploid reproductive system. To be haploid means to have only one set of chromosomes. Diploid means to have two sets of chromosomes. You, human, are diploid, since you have two sets- one from your mama and one from your daddy. Depending on whether you’re a boy or girl, your sperm or eggs are reduced by one half so that they are haploid. That way, one haploid sperm + one haploid egg = one diploid baby.

But with ants, it’s a little different. A queen lays both fertilized and non-fertilized eggs. Unfertilized eggs develop into haploid male drones, and fertilized eggs develop into diploid female workers.

These workers share 75% of their DNA, which is clearly more than the typical 50% that human siblings share. They have their haploid drone daddy to thank for this. Since he only has one set of chromosomes to offer them, they all get the exact same set of genetic material from him. They have a 50% chance of getting either of mom’s sets of chromosomes. Add this all together, and the sister workers are on average 75% genetically similar. (Sister power! See below figure.)

The distribution of genetic material in a haplodiploid system. Each "x" represents a chromosome.

Now, you may be wondering how inbreeding is avoided, since the only males around seem to be the disturbingly genetically similar queen’s sons. But an adaptation has evolved to keep this from happening: wings. Both queens and drones are born with wings. Upon reaching maturity, drones will fly away from the nest and find a new queen to breed. Meanwhile, worker ants selectively beef up a few female larvae here and there by feeding them more nutrients… and spitting on them… to produce new queens. When a little queen is ready, she’ll fly away and shed her wings after she breed. She establishes a little nesting site and boom- a few generations later, you have a new colony. Genetic dispersal is at it again…

How would you characterize a female ant from a male ant? Their sexy bits don’t matter, since the workers are sterile and essentially androgenous. Whoever guesses the gender-determining factor in a haplodiploid system gets to choose A.) to pick the topic for the next post B.) a personalized poem written by yours truly to be featured on the blog.

Hint: I already gave you a hint in the previous sentence. Don’t be greedy.