Worm Sense

I've been asked a couple of times whether Beans and Bunny know that I'm writing about them and their never ending outdoor questions. The answer is - yes, they do and they have been quite involved in the whole production since the beginning. They've now started blatantly suggesting topics for me. In fact, Bunny started her most recent question by saying "Hey Mum! I've got a blog post for you...". It's a good question too - how do worms know where they are going if they don't have eyes? And although she ended up basically answering her own question - guessing "vibrations" when she stopped to think about it a bit - there was still quite a bit to look up and discuss.

I know I've already written about worms, but she's rather fond of worms so we talk about them a lot. We also pick them up, talk to them, pet them and rescue them. We do this with potato bugs too (and they had breakfast with us once) but that's a whole other story.

The following picture should take some of us back to Grade 11 science class (or was it grade 10? or 12? It was a long time ago in the olden days of plaid and army boots) where we dissected formaldehyde soaked earthworms. I think this was one of my first inklings of my future career choices as I thoroughly enjoyed taking apart the worms and in fact did several, helping out my more squeamish classmates. I ended up taking several physiology classes in university where I took apart a variety of dead vertebrates and invertebrates and my first biology job was in an avian energetics lab where I dissected birds for 8 hours a day. If anyone needs a bird separated into all it's individual parts - I'm your girl!

But this isn't about how cool the insides of animals are (and I fear I may be making myself sound rather macabre) this is about worms. So here's a worm's insides:

Betcha didn't know worms were so complicated!
http://www.sas.upenn.edu/~rlenet/EarthwormAnatomy1.jpg

So worms have a lot of things! Notice anything they don't have? Eyes, ears and nose. That's 3 of the 5 senses missing - so they have to get by on touch and taste.

But they do have a mouth there and over there mouth is a lobe (prostomium - not in that picture) that closes it up but also has receptors to sense what's going on around it. This is what they use to feel their way around the soil. They also have a nerve cord running down their body with a larger part near the mouth, which acts as a "brain" (cerebral and subpharyngeal ganglia). This controls not only movement and digestion but also coordinates the senses in the body - they have touch, light, vibration and chemical receptors along their entire body - so Bunny was right about the vibrations. These receptors allow them to sense changes in acidity, humidity, touch and chemical composition - equivalent to our sense of smell and taste, even though there's no nose or tongue. Very key in determining the difference between "food" and "not food". Imagine if you had no sight, taste or smell - you'd have to just go around randomly eating things hoping some of it was actually food, which could be rather disastrous.

I think the coolest of all though is light sensitivity - there are light sensitive receptor cells on the body, so it does indeed know if it's underground or not. They can tell the intensity of the light and instinctively move away from strong light. The receptors sense if they are getting close to the surface and how strong the light is to help them determine if it's safe to go up (rain, night) or not (sunny day) which makes worms and vampires rather similar in their outdoor preferences, although I don't know if vampires travel in the rain so perhaps worms have an advantage.

Extra cool: these receptors are being studied in nematodes (roundworm), and they may be the precursor to the vertebrate rod and cone system of vision, which would be amazing because then this molecular similarity is in species over 540 million years ago.

Which means my eyes would be related to an ancient nematode. Awesome.


How we learned about worms:
http://cronodon.com/BioTech/Earthworm_NS.html
http://animal.discovery.com/worms/earthworm-info.htm
http://www.naturewatch.ca/english/wormwatch/resources/anatomy.html
http://www.naturewatch.ca/english/wormwatch/about/anatomy.html
http://faculty.washington.edu/chudler/invert.html

Eggnatomy

Normally the basis of these blog posts come from questions stemming from something we have seen or heard. This one came while we were eating, so we're now using a bunch of senses to figure things out and I answer questions about our dinner. Beans and Bunny love sushi. LOVE. Bunny can plow through salmon sashimi like nobody's business! But what Beans is really partial to is "fishy balls". I know what you are thinking - "but fish don't have..." I'll stop you there, what she really means is salmon roe sushi (Ikura if you happen to want to order it). A pile of squooshy fish eggs on rice, which amazingly she eats one by one with chopsticks.

Beans' coveted "fishy balls"

We've already talked a bit about eggs previously in reference to Beans' class hatching chicks in an incubator and their general breakfast and baking capabilities. But why aren't fish eggs aren't like chicken eggs? Where is the shell? AM I EATING THE SHELL?

Eggs are a pretty common thing in the world of reproducing things. Everything from insects to mammals pretty much uses an egg in some form. I mean there's a few other ways of reproducing, but eggs are by far the most common.

EGGSTRAVAGANZA!

In placental mammals (like us) they stay internal, which you can tell because well, we don't lay eggs. There's some fish and reptiles that have live young as well, and then there's a whole in between kind of internal egg but not placental thing we won't get into because we aren't doing reproductive strategies. So, for the most part, in birds, insects, fish, reptiles, amphibians, monotremes, molluscs.... you get the idea... they are external. They lay eggs, which then either hatch into offspring or get eaten, with or without rice and nori.

The key parts to have in an egg are protection (some kind of shell) and a food source for the growing chick (yolk). And obviously some parental genetic material if your goal is to get some sort of creature out of it. The eggs we eat are for the most part unfertilized, so although the female genetic material is in there it never develops which is thankfully why there are not partially developed chicks in your omelette.

There are three main kinds of eggs. Don't be scared by their titles - just think of it in this way: lecithal means "has a yolk" and the prefixes are just the size - small, medium and large. Like at Starbucks where they also name them arbitrary things and then look at you condescendingly because you can't for the life of you remember which one means medium and you really just want your soy chai latte.

Microlecithal
Small eggs with a small yolk. Usually laid in really high numbers as a reproductive strategy. This type of egg is used by things like clams, oysters, mussels, worms, sea stars, crabs, insects, butterflies... you get the idea. These usually hatch as a larva and finish developing outside the egg. 

Mesolecithal
Medium yolk. This means there can be a longer development period and some animals hatch fully formed instead of as a larva like hagfish and snails. Some still go through that extra stage though - like salamanders and lampreys. 

Macrolecithal
Eggs with a large yolk. The eggs are usually fewer in number and have enough food to get them through development. This is the one that vertebrates use, and also octopus, 'cause they are awesome like that. These are what we think of as eggs and are laid by birds, fish and reptiles. 

Shark Eggs. That's right. SHARKS. Even their eggs are cool.

In placental mammals (like us) there is essentially no yolk and there is basically a naked egg cell. We can get away with this because nourishment is provided by the mother throughout development.

So, if birds and fish are both macrolecithal then how are they so different?

All birds have the same egg structure - shell, some membranes, albumen (egg white), a yolk and some cool twisty bits (chalazae) that hold everything in place. The "blastodisc" (also called a germinal disc) is the part that would, if fertilized, start making a bird - and not some sort of nerf weapon, although it totally sounds like one. Every bird does theirs in their own special way - they come in a myriad of shapes, sizes, colours and textures. Some are even water-resistant!

Egg parts!

Fish eggs have generally the same parts, only the egg "shell" is a different substance and more of a membrane, which can be fairly tough depending on species. Fish eggs are soft until they are laid and then harden when they hit the water, but they never develop a calcium based shell like a bird. So yes, technically we are eating the shell of salmon eggs.

Amphibian eggs work much the same as fish - not surprisingly as they are both usually aquatic (although there are a few land-egg amphibians out there). The lack of a hardened shell means they all have to stay wet - either by water, a gelatinous goo or both. 

http://courses.bio.indiana.edu/L104-Bonner/Sp10/imagesSp10/L6/fishNfrog.jpg

Frogspawn

Reptiles are similar in structure to birds - they too have a shell, it's just usually leathery. Shells like this that form before laying are found in species that internally fertilize (otherwise fertilization would be really hard). The super cool thing about some reptile species (turtles and crocodiles mostly) is that the sex of the young is temperature dependent. Like in turtles - the colder eggs mean more males, females dominate for warm eggs.

I don't want to hatch, I'm cozy!

And no egg discussion would be complete without the awesomest of all egg layers - monotremes. I really should have put egg pictures but I can't resist a platypus face.

Platypus!
http://animals.nationalgeographic.com/animals/mammals/platypus/

Shortbeak Echidna
from Fir0002/Flagstaffotos

Not only are they wacky (and I mean that in the nicest way possible) they are the only mammals in the world to lay eggs. And of course they are in that mecca of all wacky animals - Australia. They do things a little differently though, the egg develops internally first and then outside for less time.  In comparison - a Ring-billed Gull will have her egg for a day before laying, then incubate on the nest for 3-4 weeks. A platypus will hold their little round white eggs internally for about 4 weeks and then incubate for 10 days. Echidnas tuck theirs in a pouch for 10 days before hatch (different than a marsupial which tucks their baby in a pouch). It's a lot like a reptile egg - soft shelled. In looking this up I learned two really important un-egg-related things though - baby Echidnas are called "puggles" and male Platypuses have poisonous hind spurs.

The only part of our egg discussion I wasn't able to answer in full was why we don't eat all the eggs and just stick to birds and fish. I'm not sure if there are people out there that eat frog or alligator eggs (I would suppose there are) but we don't. My guess would be that it has to do with the ease of harvest - catching a salmon full of roe is easy in comparison to getting an alligator egg. I know which I'd rather do!

It may be hard for an egg to turn into a bird: 
it would be a jolly sight harder for it to learn to fly while remaining an egg.

-C.S. Lewis