Raw Prawn

King Prawns were on the table today!  We spent some time observing their external features.  

Prawns are of the Arthropod phylum, which are animals that have jointed segmented bodies.  The sub group is Crustacean, which have an exoskeleton of chitin and calcium carbonate, and are generally aquatic.  The class is Malacostraca, which typically have five head segments, eight thoracic segments and six abdominal segments.  The head has a pair of compound eyes, two pairs of antennae for detection of vibrations and odours, and compound mouthparts.  Some of the appendages are specialised for food gathering.  The thorax has swimming and walking legs, and the abdomen has uropods that act like paddles.

We removed the exoskeleton from the head and dissected out the eyes which are on stalks. Interestingly, the stalks are responsible for the production of the hormone that causes moulting.  When the prawn outgrows it hard exoskeleton, it takes on a lot of water to create a gap between the exoskeleton and its soft tissues.  The exoskeleton is shed as a new one takes it place.

Orange Gorse Fungus

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One of the jelly fungus found on gorse. Probably Tremella mesenterica.  The name mesenterica refers to the intestines, so it is probably so called because it is convoluted like them.  However, the amount of water it takes up varies considerably with the climate and season and it may take on a variety of forms.  Another of its names is Yellow Brain Fungus.

This specimen was photographed near to Capel Garmon in Snowdonia

This is a flowering example of Gorse. It is a woody, thorny shrub with a profusion of bright yellow flowers that dominates some landscapes where it may be the climax community.  Often seen at the coast.

Mobbed by mallards

 Mallards are our most ubiquitous duck and due to the popularity of 'feeding the ducks' at the local pond, they have adapted their behaviour.  No sooner was I spotted approaching the pond, that a great frenzy of activity erupted in the duck population; those nearby immediately climbing out of the water and hurryingly waddling (if you can hurry and waddle) in my direction; others quacking furiously; some flying in and landing with a big splash. Presumably they thought I had food. ( I didn't. Please don't feed the ducks! Our processed food is not good for them, and food left around encourages pests like rats)

As they came near I was able to observe their features closely.  Whilst most male mallards were fairly typical with an iridescent greenish head, a white and black tail with a curl, and a bright blue/purple speculum (in case you are wondering, that is the bar or flash of colour in the secondary feathers that the birds use to signal their species to each other), I also saw this brown variant.  I say variant, because it is the size and shape of a mallard, but not the usual colour. Their are many domestic variants of mallards.  Some do breed with wild types and show hybrid features.

Jaws

You can tell a surprising amount from teeth.  Bones and teeth the most highly mineralised in the body.  They are connective tissues, which are tissues in which the living cells are suspended in a matrix of protein fibres and impregnated with inorganic minerals.  In bone the living cells are osteoclasts and the inorganic mineral is calcium.  The purpose of this is to make the tissue hard and durable, which is why bones, and teeth, remain long after a body has decomposed.

The dentition is informative about the diet and therefore a diagnostic aid to identification.
Broadly speaking, the animals can be divided into herbivores, carnivores and omnivores.  This classification is reflected in the type and number of teeth.

Herbivores have a teeth that are adapted for cutting plant material and then chewing it for long periods to break down the cell walls.  They have sharp incisors which act like scissors, and then a number of broad flat molars that grind in a side to side or round and round motion (chewing the cud).

Carnivores have large, sharp canines, for puncturing and tearing meat, and their jaw opens up and down to provide great force, but does not go side to side to chew.

Omnivores have small incisors, small canines and some molars, which is an adaptation to their varied diet.

We can deduce by the number of molars that this animal was a herbivore and most likely a sheep. (Also because it was found in the Welsh hills!)

Eye Eye, Dissection Club!

Sheep eyes are disappointingly small, unlike their testicles which we dissected last week.  If you can get hold of cow eyes or bull eyes, they are much more rewarding.  

We trimmed the muscle and fat off initially.   Then inspected the outside of the eye; the clear front (cornea), the white (sclera) and the optic nerve, which is like a nodule sticking out of the back.

The sclera of the eye is surprisingly tough. We really had to use our scissors with a lot of force to cut through this connective tissue.

We were able to remove the dark circle of tissue which is the iris and behind this was the jelly like lens. The humours (liquid) oozed out and became quite inky and messy.

Finally we turned the eyeball inside out to observe the shiny reflective tapetum lucidum, which looked like a pool of black oil with rainbows.

Dissection Club: Sheep Testicles

We dissected sheep testicles today. They were lovely specimens and an interesting tissue to dissect, being so dense.  

We first looked at the outer appearance and noted the shiny outer testis.  Exiting at the upper end is the epididymis head. The tail of the epididymis leading to the vas deferens or sperm tube is pressed against the ball sac all the way down then up again to the top.  Here we have cut through the thin membrane that attaches it to the testis and released it. We can see the associated blood vessels.

We cut into the testis and removed the outer tunica like a bag.  It was a very shiny, smooth and tough tissue with no elasticity.

Finally we cut into the testicular tissue.  When we divided it we could see the lobes or septate nature of the seminiferous tubules.

Caesarian Sections influencing Evolution

Birth weight is always the classic example of Stabilising Selection in Evolutionary Biology.  Basically, humans have difficulty giving birth; there is a trade off between large human heads and small fused pelvises required for bipedalism and human babies get stuck in the birth canal.  

Selection operates on survival of the fittest, or death of not fit.  Those babies that are too small may be born easily but are so underdeveloped they are unlikely to survive. Those babies that are too big may be well developed but they get stuck during birth, and are unlikely to survive either by being starved of oxygen or fatally haemorrhaging their mother.

So the perfect size is in the middle and selection makes sure that these individuals survive, reproduce and put their genes into the next generation.

This is an interesting article that shows that interventions in birth, ie Caesarian sections, is causing directional selection, not stabilising, and birth weight is increasing.

http://www.bbc.co.uk/news/science-environment-38210837