Yesterday a student completing a swim test mentioned to me 'the sea is so salty here' - a silly statement you may think, however it got me thinking - yes, we all know the sea is salty, but exactly why is the sea salty and why can it be more salty in some places than others? Well, here is the definitive answer as explained by the BBC. That's a lot of Salt!
Most of our planet’s surface is covered in water – salt water. The oceans that support so much of Earth’s life are around 3.5% sodium chloride – 50 million billion tonnes of salt.
But where does it come from? While some of it comes from volcanic vents or rocks on the seabed, most of it is actually from the land around us. Every time it rains, tiny amounts of mineral salts are washed into rivers, which eventually flow into the sea.
The salt in rivers is less than 1/200th the amount usually found in seawater. It becomes more concentrated in the ocean, as the Sun’s heat causes water from the surface to evaporate, leaving the salt behind. Extra salt added every year from rivers is balanced by salt which returns to the sea floor.
But salinity isn’t the same everywhere. Towards the poles, water is not as salty because it’s diluted by melting ice, while the extra heat in the tropics makes water there saltier – and denser. And that can affect how nutrients flow around the oceans.
Cuttlefish are actually not fish but molluscs belonging to the order Sepiida and class Cephalopoda, which also includes squid, octopodes, and nautiluses.
Males have four pairs of legs, whilst females have three pairs. They also have two tentacles, with suckers, for catching prey.
Cuttlefish have three hearts and blue/green blood.
Cuttlefish spawn only once in a lifetime.
They have “W” shaped pupils, can’t see colour but can see forwards and backwards, with no blind spot.
Cuttlefish normally grow from 15-25cm, however the largest species (Sepia Apama) can reach 50cm.
Cuttlefish, like Octopus, release ink as a defence mechanism. This ink was formerly used as a dye, hence the name Sepia.
They live for approximately 1-2 years and inhabit mostly shallow tropical/temperate ocean waters.
The Cuttlefish ‘bone‘ is what controls their buoyancy.
Generally Cuttlefish eat fish, shrimp, crabs, worms, molluscs but will also eat other Cuttlefish. They in turn are preyed upon by fish, seabirds, sharks, seals, dolphins and of course, Cuttlefish.
Cuttlefish have one of the largest brain to body size ratios and are thought of as amongst the most intelligent invertebrates. Cuttlefish cannot see colour but have highly sophisticated eyes, which are thought to give them sight even whilst in the womb. They are thought to learn their preferred prey in this way before they are even hatched.
Despite the fact Cuttlefish are colourblind, they are able to change their colour at will to match their surroundings.
Cuttlefish ratios mean there are usually 4-5 males for every female, which is a recipe for war. Males will either fight but more commonly trick each other, for the right to mate.
The most successful tactic is cross dressing! Males disguise themselves as females, by changing their colour, concealing their extra arms and even pretending to be holding an egg sack. The master of disguise can then sneak past the guarding male and mate with the female.
The successful male grabs the female with his tentacles, turns her face to face, then inserts a specialised tentacle into an opening near her mouth. He deposits sperm sacs there, fertilising the female, who he will guard until she lays their eggs only hours later. Check out this video of the dramatic event.
The female lays a batch of about 200 eggs and covers them with ink. After two to four months, they hatch out into perfectly formed little Cuttlefish with a yolk sac, to feed them until they make their first kill.
Cuttlefish populations are not well known, however, commercial fishermen in South Australia catch up to 71 tonnes during the mating season, both for human consumption and use as bait.
Because of their short life span and spawning only once in a lifetime, the threats of over-fishing are obvious. Currently there are no management restrictions in place to limit the numbers that can be taken but there is pressure to add the giant cuttlefish to the endangered species list.
Paul. Padi Instructor.