Strandline - secrets of the seashore

Skates and rays
Skates and rays also have a skeleton made of cartilage and are closely related to sharks. They are flat-bodied and have a disc-shaped body with winglike fins. When not swimming they rest on the seabed. Skates and rays have declined in number around the British coastline.

Skate and ray egg cases
These are leathery to the touch, but lack the tendrils present on dogfish egg cases. The outer surface of the egg case is usually covered with sticky filaments that help to anchor it to pebbles or other objects on the sea bed.

While the embryo is developing in the capsule, it lives off the yolk of the egg until the yolk is completely absorbed and the fish is ready to hatch (this varies in duration between different skates and rays). While inside the egg case, the wing-like fins are wrapped around the embryo's body..

Sharks and dogfish
Sharks differ from most other fish in that they have a skeleton made of cartilage rather than bone. They range from fast-swimming species like the porbeagle to sluggish bottom dwellers like the dogfish. Sharks have an amazing array of senses for locating prey, and skin teeth that protect them like a suit of armour.

The lesser spotted dogfish is the most common shark species around the UK, including Sussex. It can reach a length of 60-100 centimetres and feeds on the seabed in the shallow coastal waters.

Porbeagle shark


The porbeagle shark is a member of the Lamnidae family of sharks, which also includes the great white. They feed on a wide variety of fish including mackerel, herring, cod and also squid.

Unlike most sharks, porbeagles can retain the heat generated by their muscles and recycle it to heat their bodies, keeping their body temperature warmer than the surrounding sea and helping them to swim more efficiently.

The female porbeagle shark retains her eggs inside her body where they will develop and be 'born' fully formed. The mother produces lots of unfertilised eggs that are eaten by the developing embryos (oophagy) as food.

Dogfish eggs
Dogfish lay eggs - protected in a leathery capsule - between November and July. Eggs are laid two at a time.
Tendrils at the corner of each egg case are used to secure the egg to seaweed.
The egg capsule will contain enough food (yolk) for the baby dogfish while it grows inside the egg case, which on average takes 9 months.
By 4 months the baby dogfish is about half grown.
By the time the baby dogfish is ready to hatch it will have used up all the egg yolk and will be twice the length of the egg case.

Cuttlefish and Squid
Cuttlefish, octopus and squid belong to the scientific group Cephalopods, which means "head footed". It's hard to believe that cephalopods are molluscs and therefore invertebrates (animals without backbones) related to the common whelk.

Newly hatched cuttlefish
Newly hatched cuttlefish (1cm) will often change colour to match the eggs they have hatched from. After a while they will move off and change colour to match the surrounding seabed.

Cuttlefish are able to change colour for camouflage and squirt out ink to confuse predators. They have 8 tentacles covered in suckers, and two longer tentacles with suckers on the tips which they shoot out to catch prey.

Cuttlefish eggs
Cuttlefish eggs can be found on the strandline. They are often mistaken for seaweed air bladders and resemble a black bunch of grapes. The black colour comes from the cuttlefish's defensive ink. These eggs are often still alive. If you find them on the beach, put them in a tide pool or back in the sea.

Common whelks
Common whelks are gastropod molluscs, as they only have one part to their shell. These whelks are carnivorous, feeding on marine worms and other molluscs such as bivalves (which have two halves to their shell). Whelks can use the edge of their own shell to prize open bivalve shells.

Whelks also feed on dead animals, which they locate by smell using their siphon. They wave the siphon back and forth to detect where the smell of food is coming from.

Whelks lay a mass of egg capsules which are often mistaken for sponges. Large masses of egg capsules are the result of several whelks laying their eggs together. Each capsule contains hundreds of eggs, but only a few are fertilised. When these hatch, the baby whelks feed on the unfertilised eggs.

Squid



Squid are similar to cuttlefish, only more streamlined and faster swimmers. Cuttlefish and squid swim using both rippling movements of their fins and jet propulsion by squirting water out of their siphons.

Squid eggs look like the tentacle remains of a dead jellyfish. When they hatch, squid are as tiny as grains of rice.

image copyright of Judith Oakley, oakleynaturalimages.com