Strandline - secrets of the seashore
Amongst the seaweed and debris on the strandline, various animal eggs and egg cases may be washed ashore. These provide clues to how various marine animals protect their eggs to help increase the chance of their young surviving. These include the leathery egg cases of rays, dogfish and whelks. Scientists can study these beached egg cases to help discover which coastlines different sea creatures live and breed on. While many of the cases are empty, some may still have developing animals inside, especially if they have been washed up after a storm.
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.
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.
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.
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.
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