Lugworm or sandworm - Arenicola marina (Linnaeus, 1758) - Κόκκινο σκουλήκι - Cyprus

Description
The presence of this lugworm can be detected by the characteristic signs of one of its U or J-shaped burrows; depressions are formed at the head-end, and a cast of coiled defecated sediment is present at the tail-end . This segmented worm has a cylindrical body, which has two distinct regions; the thoracic region bears bristles (known as 'chaetae'), and the last 13 segments also have bushy gills . The abdominal region (the tail end), which is thinner than the thoracic region, lacks gills and bristles. The colour of this worm varies greatly; it may be pink, red, brown ...
The lugworm or sandworm (Arenicola marina) is a large marine worm of the phylum Annelida. Its coiled castings are a familiar sight on a beach at low tide but the animal itself is rarely seen except by those who, from curiosity or to use as fishing bait, dig the worm out of the sand.
From http://www.arkive.org/lugworm/arenicola-marina/image-A6872.html

When fully grown, the lugworm of the coasts of Europe is up to 9 in (23 cm) long and 0.375 in (1 cm) in diameter. Other species on the North American coast range from 3 to 12 in (7.6 to 30.5 cm). The body is like that of an earthworm: ringed or segmented. Its head end, which is blackish-red and bears no tentacles or bristles, passes into a fatter middle part which is red. This in turn passes into a thinner yellowish-red tail end. The middle part has bristles along its sides and also pairs of feathery gills. There is a well-developed system of blood vessels with red blood rich in the oxygen-carrying pigment, haemoglobin

A lugworm lives in a U-shaped burrow in sand. The U is made of an L-shaped gallery lined with mucus, from the toe of which a vertical unlined shaft runs up to the surface. This is a head shaft. At the surface the head shaft is marked by a small saucer-shaped depression. The tail shaft, 2 to 3 in (5.1 to 7.6 cm) from it, is marked by a highly coiled cast of sand. The lugworm lies in this burrow with its head at the base of the head shaft, swallowing sand from time to time. This makes the columns of sand drop slightly, so there is a periodic sinking of the sand in the saucer-shaped depression. When it first digs its burrow the lugworm softens the sand in its head shaft by pushing its head up into it with a piston action. After that the sand is kept loose by a current of water driven through the burrow from the hind end by the waves of contraction passing along the worm's body. It weighs 2 to 5 oz (57 to 142 g). Lugworms also have hairs on the outside of their bodies that act as external gills. These can rapidly increase its uptake of oxygen.

Once it burrows into the sand a lugworm seldom leaves it. It can stay there for weeks on end, sometimes changing its position slightly in the sand. But it may leave the burrow completely and re-enter the sand, making a fresh burrow for breeding but for 2 days in early October there is a genital[clarification needed] crisis. This is when all the lugworms liberate their ova and sperms into the water above, and there the ova are fertilized. The ova are enclosed in tongue-shaped masses of jelly about 8 inches long, 3 inches wide and 1 inch thick. Each mass is anchored at one end. The larvae hatching from the eggs feed on the jelly and eventually break out when they have grown to a dozen segments and are beginning to look like their parents. They burrow into the sand, usually higher up the beach than the adults, and gradually move down the beach as they get older.
From https://en.wikipedia.org/wiki/Lugworm

Underwater photos at Akrotiri  depth 12 meters 26/11/2016 by Kostas Aristeidou