This miscellaneous group includes the sea squirts, sponges, bryozoa and hydrozoa. Fastened to rocks or seaweeds, and often growing over each other, these animals can create an interesting patchwork of colours and textures. All these groups draw in food that is suspended in water. For that reason they do particularly well in places where there is a lot of wave action or strong currents to bring food to them.

Gooseberry sea-squirt Dendrodoa grossularia (Van Beneden)

At first glance this species may look colonial, as individuals often cluster together forming an encrusting, red jelly-like sheet. However, they are solitary and each individual has its own inhalant and exhalant siphon. Young gooseberry sea-squirts often settle on older individuals and this results in these dense patches. Like the star sea-squirt (Botryllus scholsseri), the gooseberry sea-squirt draws food in through the inhalant siphon and expels waste through the exhalant siphon.

Sea-squirts have both female and male sex organs and normally reproduce by self-fertilization. The gooseberry sea-squirt holds fertilized eggs in its body until they are released as larvae. The gooseberry sea-squirt live for up to two years.

• Habitat

  • Found on rocks, shells and seaweed on the lower shore and down to 600 m.

• Key Identification Features

  • Reddish-brown colour.

  • Round dome shape.

  • Two conspicuous siphons.

  • Solitary or in dense clusters.

  • Up to 2.5 cm in height.

Star sea squirt Botryllus schlosseri (Pallas)

Sea squirts feed on particles in the water. Water and food is drawn through an inhalant siphon and waste is expelled through an exhalant siphon. The name sea squirt comes from the jet of water these animals produce by contracting their bodies when they are disturbed.

Each individual sea squirt is part of a closely organised colony. Groups of three to twelve individuals form the characteristic star shape. Each individual has its own inhalant siphon but the groups share a common outflow opening in the centre of the star. The colonies are embedded together in a flat jelly-like layer.

• Habitat

  • Found encrusting rocks or seaweed on the lower shore and in shallow water.

• Key Identification Features

  • Colour very variable, usually a dark colour with the stars a contrasting yellow.

  • Smooth to touch.

  • Individuals arranged in star-like groups.

  • Colonies from a few millimetres to 15 cm across.

Sea oak Dynamena pumila (Linnaeus)

At first glance the sea oak appears to be a seaweed but in fact it belongs to a group of animals called the hydrozoans. These animals are related to the sea-anemones. They are colonial animals consisting of many individuals joined together by connecting branches. Some of the individuals are specialised for feeding while others are for reproduction. The feeding individuals look like small sea-anemones and have tentacles surrounding a mouth that leads to a sac-like body. Feeding individuals are contained in minute cup-like structures that need to be observed using a hand lens. Pear-shaped structures may also be apparent. These are the individuals specialised for reproduction.

Like most hydrozoans the sea-oak has two forms in its life cycle. The other form is jellyfish-like in appearance. These latter forms bud off from the reproductive individuals and in the case of the sea-oak remain attached to the colony. Sperm and eggs are released from these individuals. Free swimming larvae eventually settle on the seabed to form a new colony.

The sea oak is found on shores all around Britain, often growing on wracks. Colonies found on the lower shore have been observed to have more branches.

• Habitat

  • Found on the middle and lower shore attached to rocks and wracks.

• Key Identification Features

  • Pale brown colour.

  • May be sparsely branched and only about 1 cm in height or profusely branched and up to 5cm in height.

  • Tiny cups containing the feeding individuals are found in opposite pairs on the stem and branches.

Breadcrumb sponge Halichondria panicea (Pallas)

These animals have amazing powers of regeneration. In a classic experiment a sponge was forced through a fine mesh net. After a short while, the sponge cells were observed to aggregate to form many new sponges. As well as regenerating from fragments sponges also reproduce sexually. Sponges release sperm into the sea which is then drawn into other sponges. Fertilized eggs then develop into larvae which are released to disperse in the currents before settling on the seabed.

The breadcrumb sponge is probably the sponge most likely to be observed on the shore. However, both its colour and shape are variable. If the sponge is exposed to light it is likely to be green but if it is in the shade it is yellow. The green colouration is due to the presence of algae within the sponge’s tissue. The algae may gain protection and obtain nutrients and carbon dioxide from the sponge. The sponge, meanwhile, is likely to benefit from the production of organic compounds by the algae. This algae like other plants is unable to survive without light and is therefore not present in sponges found in the shade. The shape of the sponge depends on the strength of the current. In exposed areas the sponge is much thicker than in sheltered areas. This is likely to be due to the enhanced flow of water through the sponge in exposed areas. The breadcrumb sponge may live for about three years.

• Habitat

  • Found in gullies, under rocks and among seaweed holdfasts on the lower shore.

• Key Identification Features

  • Very variable in shape. Encrusting.

  • Usually olive-green colour but also can be yellow.

  • Smooth surface with large holes.

  • Up to 2 cm thick.

Sea mat Membranipora membranacea (Linnaeus)

The sea mat belongs to a group called the Bryozoa. Bryozoa are small colonial animals. Each individual is called a zooid and is normally no more than 1 mm in length. The zooids are usually contained within box-like structures. bryozoans feed on bacteria and single-celled plants. Food is caught by a circle of tentacles surrounding the mouth.

A sea mat colony will contain both males and females and cross fertilization occurs. Sperm released by one zooid enters neighbouring zooids. The larvae develop in the sea for several weeks. After finding a suitable surface, the larvae change into a zooid to form new colonies. Sea mat is often found on the fronds of seaweed. The colony always grows towards the base of the seaweed so it is not lost as the fronds of the seaweed erode. The colony may live for several years.

• Habitat

  • Found on the low shore on the fronds of oarweed (Laminaria digitata) and serrated wrack (Fucus serratus).

• Key Identification Features

  • Whitish colour.

  • Lace-like appearance.

  • Colony is made up of rectangular compartments containing zooids.

  • Each zooid is about 0.4 mm long and 0.13 mm wide.