Bioblitz Battle Plymouth (May 2025)
17 May
2025
1:45pm to 4:15pm
March 4, 2025
Wrack are a family of common seaweeds found on many UK coasts. The name ‘seaweed’ is rather deceiving as the wracks are actually a type of algae. There are three main types of seaweed; red (Rhodophyta), brown (Phaeophyta) and green (Chlorophyta), wracks are brown seaweed and are branched and have ribbon-like fronds. Seaweeds can have similar structures to terrestrial plants with a thallus or frond (the entire body), stipe (the stem), lamina or blade (leaves) and a holdfast (anchor to the seafloor, like roots on a plant). Some wrack species have gas bladders or pneumatocysts which help them float closer to the surface so they can photosynthesize better. There are several different kinds of wrack: bladderwrack, egg wrack, spiral wrack, serrated wrack, channel wrack, and rainbow wrac
Bladderwrack (Fucus vesiculosus) is one of the most common wracks found on the shore and are found on all rocky shores along the UK mainly on the middle littoral to lower intertidal zone. They are olive-brown and the thallus can be 90cm long and 2.5cm wide! This wrack has gas bladders to keep it near the surface for gas exchange. This seaweed reproduces by releasing its gametes (reproductive cell) into the water column and are fertilized externally. Did you know that the bladderwrack was the first source of iodine?
Egg wrack (Ascophyllum nodosum) is found on sheltered shores along the mid-shore zone. They are more yellow-brown in colour with long tough and leathery fronds with egg-shaped air bladders regularly spaced along the frond. They can get to 2 meters in length and are very slow-growing – only 0.5% per day, but they can live up to 15 years. Did you know that this is the only species in the genus Ascophyllum?
Spiral wrack (Fucus spiralis) is found on the upper littoral zone underneath the high water mark on rocky shores and so is resistant to desiccation. They can be 30cm in length and have flattened blades with a mid-rib and has a spirally twisted frond, which gives the seaweed its name. These do not have any gas bladders but their reproductive bodies can be found in pairs at the end of the blade which are round in shape and can look like grapes. Did you know that the spiral wrack is the only species in the Fucus genus that is a hermaphrodite, meaning each plant has both male and female reproductive bodies!
Serrated wrack (Fucus serratus) or toothed wrack is found on rocky shores and grows best when they are slow draining, easily found on the lower shore around the low water mark. The holdfast can be 180cm long with flat fronds and blades, the frond can get up to 1m in length. You can easily identify this wrack from others by its midrib and serrated edges, this seaweed has no gas bladders. The serrated wrack reproduces by swimming sex cells that are fertilized externally. Did you know that F. serratus is used in Ireland and France for cosmetics and thalassotherapy – which is a type of therapy that uses seawater!
Channel (Pelvetia canaliculate) is another common wrack seaweed and is found on upper rocky shores around the high water mark in sheltered shores, which means it is tolerant of desiccation (drying out) and can lose up to 90% of its water. This is a much smaller species of wrack as they grow to a maximum of 15cm where their fronds are channeled on one of the sides. These channels, plus a special mucus layer, is what helps this seaweed from drying out and just like the spiral wrack, the channel wrack is a hermaphrodite. Did you know that if this seaweed spends more than 6 hours submerged in water they start to die? This is because they can completely rehydrate in 25 minutes!
Rainbow wrack (Cystoseira tamariscifolia) is a beautiful species to find submerged when rockpooling. It is more common in southwest Britain on the lower rocky shore but it can also be found on gravelly flats. This wrack can get up to 60cm in length with cylindrical fronds. They tend to look bushy, with lots of small branchlets. When on land, this seaweed has dark brown colourations, however, when in the water it has blue-green iridescence. These iridescent tips are used by the Rainbow wrack to control how much light it receives – they reflect the light back which gives it its fantastic colour.
