December - Mearl Beds

Our habitat highlight for this month is the wonderful MEARL BEDS!

Mearl beds is a relatively unknown marine habitat, yet their important role as a habitat can’t be understated. For Scotland especially, maerl beds are listed as a priority marine feature and are protected in 11 different locations!


So, what is maerl? Mearl is the overarching term used to describe a group of red coralline algae belonging to the Class Rhodophyta with a pink-purple colouring. Maerl has a distinct three-dimensional limestone skeleton formation which uniquely combines together on the seabed to crate what’s known as ‘maerl beds’. This skeleton is formed from the deposition of lime into maerl cell walls as it continues to grow, forming this hard yet brittle exterior. In Scotland specifically we have two common types of maerl species; Phymatolithon calcareum and Lithothamnion glaciale.


Across the west coast and sea lochs of Scotland, maerl tends to form on top of deep gravel formed from dead white maerl. Maerl beds are highly significant ecologically and are often considered one of the most biodiverse habitats in the world. The beds provide a vital habitat for a large variety of species such as sea anemones (our species of the month!), bivalves and scallops. Scallops specifically utilise maerl as a nursery habitat when young, emphasising the important role the habitat provides for many species. Maerl beds also act as a key blue carbon store; a vital role.


Regarded as a natural heritage feature of Scotland, mearl is undeniably one of the important natural marine features the country has. However, the distribution of maerl beds is not unique to Scotland, with maerl in fact being found in coastal waters all over the globe in intertidal as well as deep subtidal waters.

Maerl beds are under threat like many other critical habitats due to climate change. A naturally slow growing species, maerl is often unable to respond quickly to environmental changes such as rising water temperatures and ocean acidification. Maerl beds are additionally highly at risk to damage from boats, dredging and anchors due to its fragile structure, emphasising the need to protect these amazing habitats.


So lets look at our species of the month for December - SEA ANEMONES!!!!

Sea anemones (Actinia equina) are invertebrates and belong to the group cnidarians. Over 70 species of sea anemones can be found in Scotland on our maerl beds as well as rocky intertidal zones. Anemones attach heir selves to hard substrate such as rocks or coral. The most common anemone that can be seen in Scotland is the beadlet anemone. The beadlet anemone can survive both in and out of water as the tide rises and falls. The anemone curls up its tentacles inside its body to prevent drying out.


Sea anemones have 192 tentacles, these long tentacles of the anemone act as arms which catch passing food such as plankton or small fish. The captured prey is stung by the tentacles by nematocyst, this paralyses the prey which then gets fed to the anemones mouth. Some sea anemones reproduce asexually by a method called budding: in which segments of the anemone split away from the main body and produce little offspring. Or by Longitudinal fission where the anemone splits down the middle and creates two new anemones. The sea anemone can have up to 100 embryos in their body at one time!


Sea you next month!

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© 2020 by Lauren Kennedy, Glasgow University Sea Society