If there is one organism on the shore that will drive you mad it is algae. You think you know a seaweed when you see one, it’s the smelly stuff washed up on the strand-line or the slippery bits on the rocks. What about that black stain on a rock or the red one, perhaps the green is something?
Seaweeds are basically split into three groups, red, brown and green. The theory has the greens high up the shore the browns in the middle and the reds lower down. Nice and neat, this makes identification easy surely. The reality is that seaweeds haven’t read the book.
We will start with the basics, seaweeds are algae, just the same as the green stuff in your pond. There it basically ends, the three groups are not even closely related to each other. Each has a different strategy of food production through photosynthesis with the use of accessory pigments in addition to chlorophyll. It is these pigments that determine the groups.
Seaweeds are variable in their ability to withstand being out of the water, variable in their ability to absorb sections of the light spectrum and variable in the degree of salinity they can cope with. Variable is the keyword. They change physical form in exposed areas compared with sheltered shores, in different stages of the life cycle and seasonally. An example is Grape pip weed (Mastocarpus stellatus) it starts as a petrocelis form, a glossy black paint like patch on a rock. The adult plant is a red /black tough recognisable seaweed shape. The difference is so extreme that it was thought to be a different species. Slightly higher up the shore, another black stain is visible, this, however, is not glossy and it is slightly textured. What kind of seaweed could it be? It isn’t it is a lichen (Black tar lichen). The red/pink patch in the rock pool is a seaweed, the green one is another lichen. The wiry tuft on the rock is another lichen (Lichina pygmaea). If it was reddish and slightly softer it would be a creeping chain weed, a seaweed.
So this becomes slightly confusing, let’s go to the big smelly weeds on the strand-line, everyone knows bladderwrack, it is brown and has the bladders that you pop as a kid. Easy to identify, but not if you have a wave-beaten shore, it doesn’t have bladders. These are used to keep it buoyant in the water and so optimises the chance of gathering sunlight. Being buoyant in heavy waves means you are torn off the rocks, it has evolved into a different form simply to cope with differing environmental conditions. This altered form looks very much like another species of wrack.
Let’s try and pick an easy one, it looks like limp lettuce, mostly in rock pools and is very much green, Sea lettuce, good guess, but which one of the at least thirteen species is it? A PhD and a very good microscope might help, but even the experts struggle with weeds, they keep changing the names as they realise they have got the families wrong. What hope do you and I have of ever getting a correct identification?
Seaweeds will drive you mad but once you see a stringy bit of weed under a microscope you will be hooked.