Wracks

 

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Wracks are members of the fucus family. They can be found on any rocky shore in one form or another. They are tough plants and can colonise the most exposed of shores. However medium exposed shores tend to have more wrack species, on which every species will colonise their own niche on the shoreline. This makes them good biological indicators as depending on where they are found one can zone the shoreline.

 

Physiology of Wracks

The wrack plant can be divided into two parts the holdfast and stipe (or frond). The holdfast is the method of fixing the seaweed to the rock face. The stipe flattens and forms ribbon like fronds. The plant grows dictomously that is every time it subdivides into two equal branches. What the wracks are famous for are their bladders to provide buoyancy and bring lift the fronds up so that they are in sunlight. However not all species possess bladders.

 

Reproduction of Wracks

Wracks have both and female reproductive bodies as they reproduce sexually. The reproductive bodies are located in small cavities called conceptacles. Conceptacles depending on the species are found on the end of fronds in receptacles or on the fronds themselves. The conceptacles have a "goose bump" appearance.

In spring the reproductive bodies rupture, both male and female cells are spewed into the sea. The male cells antherozoids have two flagella enabling them to swim. The males are attracted to the females (oospheres), and will swim to them. Fertilisation takes place, and the process of cell division takes place until a sporeling forms. The sporeling then settles on a rock face.

 

 

 

Conceptacles on Fronds

 

Conceptacles on Receptacles

 

 

 

Conceptacles on Fronds

 

Conceptacles on Receptacles

 

Zoning the Shore Using Wrack Species

This zoning only remains true for moderately exposed rocky shores.

Upper Eulittoral Zone

Whatever grows here has to be able to withstand long periods of drying out as it is only covered by the tides for a short time. The two species that fit this niche are Channelled Wrack, Pelvetia canaliculata, and the Spiral Wrack, Fucus spiralis. The channelled wrack colonises higher up the shore than the spiral wrack. The channelled wrack will completely dry out on hot summer days appearing dead but springs back when wetted by the tide.

Mid-Eulittoral Zone

Bladder wrack Fucus vesiculosus is the dominant seaweed in this zone. Knotted wrack Ascophyllum nodosum may be found in this zone and can form extensive beds. The proportion of both species sharing this zone depends on the degree of exposure, the more exposed the beach the higher the proportion of Bladder-wrack.  Both of these seaweeds may be replaced where there is a flow of fresh water by the brackish tolerant horned wrack Fucus ceranoides.

On exposed shores the bladder wrack can be found in the form Fucus vesiculosus linearis. This is a bladderless stunted form that is badly worn from severe wave action. The lamina may be worn away leaving only the mid rib and even when the plant does grow back it may lead to a distorted form.

Low-Eulittoral Zone

The serrated or toothed wrack, Fucus serratus, colonises the lower part of the shore.

Problems of Identification Between Species of Wrack

Where the plants are heavily exposed to the elements the dichotomous branching will not be seen. The fronds wear away and when they grow back they can produce asymmetrical branching.

Interbreeding between the species produce hybrid forms which are difficult to identify unless an expert.

On heavily exposed shores species may only be there as a specific form as in the case of Bladderwrack with its bladderless variety Fucus vesiculosus linearis.

 

Species of Wracks In Cornwall

There are a wide variety of wracks, each type inhabiting different zones on the shoreline. The most distinctive forms are those with air bladders. The air bladders hold the plant up to the light when submerged. The buoyancy they provide, can lead to the lifting of the stone that the wracks are attached too during storms. Wracks are a very common sight on rocky beaches.

 

      Channelled Wrack Pelvetia canaliculata

       Spiral Wrack Fucus spiralis

      Bladderwrack Fucus vesiculosus

       Toothed Wrack Fucus serratus

     Egg Wrack Ascophyllum nodosum

       Horned Wrack Fucus ceranoides

 

 

 

Channelled Wrack Pelvetia canaliculata

This wrack grows at the top of the shore and has to be resistant to dessication. On hot summer days this seaweed dries out to a black crisp mass appearing dead. However it is rejuvenated when the tide hits it.

The fronds curl at their edges so creating a channel, hence the name. It is a bladderless wrack, the bladder like organs are receptacles.

 

Channelled Wrack Pelvetia canaliculata

 

Bladderwrack Fucus vesiculosus

Bladderwrack is the most well known of the wracks. Can be found in large beds on Cornwalls’ rocky shores. With a quick look at bladderwrack it appears that there are two types of bladder. The knobbly bladder like organs at the end of some of the fronds, are receptacles. In spring the female receptacles are covered by orange mucus and the males by green mucus.

 

 

 

Knobbly bladder like organs at the end of some of the fronds, are recaptacles.

 

Bladderwrack Fucus vesiculosus

 

Egg Wrack Ascophyllum nodosum

The name comes from the egg shaped bladders that are regularly placed along the fronds. Unlike most wracks, the frond lacks a midrib, without this strengthening, the egg wrack cannot flourish on exposed locations. During the spring small grape like receptacles appear on the seaweed.

 

 

 

Egg Wrack Ascophyllum nodosum. Small grape like receptacles

 

Egg Wrack Ascophyllum nodosum

 

 

 

Spiral Wrack Fucus spiralis   

Spiral wrack colonises high up the shore.

At first glance you may think it has bladders, but on closer observation you will find the knobbly bladder like organs at the end of the fronds, are receptacles.

 

 

 

 

Toothed Wrack Fucus serratus

This bladderless toothed wrack colonises the lower shore. It gets its name from its flat serrated fronds.

The colour of the seaweed can be various shades of brown, but a bright coppery brown is not uncommon in Cornwall.

 

Toothed Wrack Fucus serratus a bright coppery specimen.

 

 

 

Horned wrack Fucus ceranoides

A bladderless wrack which tolerates brackish water and is commonly the dominant wrack in such areas.

 

 

 

 

 

 

 

 

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