Marine animals that can cope with desiccation and wave action begin to appear on in greater
numbers and variety on the upper shore. Animals in this zone have to be able to withstand
desiccation and most of them do this most obviously by possession of a shell.
Barnacles are shelled crustaceans (like lobsters and crabs) which filter their food from the water and
are found in this zone. On very exposed shores with more or less continuous splash and spray some
species of barnacles can survive high up in the upper shore. Limpets (Patella vulgata) and rough
periwinkles are snails that graze on the algae and lichens.
The soft bodied beadlet anemone (Actinia equina) may be found in suitable microhabitats like moist
This greater variety is also seen in the animal life; less extreme adaptations are needed to
survive the shorter periods of exposure to the air. However, the decrease in environmental
pressures mean that there are a wider group of organisms all competing for the same area and
this competition starts to become more a more important influence on the distribution of
organisms than the effects of the tidal cycle.
Rough and small periwinkles Limpets (Patella vulgata) graze on the algae and lichens.
Acorn Barnacles settle in this zone.
Barnacles are shelled crustaceans (like lobsters and crabs) which filter their food from the water and
are found towards the bottom of this zone. On very exposed shores with more or less continuous
splash and spray some species of barnacles can survive high up in the upper shore. Barnacles-
crustaceans, reproductions, larvae are planktonic, like crab and shrimp etc but settle onto rock and
grows a pointed shell of interlocking plates with a hinged mouth at the top,. When submerged, the
mouth opens and the barnacle extends 6 pairs of feathery legs to filter out passing plankton. 2 types
compete. Cthalamus dominates higher up and semibalanus lower down. Long extendible penis for
cross fertilisation Semibalanus balanoides- MLWS-MHWS
Cthalamus stellatus MTL and MHWS – upper limit determined by dessication. Lower limit by
competition with semibalanus and predation by the dogwhelk
The soft bodied beadlet anemone (Actinia equina) may be found in suitable microhabitats like moist
Upper shore- animals that are almost land animals- chitons and sea slaters- like woodlice – scavenge
Mussel beds will form and both limpets and periwinkles will graze the rocks. Beadlet Anemones are
resident and In pools, under stones and seaweed there are common shore crabs (Carcinus maenas
will be found from spring to autumn.
Sheltered shore -More food results in more herbivores to eat it and more herbivores mean
carnivores to eat them. Limpets are still much in evidence grazing on the small stages of seaweeds.
Top-shells (Gibbula umbilicalis, Osilinus lineatus) and different winkle species flat and edible winkles
now also begin to thrive. Carnivores like the dogwhelk (Nucella lapillus) may now be found in
abundance feeding on the dense assemblages of barnacles (or sometimes mussels if there are
some). These are very important predators, particularly on the molluscs. The sheltered conditions
will allow many animals that are not attached (like the crabs) to appear here including fish like the
blenny or shanny.
The middle part of an exposed shore is subject to heavy wave action and is bathed in strong sunlight
for much of the day often dominated by species which are capable of holding on tight against the
wave action and withstanding the heat of the sun and the drying of the facing into the prevailing
wind. Limpets and barnacles are two such creatures and they sometimes dominate the middle parts
of exposed shores. Limpets are herbivores so there must be some seaweed or lichen for them to eat.
Presumably juvenile algae settle (from the plankton) but get eaten before we can see them.
Carnivores like the dogwhelk (Nucella lapillus) may now be found in sheltered microhabitats (cracks
and crevices) feeding on the dense assemblages of barnacles (or sometimes mussels if there are
Acorn barnacles- dog whelks, rough and smooth periwinkles and beadlet anemones.
Winkles and crustaceans well defined zonation on low tide shore. High tide is another story, range
much more widely. Shore Crab moves up with the tide and back as do some snails. No evidence as to
why. Blenny. Starfish. Sea-urchins.
Periwinkles and topshells.
Mussells attached to the rocks by strong threads-intertidal filter feeder- predated on by dog-whelks
in the absence of barnacles, starfish, crabs, birds.
For more on animals and information relevant to text boxes- see The Sea Shore – CM Yonge-
chapters 9-11 & Seashore – Peter J Hayward
The range of crabs, molluscs, small fish and prawns is much greater in this zone.
Greater productivity of algae means more herbivores can survive (eg. Blue rayed limpets (Helcion
pellucida), grey topshell (Gibbula cineraria), and more herbivores means more carnivores like shore
crabs (Carcinus maenas), blennies (Lipophrys pholis) and starfish (Asterias rubens). Not only that but
the food available to filter feeders is increased due to longer feeding time so sponges like the
breadcrumb sponge (Halichondria panicea) can do very well in these regions. Filter feeding tube
worms like the spiral tube worm (Spirorbis borealis) and the keeled tube worm (Pomatocerous
triqueter) also can be found in great quantity here.
All these creatures die at some point and detritivores (who eat dead stuff) also abound here.
Porcelain crabs (Porcellanus platycheles) use their massive front claws to stir up the sediment and
feed on decaying material trapped on the bristles. Soft bodied predators/detritivores like the snake
locks anemone (Anemonia viridis) and the beadlet anemone (Actinia equina) also do well here
(although A. equina can live on the higher levels of the shore (see sheltered rocky shore upper shore
section). Shrimps and prawns are generalist feeders who will consume large amounts of dead animal
material. All this means that factors like competition and predation and interactions between
organisms become more important as we move down this zone.
In fact the list will go on and on. A search around the kelp base will show and incredible variety so
have a look at the species list for rocky shore under the headings of the main phylum groups like
echinoderms, crustraceans and molluscs.
Infralittoral Fringe (around Low Water Springs):
This is the exciting area with conditions in the larger pools matching the shallow seas, and almost
any of the fish and invertebrates that live around the British Isles can be found on occasions.
Sublittoral (below Lowest Astronomical Tide, or Chart Datum):
In most literature this refers to the submerged sea area below the low water mark.
Refuge or stressful trap depending on size etc O2 concentrations more variable, temperatures,
salinity, variation in life determined by time isolated from sea.
Above MHWS enteromorpha intestinalis and little else.
Between MHWS and MHWN encrusting corraline algae and the branching species corralina vulgaris.
Limpets common and graze on enteromorpha. Beadlet anemone- dark red with a blue dot at the
base of each tentacle- stinging cells. MHWN and MLWN pools contain laminaria and other large
brown algae with corralina as undergrowth. Small invertebrates common especially those inhabiting
the corralina. Snake locks and daisy anemones (same as dahlia?)- preyed on by grey sea-slug which
eats solely anemones.
Below MLWN more enriched algae and similar to sublittoral but more blue rayed limpets on
Laminaria Hyperboria. Sublittoral aquaria- species not otherwise observable without venturing
offshore. Well grown kelps, fish etc
Prawns, shrimp, crabs, lobster
Dog whelks- strong enough to survive rough seas – lay eggs in vaselike capsules on underside of
stones and in crevices- capsules. Mainly prey on barnacles and mussels-Exude a muscle relaxant –
whelk then forces open the barnacles plates- exudes digestive enzymes and sucks up the resulting
soup. Mussels Whelk climbs onto limpets and rasps a hoel into the top of the shell. Up to 2 days. –
can be eaten by shore crabs (apex removed) or smashed open by lobster or edible crab.
Shore crabs the main predator in our tidal zone. Specialise in mussels and dog whelks. Also eat a
variety of other intertidal organisms and even seaweed. Other crabs will eat but not in numbers
significant enough to have an impact.
Starfish- shape and ability to exert prolonged pressure- effective predators of shellfish- evert
stomach – low shore
Sea urchins- pelagic larvae-Echinus esculentus- graze on algae and invertebrates associated or dead
plant material (which for EE?)
Anemones – anthozoa- flower animals- watch in rock pools – attached to rocks by muscular disk-
absorb o2 through skin- bag like body with a ring of hollow tentacles around mouth, catch small
crustaceans and molluscs
commonest is beadlet-across entire littoral zone- blue patches at base of tentacles – contain stinging
cells- appear sessile but do move around- Mostly clonal reproduction- like to be spread out- attack
Snakelocks anemone- long non-retractable tentacles- tinged green with symbiotic algae- also clones-
splits into two- like space also but more tolerant of clones.
Dahlia- sheltered crevices up to mhwn- much more sociable- still need some space.
Limpet Patella Vulgata- most common british limpet-Conical shell-Live on rocky shore, feed on
algae, suckerlike foot to cling to rock, crawl around on it too- rippling muscles in waves Gastropod.
Radula to scrape algae- ribbon like tongue with rows of teeth, keystone species in controlling algal
levels (also small larvae)- seaweed spores Live up to 15 years. Good food supply= faster growth but
5 year life span Low shore grow faster and lower profile- higher shore- grow more slowly, taller
shell- hold on more tightly. This advantage is negated on exposed shores. Better to be in a crevice.
More vertical shell on sheltered shores- dessication= harder clamp= more muscle Homing
behaviour home-scars- formed by rubbing the rock- good seal (harder rock- the shell is rubbed to
fit), return by following chemical cues in their mucus trakc- use shell like a bullsozer Mature as
male at 9months and then later change to be female after a year or 2 Spawning – once a year,
rough seas trigger, eggs, sperm, pelagic larvae, settle after a couple of weeks. Limpets- sheltered
shores, a few large limpets one species. Exposed shores, lots of smaller limpets and 3 species.
Crabs, starfish, dogwhelks, gulls and other birds
Oystercatcher- levers shell off with a quick thrust to the slightly lopsided head portion of the cone.
Dog whelks Nucella eat barnacles, mussels. Empty shells are often inhabited by young hermit
crabs- older go on to live in whelk shells. Dog whelk different in different exposure areas.
Sheltered are long. Exposed are short and squat. Exposed have a larger aperture to allow a larger
foot for better sticking.
Periwinkles-feed on softer food than the limpet
Common Periwinkle- eaten by winklepickers- eats sea lettuce and gut weed- which is therefore
rarely seen after spring except around fresh water or in rock pools containing shore crabs which eat
common winkles. Not so common on sheltered shores- perhaps because of number of crabs
sheltering in seaweed or because egg wrack survives better here and they don’t eat it. Small red
seaweeds would lose out in competition to greens were it not for the presence of the common
winkle. On More exposed shores, their small foot and relatively large bodies mean that they are
vulnerable to wave action and will generally be found only in small clusters in sheltered spots and
Flat winkle- 2 closely related species- virtually indistinguishable- green, yellow, brown, olive,
chequered,camouflaged amongst bladders of seaweed- outnumbers common on weedier shores-
eat brown seaweeds and the microscopic plants which may be growing on their surfaces. – eaten by
shore crabs and blennies
Small periwinkle – small shiny black feeds on lichens and able to live high above the tideline within
the occasional splash of seawater.- but still needs to reproduce via a plankton stage
Rough periwinkle- lives on the upper shore not going into the splash zone. But is oviviviparous- small
with a widely flared outer lip, lots of variability in colour and distribution- whole spectrum of
Middle and lower shore-
Ssea slugs- predators- can be colourful and spectacular but are generally not particularly striking-
camouflaged, found under boulders and overhangs to avoid dessication and attack sessile prey such
as sea anemones or sponges
Fish- shanny, goby- wide ranging diet of shore animals and algae.
Mytilus edulis (The common or edible mussel)
Molluscs are conspicuous invertebrates, including familiar forms such as clams, oysters, mussels
squids, octopus, snails. In abundance the molluscs comprise the largest invertebrate phylum aside
from the arthropods Ð 80,000 living species and 35,000 fossil species. Molluscs are a heterogeneous
assemblage but all built on the same fundamental plan (Figure 1).
Molluscs are unsegmented coelomate animals with a head, ventral muscular foot, dorsal visceral
hump, soft mantle covering visceral hump which secretes a calcareous shell, mantle cavity, anus and
kidneys opening into mantle cavity, a pair of ctenidia (gills) (originally for breathing) within the
mantle cavity (Figure1).
Class Lamellibranchiata (Bivalves)
Bivalves are molluscs with bilaterally symmetrical body, laterally compressed and enclosed by a shell
that develops two valves hinged dorsally (Figure 2). The foot like the remainder of the body is also
laterally compressed, the mantle cavity is capacious and the gills are large having assumed the role
of food collecting as well as respiration. (Figure 2)
Most of the characteristics represent modifications that enabled Bivalves to leave the hard
substratum, to which the ancestral molluscs were confined, and take up an existence in the much
more numerous soft bottom habitats. The lateral compression and development of a strong
muscular foot for burrowing led to a degree of specialisation that confined almost all Bivalves to the
soft sediment environment. Only a few groups have subsequently migrated to other habitats.
The development of the gills (ctenidia) is the outstanding morphological and physiological character
of the Bivalves (Figure 3). The arrangement of the valves allows the mantle cavity to extend the
whole length of the body and makes possible a great extension of the gills. There are two limbs of
the gill filaments on either side of the body mass, hanging down into the mantle cavity and joined
together by ciliary junctions. The joining of the gill fillaments divides the mantle cavity horizontally
into an upper and lower chamber. During activity a constant stream of water is maintained entering
via an inhalent ventral siphon, passing through the gill lamellae into the dorsal chamber and out
through a dorsal exhalent siphon. This system facilitates respiration and feeding. In some bivalves,
including mussels, there has been morphological sealing of the mantle edges and the development
of inhalent and exhalent siphons (Figure 4)
Filter feeding is achieved by the separation of minute plants and organic debris from the water
current onto the surface of the gill lamellae. Inhalent water is largely a function of ciliary activity. As
the water enters the mantle cavity larger particles fall to the floor of the cavity whilst smaller
particles are filtered onto gill lamellae. These smaller particles are entrapped in mucous and
transported in the direction of the mouth in ciliated grooves by ciliary action. Ciliated palps between
the gills and mouth have the function of sorting food particles. Lighter particles are transported to
the mouth whilst heavier particles are transported downwards to the bottom of the mantle cavity.
Rejected particles which accumulate in the bottom of the mantle cavity are periodically ejected
through the inhalent siphon. This achieved by closure of the two valves of the shell (Figure 5)
The lamellibranchs are divided into 4 Orders according to structure, degree of development of the
gills and the extent to which gill filaments are joined together. Mytilus edulis belongs to the Order
Filibranchiata with reflected gill filaments and adjacent filaments joined by ciliary junctions.
Whilst the majority of lamellibranchs are semi-sedentary, the sea mussel has developed the
sedentary tendency and marks a half way stage to the oyster which remains fixed throughout adult
life. The mussel lives in associations in beds between the tide marks where conditions are
favourable. The reduced but very extensive foot is tongue like in shape with a groove on the ventral
surface, which is continuous with the byssus pit. In this pit a viscous secretion is poured out which
enters the groove and hardens gradually when it comes into contact with sea water. The tip of the
foot is pressed against the surface to which the mussel attaches itself, and in a cup like hollow which
ends the groove the attachment plate is formed at the end of the byssal thread. When one byssal
thread has formed the foot changes position and secretes another thread in another place. The
byssus thus consists of a mass of threads arising from the byssus pit and by means of it the mussel is
firmly attached to rock, stones or other mussels etc. (Figure 6). However, mussels, particularly when
young, creep about both by using the cup at the tip of the foot as a sucker and by forming a path of
threads along the substratum. The byssus is the most outstanding characteristic of the mussel but it
is additionally worth noting that other features include a pair of simple eyes at the anterior end of
the gills and invasion of the mantle by the gonads. Note also that most bivalves, including mussels,
are dioecious. There is no copulation and fertilisation takes place in the surrounding water. The
development of free swimming larvae is typical of bivalves.
The general biology of mussels
Mussels dominate in much of the low and mid intertidal region in temperate seas of the northern
and southern hemispheres. There are many genera, all with a narrow anterior end and the anterior
abductor reduced. The pointed umbo is at the anterior end of the shell in Mytilus, and slightly set
back in other genera.
Mytilus edulis is essentially an intertidal organism, which can form extensive beds dominating the
rock surface. It can also form strips or patches. Competition, predation and physical factors are
important in determining the distribution of Mytilus. Mytilus edulis can live high on the shore
because it is tolerant of desiccation and frost. It is, however, susceptible to predation by starfish,
dogwhelks, shore crabs and various birds. Mussels on exposed coasts are often free of predators
while those in sheltered waters may suffer extreme predation.
The first larval stage of Mytilus edulis is called a trochophore, which lasts about one day. It is
followed by various stages of veliger larvae lasting in all about one month. The veliger larvae has a
pair of shell valves and carries a ciliated swimming organ or vellum. It settles first on filamentous
seaweeds, with maximum settlement in June ÐJuly. In July-August these early plantigrades detach
themselves and are carried once again by water currents. They settle again in their final position in
mussel beds in the same months.
Growth rate of mussels varies greatly and is dependent largely upon the amount of time available
for feeding i.e. in proportion to immersion. The position within mussel beds is also important:
individuals within the centre grow slowly, while those at the edges grow faster but may suffer the
penalty of higher predation. Growth can be measured by disturbance rings on the shell amongst
Adaptations of Mytilus edulis to life in the intertidal zone
1. An attached sedentary mode of life in dense communities enables mussels to withstand the
physical exposure of waves. Their sedentary mode of life is facilitated by (1) nutrition by
filter feeding which does not require locomotion; (2) reproduction without copulation and
by pelagic larvae which achieves dispersal of the species without locomotion by the adult.
Filter feeding is particularly effective in the intertidal zone where wave action causes the
suspension and re-suspension particulate nutritive material which is readily available to the
2. Closure of the valves which enables mussels to withstand the physical, chemical and biotic
factors associated with exposure to the atmosphere during low tide by trapping water in
their valves and restricting water loss*. (Factors associated with exposure to atmosphere
include dessication or water loss, light, heat, lack of dissolved oxygen, predation etc.).
3. Important in the context of 2 above are adaptations to facilitate (1)survival in the absence of
a supply of dissolved oxygen; (2) survival of the build up carbon dioxide and toxic
metabolites. Mussels can trap water within their valves and respire anaerobically, building
up an oxygen debt which is paid off by aerobic respiration on the next incoming tide.
Furthermore, the build up of carbon dioxide within the mantle cavity during low tide
depresses ciliary activity and finally causes ciliary activity to stop so that the store of oxygen
within the cells is conserved. In mussels ammonia constitutes a relatively small proportion of
nitrogen excretion, much of it comprising less toxic amino acids*.
4. Closure of the valves affords mussels protection against many terrestrial predators during
exposure to the atmosphere at low tide.
*Water loss can be fatal either because of a change in the internal environment as a result of an
increase in body fluids (this affects the osmotic potential of the body, which may assist desiccation
as the cells lose water by osmosis to their surrounding fluid), or because of lack of oxygen to the cells
because of lack of water flowing over the surface of the gills(and so reducing gas exchange). Aquatic
animals may suffocate even if they are surrounded by oxygen rich air because their gills will stick
together. Water is also lost by excretion; many marine animals excrete ammonia which is highly
toxic and must be diluted with large quantities of water. This is a serious problem on the sea shore
where water loss is to be avoided at all costs.
Mytilus edulis as an indicator organism in marine pollution assessment
In many respects Mytilus edulis is the ideal marine indicator organism. It fulfils many of the basic
prerequisites for the selection of an indicator organism (see Appendix ). Additional to the fulfilment
of these requisites the following are particularly pertinent factors:
1. Mussels, like all filter feeding bivalve molluscs, process large volumes of water. This is
necessary because the amount organic matter in sea water is low (average 1mg/l). Mussels
filter, on average, 7.5 litres of sea water/hour. As a consequence of this they accumulate and
concentrate many pollutants in sea water, particularly those which are particulate or are
associated with particles. Like all bivalves, mussels are notorious for their ability to accumulate
very high concentrations of metals. They also accumulate other pollutants such as faecal
bacteria and radionuclides. This ability to accumulate materials facilitates the detection and
measurement of pollutants that may be in the water column at very low concentrations.
2. The fact that mussels can accumulate faecal bacteria and viruses and the fact that mussels are
harvested for human consumption has led to their use in routine determination of the quality
of mussel flesh and water in terms of hygiene and public health.
3. Because mussels satisfy so many features of a good indicator organism they have been used
widely in pollution assessment work, including survival (LC50), behavioural changes,
reproductive changes, transplant experiments, and scope for growth experiments.
4. The scope for growth test was developed for use with bivalves, particularly mussels, to give a
general measure of the overall physiological health of the animal based on its energy budget.
The scope for growth is the difference between the energy assimilated from food, and the
energy used in respiration, excretion and other maintenance activities. Any surplus energy is
available for growth and reproduction. A reduced or even negative scope for growth results
when energy intake from food is reduced, as it may be in winter, or if the energy expenditure
on maintenance activities is increased by environmental stress.
5. Because of the filtration mode of feeding mussels also readily become tainted by
hydrocarbons present in the water. This is of particular significance because of their
commercial value for human consumption. Tainting of shellfish and finfish flesh is evaluated by
Effects of different exposure
Sheltered Semi exposed- still abundant kelps but mostly L digitata. Scarcer algae in eulittoral
patchy f vesiculosus patchy instead of extensive. More seimbalanus. Mosaic of fucus, barnacles
and bare rock. 2 other limpets. Littoral fringe Verrucaria Maura and a winkle Melaraphne
Very exposed- much less algae except in sublittoral (Alaria esculenta) Mastocarpus stellatus
replaces Fucus serratus and there are turfs of other red algae especiall y corraline. Patches of
barnacles, small mussels, nucella and abundant limpets.
shores= rocks obscured by weed. Exposed shores= less weed and dense cover of barnacles and
mussels with red algae
Sheltered shores= algae cover the shore completely. Hard to spot a 3 zone structure and no littoral
fringe. Abundant animals in the weed. Patches of barnacles and dog whelks predator. Scarce large
patella vulgate. Littorina Mariae on focus serratus and L obtusata on Ascophyllum littorina
saxatilllis (rough winkle) on upper fucoids and large edible winkle (litorina littorea) common
throughout eulittoral. Delicate algae unable to survive on exposed headland gastropods unable to
stick on. Warmer water in summer, more turbid, less saline and less oxygenated. Deposited
As exposure increases, zones expand further up the shore – wider.