Download Additional Species Info – Rocky Shore

Additional Species Info – Rocky Shore.
LIMPETS:
Patella spp
The common limpet is one of the best known intertidal organisms, found on rocky
shores of all degrees of wave exposure, from the high shore to the sublittoral.
There is considerable variation in shell shape between individuals from different
shore levels. During low tide the shell muscles are contracted, keeping the
animal firmly attached to the rock and thereby reducing desiccation. This is
believed to have the effect of pulling in the shell secreting muscles and thus
affecting the shape.
Limpets feed on a wide range of microorganisms and algae, including Fucus spp.,
and encrusting red algae. These are removed by a radula (known as a
docoglossan), the teeth of which are hardened by iron and silicate compounds,
leaving zigzag impressions in the rocks. They undergo feeding excursions,
returning to the same ‘home scars’, this homing behaviour is achieved by
following a mucus trail deposited on the rock surface and controlled by an
endogenous circadian rhythm. The home scar is a slight depression formed by
chemical reaction and abrasion between the limpet and the rock. The edge of the
shell fits tightly into the depression and thus reduces the rate at which water is
lost from the mantle tissues during emersion. Limpets need water even when
emersed as they breathe through gills in the mantle cavity.
Patella spp regularly undergo the phenomenon of sex change; small individuals
being predominantly male (about 1 year old and 20mm), the remainder being
neuter or female. Larger and older individuals are progressively more female.
Thus they are referred to as protandrous (male first) hermaphrodites. Spawning
occurs, possibly correlated with rough seas and bad weather, from early
September to late October. After a planktonic period the larvae settle as ‘spat’
low on the shore. Limpets grow by laying down extra shell during submersion.
Desiccation in very important to limpets, Patella vulgata from the upper shore
lose water slowly and half will die when lose 65% of their body water. Patella
vulgata from the lower shore desiccate more rapidly and half will die 50% water
loss. The situation is made complicated however as upper shore limpets grow
much larger than low shore limpets and therefore have a lower surface are to
body volume ratio and this will reduce water loss.
Crepidula fornicator
The slipper limpet is originally a North American species now common around
Britain (believed to have entered attached to imported Oyster shells). Curved
chains of up to 12 individuals can be found on lower shore down to sublittoral.
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They feed on small particles which are drawn into the mantle cavity with water
and trapped by a mucus filter. Slipper Limpets are protandrous hermaphrodites;
in a chain of individuals the smaller, younger animals at the top are males and the
larger, older ones at the bottom are females. Internal fertilisation results in eggs
being laid in stalked capsules attached to the female. After hatching there is a
pelagic life stage then the individuals settle on an existing chain as males or if not
on some other substrate as females.
TOPSHELLS:
Gibbula cineraria
Sheltered shores from the lower shore down to about 130m, feeds mainly on
detritus and microalgae. The radula is not hardened with iron compounds and is
therefore apparently unable to graze red encrusting alga or tough Fucus spp.
The sexes are separate and fertilisation is external (spawning is thought to occur
in response to and increase in seawater temperature). It has a very thin delicate
shell and is very susceptible to desiccation stress.
Gibbula umbilicalis
Found on upper shore down to low water on sheltered rocky shores. Feeds on
microorganisms and detritus. Separate sexes and external fertilisation, breeding
takes place in the summer with a short pelagic life. The have an innate
behavioural response (negative phototaxis) which allow them to avoid desiccation
by seeking out dark moist microhabitats e.g. in large seaweed mats.
Osilinus lineatus (formerly Monodonta lineata)
A southern species, extending only as far north as N Wales. Found on rocks and
boulders from upper to lower shores and as above feeds on detritus and
microorganisms. Sexes separate, fertilisation external and breeding in summer.
Short pelagic life then juveniles settle on upper shore under boulders and stones.
Thick shell allows resistance to desiccation and temperature stress as reduces
water loss. Low SA:VOL which means they have a large mantle cavity with which
to store water in comparison to the surface area available to lose water.
PERIWINKLES:
Littorina littorea
Widely distributed on rocky coasts in NW Europe, from the upper shore into the
sublittoral (also on estuaries and mud flats). It is a grazer of microorganisms and
detritus and green algae. Sexes are separate with internal fertilisation;
copulation takes place in early spring and eggs are shed in gelatinous capsules in
conjunction with spring tides. The older, larger individuals tend to live higher up
the shore as they have a lower SA:VOL which can aid surviving desiccation stress.
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They also tend to aggregate in suitable habitats so you find masses of them under
rocks.
Littorina saxatilis
The Littorina saxatilis species complex is a taxonomic confusion as scientists still
cannot decide which are species and subspecies out of L. saxatilis, L. arcana
L.neglecta and L. nigrolineata. For the purpose of A level we probably ought to
stick to calling them L. saxatilis.
These rough periwinkles are widely distributed around Britain on the upper
reaches of shores of all exposures, living in cracks, crevices under stones and with
seaweeds such as Fucus spiralis. Variation in shell colour is believed to be
maintained by predator selection. It feeds on microorganisms, detritus, seaweed
and some lichens. The sexes are separate and the fertilisation is internal and the
embryos develop into shelled, crawling young in a brood chamber inside the
female (ovoviviparity); therefore rough periwinkles do not need any water to
breed. They also avoid desiccation by conserving water via releasing nitrogenous
waste as a solid (Uric acid) and having eurythermal enzymes. On rock cliffs that
are exposed to wave action the shells of L. saxatilis have a relatively large
aperture and are thinner than shells found on more sheltered shores.
Littorina obtusata
Widely distributed all around the coast of Britain and always associated with the
seaweed Fucus vesiculosus and Ascophyllum nodosum on which it grazes. Shell
colour is related to shore exposure (sheltered = olive green / exposed = brown
criss-cross). Sexes are separate and internal fertilisation produces egg masses laid
on Egg Wrack.
Littorina marie
Widely distributed the same as above although lower on the shore than L.
obtusata. Associated with Fucus serratus, the shell colour is again associated
with exposure (sheltered = yellow / exposed = Brown criss-cross). The sexes are
separate with internal fertilisation resulting in egg masses laid on Saw Wrack.
NB: Prior to 1966 the flat periwinkles were lumped together and called either
L.obtusata or L.Littoralis. In 1966 Sacchi and Rastelli split the group into above
spp, this is widely recognised.
DOG WHELKS:
Nucella lapillus
Occurs on middle shore down to shallow sublittoral on all rocky shore coastlines in
Britain except the most sheltered. Exposed shells tend to be shore, squat shells
with a large aperture; whilst sheltered shells tend to be taller with a relatively
narrow aperture. These variations have been related to exposure and predation
from crabs. The Dog Whelk is a carnivorous snail eating acorn barnacles and
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mussels. Using a mix of mechanical and chemical (carbonic anhydrase) means the
protective plates (in barnacles) are pushed apart or shells (in mussels) are bored
into. After affecting an entrance, a narcotic is injected, followed by secretion of
digestive enzymes. The dissolved tissues are then sucked up; this whole process
takes about a day for a barnacle or up to a week for a mussel. The sexes are
separate and internal fertilisation produces egg masses known as nurseries. Dog
whelks are cited as examples of the development of ‘imposex’ in response to
pollution of inshore waters from TBT (Tributylin) leached from anti-fouling paints.
CRABS:
Carcinus maenas
One of the most common shore crabs in NW Europe, it is found on all types of
shores from high water to sublittoral at 60m of depth. It shows a wide range of
colour variations from green through orange and reddish-purple. Maturity is
generally reached at about 1year, and then the male will seek out a female about
to moult and carries her under his body for a few days, when the female moults
copulation takes place. Eggs are then produced which can often be seen on
female crabs as an egg mass on the tail section. Larvae life is planktonic (Zoea)
for about 2-3 months then juveniles settle on shore. The shore crab feeds on a
variety of invertebrates including: polychaete worms, molluscs (especially Mussels
and Dog Whelks) and crustaceans.
In lab conditions without obvious environmental clues the crabs show rhythmic
activity, demonstrating they have intrinsic internal rhythms. The crabs seems to
move up shore as the tide rises and retreats again as the tide falls – but how does
it know when the tide will turn. Studies show the crabs exhibit activity every 12.4
hours when maintained in constant conditions, so it is thought these circatidal
rhythms help to maintain the animals ‘correct’ zone because they switch of
activity just before the tide falls stopping them from being stranded.
Pocellana platycheles
The Broad-Clawed Porcelain Crab is a suspension feeder which is common under
stones on middle and lower shores, particularly those in muddy gravel.
Pagarus bernhardus
The common hermit crab is common on most British coasts during spring and
summer, most frequently occurring in Edible Periwinkle and Dog Whelk shells.
The hermit crab is a scavenging omnivore but also filters small particles from the
water. The crabs must find larger shells to live in as they grow and are incredibly
vulnerable during these moves.
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BARNACLES:
Semibalanus balanoides
Occurs extensively on British coasts on shores on all exposures and tolerates
estuarine salinities. Together with Chthamalus spp, Semibalanus forms the
characteristic barnacle zone on most rocky shores, generally with the latter
further down the shore. Semibalanus has been shown to be less tolerant of
desiccation stress than Chthamalus spp and this has been shown to be related to
the greater permeability of its shell plates. On shores dominated by Fucoids the
density of barnacles is greatly reduced, the seaweed population preventing larval
settling. Semibalanus has a single breeding season with copulation and cross
fertilisation taking place in the autumn, but the nauplii are held in the barnacle
to be released into the sea from February onwards when the planktonic diatom
production is high.
Chthamalus spp.
Prior to 1976 British Chthamalid barnacles were all regarded as members of the
same species with wide morphological variation. They are now regarded as two
species: Chthamalus montagui which tends to be found at the top of the middle
shore and upper shore, and Chthamalus stellatus which overlaps in distribution in
between the other two.
Elminius modestus
This Australasian barnacle was first discovered in Europe in 1945 after coming
over attached to ships bottoms in 1939, from where they have spread around the
coast of Britain. Elminius competes for space with Semibalanus on sheltered
shores and also thrives in dirty harbours and muddy estuaries.
BROWN SEAWEEDS:
Fucus serratus
This brown alga is a widely distributed organism on sheltered shores around NW
Europe. It is found in the lower shore in a distinct zone below A. nodosum and F.
vesiculosus. It is dioecious (has male and female reproductive structures on
separate individuals) and has a lifespan of about 3-5 years. It reproduces in the
autumn and after fruiting shows marked defoliation and the fronds that bore
receptacles are shed after winter, reducing the alga in size. It has incredibly thin
fronds that are wide and serrated for increased surface area; this allows the alga
to float up to the surface of the water forming an interconnecting mat.
Fucus vesiculosus:
An abundant seaweed in sheltered conditions found on the middle and lower shore
along with A. nodosum in a zone above F. serratus. It is tolerant to a wide range
of wave action and the morphology of the seaweed varies in response to
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environmental conditions (F. vesiculosus var.linearis the bladderless bladder
wrack). These seaweeds are dioecious and it forms receptacles in January and
release occurs from March to August each alga release up to a million eggs per
year. It is a fast growing, opportunist colonising cleared areas rapidly and its fast
growth (2-3 times faster than A. nodosum) means that it will outgrow other
species in the middle shore but will only live for up to 4-5 years.
Ascophyllum nodosum
Very abundant species on the middle and lower shore of very sheltered shores,
competing with F. vesiculosus and only outcompeting its fast growing rival on the
most sheltered of shores. It is dioecious with gametes being shed in spring after
which the reproductive bodies and stalks are shed. The fronds produce one air
bladder a year (after the first year or two?) providing a means of aging the
seaweeds. A. nodosum often bears tufts of the filamentous red algae
Polysiphonia lanosa. A. nodosum has a different strategy for living on the middle
shore than F. vesiculosus as it grows slowly taking a long time to colonise rock
space but may live for up to 15 years.
Fucus spiralis
Found widely distributed around British coasts in a distinct band below Pelvetia
canaliculata and above the two common brown algae of the middle shore. It is
usually absent from the most exposed shores. It is hermaphroditic, each
individual developing male and female spores in the summer, they also self
fertilise which may be an adaptation to life high on the shore as it negates the
need for immersion. F. spiralis is tolerant of dry conditions allowing it to live
high on the shore however its vertical distribution is limited – when transplanted
into the Pelvetia zone (just 10cm in one experiment) all died within 8 weeks.
Pelvetia canaliculata
This alga is found highest up the shore of the common brown algae forming a
distinct band. Pelvetia may remain out of water for several days during neap
tides and in summer it can be become dry and brittle, losing up to 96% of its
water content (this is called ‘drought hardening’). When transplanted down to
the middle shore in experiments Pelvetia initially grew rapidly but then started to
rot and although it has not been definitively proven why this is it is clear that the
algae cannot live further down than its current habitat. The seaweed is
hermaphroditic and reproduction occurs in the summer.
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