Lecture 4 Biodiversity of Animals on the Rocky Shore
Total Page：16
File Type：pdf, Size：1020Kb
LECTURE 4 BIODIVERSITY OF ANIMALS ON THE ROCKY SHORE NB It is possible to preface this lecture with an introduction to zonation. Please select COMBINATION OPTION 2 (118 slides) NB If you wish to present a complete lecture on zonation, plants and animals please select COMBINATION OPTION 3 (197 slides) Slide no. 1 Title Slide. 2 Introductory. Illustrated: common periwinkle Littorina littorea ; shore crab Carcinus maenas ; beadlet anemone Actinia equina ; encrusting red algae such as Lithophyllum spp. all in a small rock pool near Plockton in NW Scotland. The colours are real! 3 Rocky shore study in progress. 4-5 An invitation to examine the undersides of stones etc. with the important message: PLEASE TURN THE BOULDERS BACK to preserve the organisms being being studied. 6 Animals of the upper shore. 7 Sea slater Ligia oceanica , a large woodlouse which can be found beneath stones during daylight, but scuttles about the surface in large numbers as it feeds at night. 8-15 Sand hoppers Orchestia gamarellus (NB often mis-spelled 'Orchestria') and Talitrus saltator . I think this is Orchestia although the tell-tale large 'claw' on the second leg (uropod) cannot be seen in any of the pictures. Yonge (1949) wrote: "These animals may fairly be grouped with the sea-slater and the rough periwinkle [slide 36 et seq. ] as creatures which have very nearly crossed the threshold of the shore on to dry land". He quotes from Paley's Natural Theology wherein is described a scene I doubt we might witness in these days of anthropogenic reduction in diversity and numbers of wildlife: "Walking by the sea- side, in a calm evening, upon a sandy shore, with an ebbing tide, I have frequently remarked the appearance of a dark cloud, or rather very thick mist, hanging over the edge of the water, to a height perhaps of half a yard, and a breadth of two or three yards, stretching along thew coast as far as the eye could reach, and always retiring with the water. When this cloud came to be examined, it proved to be nothing else than so much space filled with young Shrimps , [Yonge presumes sandhoppers] in the act of bounding into the air from the shallow margin of the water, or from the wet sand". 16 Animals of the middle shore. 17 A second invitation to examine the undersides of stones etc. with the message: PLEASE TURN THE BOULDER BACK to preserve the organisms being being studied. 18 The centipede Strigamia maritima is not well known, but in some areas is very common. It doesn't look much like garden centipedes, whereas the non- Ligia woodlice with which it frequently shares this habitat seem very familiar. 19-21 Where there is a modicum of sea water Gammarus spp. can be found. They lurk in huge numbers under suitable stones and can be found beneath seaweed and in rock pools: just about everywhere. Sorting them our to species is a job for the specialist or the meticulous enthusiast. 22-23 Acorn barnacles can inhabit rock surfaces in unbelievable numbers. There are only two main species to trouble us on the seashore, but others may be found below low water or in specialist situations, such as piggy-backing on periwinkles (Balanus hameri ?). Look closely and you will discover that such barnacles are different from the commmon species such as Semibalanus balanoides which also grows on molluscs, but those that don't move e.g. mussels. As adults, these are static crustaceans. The larva (cypris) attaches itself to rocks etc. by its head and develops a shell with an aperture through which it spreads its paddling legs when under water to collect suspended food material. Surprisingly, fertilisation does not occur at random in the sea, but the male finds females with its enormously long penis and fertilises them in situ . 24 Not far below high water on exposed shores there is a relatively narrow band of barnacles that, on very close inspection (a hand lens is essential, and prolonged stooping and painful, barnacle-pitted knees are inevitable), can be identified as Chthamalus montagui . (Careful of spelling and pronunciation; this is not 'Chthalamus'). It has a relatively well-constructed shell, not easily damaged with a finger nail, which is much more robust than that of the neighbouring barnacle species. 25 Two main features characterise C. montagui . The aperture is shaped like an old- fashioned kite (diamond-shaped, but longer below and shorter above the mid line) and the rostral plate underlaps those on either side (compare with description and illustration of S. balanoides , slide 27). 26 The most numerous barnacle on British shores is Semibalanus balanoides which forms a wide zone lower down the shore than C. montagui . It is absent from exposed shores and can be incredibly numerous on sheltered shores. It is poorly constructed, and breaks up fatally when picked at by a curious biologist. This fragility enables populations to compete for space, and vigorous individuals can exclude others, weaker than themselves, gaining more room in which to grow. Sometimes these barnacles do not evict one another, instead growing to great height (in barnacle terms) side-by-side. 27 Two main features characterise S. balanoides . The aperture is a regular, equilateral diamond shape and the rostral plate overlaps those on either side (compare with description and illustration of C. stellatus , slide 25). 28 The common limpet Patella vulgata is a gastropod with an uncoiled body, much like early molluscs, though this is very much a 21st century animal. It tends to inhabit a specific place on its rock, crawling away to graze when covered by the tide, but returning for the intertidal period of exposure. 29 Lower shore limpets tend to get covered in barnacles or seaweeds which may help disguise them from gulls which certainly feed on them. 30 For various reasons, perhaps poor camouflage, bad weather or the attentions of a large fish, some never make it back to base after a feeding trip. 31-32 The common, edible, mussel Mytilus edulis can be found in large colonies on solid rock and large boulders, particularly near fresh water or where organic pollution makes suspended food available to this filter feeder. It can also be found on particulate shores where it exists as lone individualsor small groups, often atttched by its byssus to stones, vacated shells or other sedentary molluscs such as cockles. The byssus is a feature shared by a number of other bivalves and is a tuft of exceptionally strong proteinaceous fibres which rapidly cement themselves to anything within reach. Small mussels of this and other species can be found densely packed along cracks in rock where few can attain maturity. 33 Periwinkles come in variety, tend to occupy overlappping, but definable zones and can be difficult to identify with certainty. However, if one eliminates all but the commonest four species, identification is possible and ecological data may be successfully gathered. 34-35 Common periwinkles Littorina littorea may be found from the middle of the shore downward. In counting them, care should be taken to look under stones and seaweed otherwise significantnumbers will be overlooked. (For identificationguide see slides 42-43). 36-37 Rough periwinkles L. saxatilis occupy the upper regions of the shore, being so tolerant of conditions above the tide that they are almost terrestrial animals (see notes to sldes 8-15). (For identification guide see slides 42-43). 38-41/44 Until relatively recently, flat periwinkles were considered to be a single species L. littoralis . However, these days there are two common species which, if you look closely, are not difficultto tell apart: L. obtusata and L. mariae . (For identification guide see slides 42-43). They can usually be found grazing on the fronds of the wrack seaweeds from mid-shore downwards where some colour morphs successfully mimic bladders, perhaps escaping the attention of fish that certainly feed on winkles. 42-43 Identifying four of the commonest periwinkles (hopefully, self explanatory). 45-56 Unlike the periwinkles that graze, dog whelks Nucella lapillus are predatory carnivores. They can be found wherever their favourite food, mussels and barnacles are and, if you lift them off their prey, you can often see what they have been up to, particularly on a live mussel (see slides 51-56). At first sight they may be mistaken for common periwinkles (slide 47), but the the shell is rougher and more 'spired', and the opening has a distinct channel where the occupant's respiration tube (siphon) emerges. Dog whelks have a radula for boring through the shells of prey and produce hydrochloric acid (dissolves the calcium carbonate of which shell is mostly constructed) to assist in this process. 57-59 The collembolan Anurida maritima (formerly known as Lipura maritima )isa minute primitive insect, fossilised ancestors of which have been discovered associtated with some of the first land plants in Devonian rocks (rhynie chert) which are some 450 millionyears old. Truly marine insects are very unusual and Anurida is no exception, for it never really comes into contact with sea water, being coated in wax and totally unsinkable. It never uses the sea as other real marine animals do for respiration, bodily mositure and reproductive processes, and might as well be considered a terrestrial insect did it not live exlusively on the sea shore. 60 Pomotoceros triquetur is just one of several sea shore worms that build a protective, calcareous tube to live in, but it is featured here because it is probably the commonest and most wide-spread. It is used in the laboratory to demonstrate animal fertilisation and embryology, for if evicted from their tubes and set aside, both male anf female worms release their gametes into the water, as they do from within in the wild.