Life by the seashore : an introduction to natural history

Front matter

22102077391

Med o I

3Ltt ifntroimrtton to Natural History

Contents

CHAPTER PAGE

I. The General Characteristics of Shore Animals 1 II. The Study of Shore Animals . . . 19

III. Sponges, Zoophytes, and Sea-Firs . 36

Index

LIST OF FIGURES.

FIG.

1 . Pliolas crispala, a burrowing Mollusc

2. Sand-launce ( Ammodytcs tobianus). After Day

3. Hermit-crab and Eydradinia. After Allman

4. Dolo coronata. After Alder and Hancock .

5. Dead Men’s Fingers (Alcyonium cligitatum)

6. Swimming-bell ( Sarsict ). After Allman .

7. Lump-sucker (Cycloptcrus lumpus). After Day .

8. Edible crab (Cancer pagurus) . .

9. Plumularia sctacca. After Hincks

10. Fisherman’s lob-worm (Arcnicola jnscatorum)

11. Common scallop ( Pccten opcrcularis)

12. Diagram of Hydraclinia. After Allman .

13. Clava squamata. After Allman

14. Syncoryne eximia. After Allman

15. Sarsia, or swimming-bell. After Hincks .

16. Tubularia indivisa. After Allman

17. Obelia geniculata. After Hincks

18. Campanularia Jlcxuosa (magnified). After Hincks

19. Halccium halccinum (magnified). .After Hincks .

20. Sertularia pumila and branch magnified. After Hincks

21. Bottle-brush coralline (Thuiaria thuia). After Hincks

22. Plumularia (magnified branch). After Hincks

23. Common sea-anemone (Actinia mcscmbryanthcmum)

24. Tcalia crassicornis. After Tugwell

25. Sagartia troglodytes . . ...

26. Adinoloba dianthus. After Tugwell

27. Ealiclyslus odoradiatus . ...

28. Nereis pclagica . . . . .

29. Foot of Nereis pclagica. After Elders

30. Introvert of Nereis pclagica. After Lang .

PACK

31 41

47 49

LIST OF FIGURES.

VII

Fro. PAGE

31. Dissection of Arenicola. In part after Gamble and

Ashworth . . . . . 92

32. Sthenelais boa, a sand Polynoid. After Johnston . • . 97

33. Paddle-worm ( Phyllodoce lamelligera ) . . . 99

34. Head and introvert of paddle-worm. After Ehlers . 100

35. Introvert of Nereis pclagica to show teeth. After Ehlers 104

36. Foot of Nephthys hombergii. After Ehlers . . 109

37. Sand-mason {Terebella) removed from tube . .113

38. Pedinaria belgica. After Malmgren . . .115

39. Scrpula vcrmicularis in tube . . . .117

40. Sea-snake {Linens marinus ) . ... 121

41. Solaster papposus, or sun-star . . . . 128

42. Common brittle-star {Ophiothrix fragilis) . . 129

43. Disc of sand -star (Ophiura) . . . . 130

44. Aristotle’s lantern from sea-ui’chin . . .136

45. Sea-urchin ( Echinus esculentus ) . . . .138

46. Sea-cucumber {Cucumaria planci). After Bell. . . 144

47. Prawn ( Palcemon squilla) . ... 151

48. Lobster {Romanis vulgaris) . . . . 152

49. Shore crab ( Garcinus mcenas) . ... 153

50. Foot-jaws of lobster and of crab . . .160

51. Scaly Galathea {Galathca squamifera) . . . 177

52. Hairy porcelain-crab {Porcellana platychdcs) . .179

53. Hermit-crab ( Pagurus bernhardus) . . . 184

54. Masked crab {Corystcs cassivelaunus). In part after

55. Spider-crab {Eyas araneus ) . . . . 197

56. Long-legged spider-crab {Stenorliynchus phalangium) . 198

57. Wrinkled swimming crab {Portunus depurator) . . 202

58. Megalopa of shore crab. After Brook . . . 206

59. Mysis stage of Norway lobster. After Sars . . 206

60. Opossum-shrimp {Mysis flexuosa) . . . 209

61. Head and tail of Mysis. After Bell . . . 212

62. Idotea tricuspidata. In part from Bate and Westwood . 216

63. Gammarus locusta . . . . . 217

64. Gaprella linearis . . . . . 218

65. Sea-spider ( Pycnogonum littorale) . . . 221

66. Under surface of limpet {Patella) . . . 226

67. Chiton marginatus . . ... 228

68. Tortoise-shell limpet ( Acmcea testndinalis) . . 233 vm

LIFE BY THE SEASHORE.

FI°- PAGE

69. Trochus zizyphinus . . ... 235

70. Common whelk (Buccinum undatum) . . . 244

71. Sea-hare (Aplysia hybrida). After Gosse . . . 250

72. Boris johnsloni. After Alder and Hancock . . 252

73. Goniocloris nodosa. After Alder and Hancock . . 255

74. Ancula cristata. After Alder and Hancock . . 256

75. Spawn of Doto coronata. After Alder and Hancock . 259

76. Eolis rufibranchialis. After Alder and Hancock . . 261

77. Common mussel ( Mytilus cdulis) . . . 267

78. Tapes pullastra . . ... 268

79. Shell of Cyprina islaiulica . . . 279

80. Mactra stultorum . . ... 280

81. My a truncata . . ... 283

* 82. Razor-shell ( Solen siliqua) . ... 284

83. Corclla paralldoyramma . ... 292

84. Polycarpa rustica . . ... 295

85. Saitlie, or coal-fish ( Qadus virens). After Day . . 298

86. Sea-scorpion ( Cottus scorpius). After Day . . 301

87. Common shanny (Blennius pholis). After Day . . 306

88. Gunnel ( Gentronotus gunnellus). After Day . . 308

89. Herring-bone coralline ( Halccium halecinum). After

90. Swimming-bell of Clytia jolinstoni. After Hincks . 326

91. Nauplius of Pencils. After Midler . . . 327

92. Zoea of crab ( Thia polita). After Claus . . . 328

93. Sea-gooseberry ( Plcurobrachia) . . . 330

Life by the Seashore.

Chapter I. The General Characteristics Of Shore Animals

Conditions of shore life — The abundant food-supply — The physical conditions — Influence of the tides — The peculiarities of shore animals — Passive means of protection — Shells and tubes — The habit of burrowing — Protection against organic foes — Weapons of ofl'ence and defence — Self-mutilation — Partnerships — Colour resemblances—Masking — Dangers of storms and floods — Means of distribution— Characters of young and larval forms.

fT^HERE are perhaps few localities where the extraordinary X abundance of life is more striking than on the seashore. From the birds which circle and cry overhead to the countless myriads of sand-hoppers which spring up at every footstep, there seems to be life everywhere, life in a careless and wanton profusion the secret of which is known to the sea alone. Nowhere else does one find animals in such number and variety within a limited area. It is therefore all the more remarkable that while so many people take an interest in terrestrial animals, such as insects and land shells, relatively so few are interested in marine animals, where the field is so much wider, and the phenomena so much more striking. For every person who could name a common anemone there must be dozens who could name a common butterfly, and this in a country not a little proud of its encircling ocean. The opportunity for shore -hunting is n

LIFE BY THE SEASHORE.

nowadays given to very many people for at least a few weeks in every year, and even in this brief time it is possible to acquire not a little knowledge of the ways and structure of the common shore animals.

We shall not at present seek strictly to define the meaning of the word “shore,” but in beginning a preliminary study of the conditions of shore life, may conveniently start from that commonplace of observation, which shows that all parts of the shore area are not equally productive. It is true that wherever the ebbing tide leaves bare long stretches of sand, there will be found some of the inhabitants of the littoral waters, living or dead, according to the force of the waves which have torn them from their rocky homes ; but we all know that to find these animals in their natural conditions we must forsake the sandy beach for the weedcovered rocks. In order to understand why it is that the majority of shore animals live in the vicinity of rocks, let us watch what happens when some change of current uncovers a ridge of rock hitherto concealed by the sand. We find that the first organisms to appear are usually Algae of various kinds, the coarser kinds being often the most obvious at first. Then come acorn-shells and vegetable-eating Molluscs, and as these thrive and multiply they are followed by carnivorous whelks, buckies, and starfishes. As the weeds grow, crabs and other Crustaceans make their appearance, and the new settlement thrives apace until it contains most of the animals inhabiting the parent area. How the animals reach the new area is a question to which we shall return later ; our special concern now is what determines the. gradual colonisation, and why does it only occur where there is a solid substratum of some kind? The answer is simple; it is essentially a question of food, and the food upon which shore animals depend is most abundant in the vicinity of rocks.

Let us for-ar-moment- consider generally the food-supplies of marine animals. The simplest case is probably that of the pelagic animals, or those animals which live in the open waters of the sea. These all depend ultimately either upon the microscopic plants with which the water is filled, or upon microscopic animals which because they contain green colouring matter are able to feed like plants. The dependence is primary when, as in many pelagic worms, molluscs, | and sea-squirts, the minute plants are actually taken as food ; it is secondary when, as in many fish, the food consists of the worms, molluscs, sea-squirts, etc., which themselves feed upon the Algie. Abundantly supplied with air and sunlight, the little plants grow and multiply rapidly, and constitute the great basal food-supply of the animals of the open sea.

Many of those minute plants, or plant-like animals, occur also in the shallow shore waters, and there again constitute an important part of the food-supply, but this is supplemented in two ways. First, we have an enormous amount of material carried into the sea by rivers. It is a fact of common experience that mudbanks of varying size usually occur about the mouths of rivers. The constituent mud is brought down by the river, and it contains an abundant supply of nutrient material, of which very many shore animals avail themselves. Second, we have the large fixed seaweeds, which can flourish only in water shallow enough for the light to reach them, and which occur in great variety and abundance around our shores wherever there are rocky surfaces to which they can affix themselves.

According to their diet we may divide the shore animals into three sets: (1) those which are vegetarian in habit, living upon the large seaweeds ; (2) those which feed upon minute food-particles contained in the water or in sand and mud ; (3) those which are carnivorous and depend upon the two preceding sets for food. All these three sets find food most abundant in the vicinity of rocks. The first obviously do so because the large seaweeds grow well only when fixed to a solid base. It may not be quite so clear why the statement is true of the second set, but it is a fact that shore animals which feed on microscopic particles are sedentary animals, not capable of resisting by their own activity the force of shore currents and shore waves. In consequence they usually cannot flourish unless, like the shore weeds, they have a firm basis of attachment. The chief exception arises in the case of borrowers which often live in sand quite away from rocks. As the carnivorous animals depend upon the preceding two sets, it is obvious that they can abound only in the vicinity of the rocks

LIFE BY THE SEASHORE.

haunted by these. A little experience on the shore will soon convince you that shore animals are not quite so sharply differentiated from one another as regards food as this description seems to suggest, for some forms seem to indulge in a mixed diet; hut at the same time it may he helpful at first to look at the food-supply in this way.

So far we have seen that the shore area is above all distinguished by its abundant food-supply, hut it must not bo supposed on this account that life within this area is necessarily easy. It is indeed rather the reverse that is true. In the first place the abundant food-supply has led to a great increase of population, and a consequent increase in the intensity of the struggle for existence among the shore animals, and in the second place the physical environment is so variable as to make heavy demands on the adaptability of the organism. Look at the wreckage which almost every tide strews upon the beach, and you will realise how fierce is the struggle against inorganic nature which goes on in the shore area.

Let us look for a little at the special peculiarities of the physical environment of shore animals. Round our coasts one of the most striking of the natural phenomena of the littoral region is the daily ebb and flow of the tide. Twice in each twenty-four hours the waters retreat and leave bare a great stretch of the shore, twice they return, the breakers thundering on the rocks as they advance. As everyone who has had anything to do with the sea knows well, not only does the extent of the rise vary according to the locality, hut for the same locality it varies from day to day. Twice in every lunar month occurs the phenomenon of spring tides, when the water rises to an unusual height and sinks to a correspondingly low level. Even these spring tides are, however, not constant, certain tides in spring and autumn rising to a much greater height than the ordinary springs. Later, we shall discuss the importance of these facts to the naturalist, at present we are concerned merely with their importance to the shallow-water animals. The shore area is populated by truly marine animals from high-tide mark downwards ; indeed, certain periwinkles seem to live above the level of all but the highest spring tides. If we begin with these hardy forms and pass downwards to the region

THE GENERAL CHARACTERISTICS OF SHORE ANIMALS.

which is uncovered only at the lowest springs, we find a complete series of gradations in regard to exposure to air. The periwinkles mentioned are really under water only for a brief period daily, during perhaps a few days every six months. Then we may have other forms which are covered by water only for a short time at spring tides, and so on down to the animals which are wrccovered only for a brief period during the very lowest springs. But, as all seafaring people know, the times and heights of the tides as indicated in the calculated tables are in many localities liable to considerable variation on account of winds and storms, so that one must beware of ascribing too great constancy to tidal movements. All the animals which belong to the shore area, with a few exceptions which need not concern us here, breathe air dissolved in water, so that the fact that they are periodically exposed to the action of the atmosphere, necessitates special means of protection for the delicate breathing organs. The amount of protection required must necessarily vary with the amount of exposure.

The risk of injury to the breathing organs is not the only danger to which the ebb of the tide exposes shore animals, for the removal of the water makes feeding impossible to not a few of them, and it also exposes them to variations of temperature — the frosts of winter and the sun of summer — and to the keen eyes of the birds which flock to the rocks as the tide ebbs. Furthermore, as the water returns its waves batter furiously against the rocks and their denizens, so that these have manifold dangers to guard against.

Among the general characters of shore animals we should thus expect to find that they usually possess some means of protection against the risk of exposure to the atmosphere, with the correlated risks of freezing or drying up, and also against the force of the waves, which tend to tear them away from their rocky homes. In point of fact, we do find that shore animals show many adaptations to these conditions of shore life. In the first place, very many of them possess shells into which the animal can retire, and which serve to protect it against variations of temperature and the risk of drying up. Shells are especially characteristic of the greater number of the Mollusca, or “ shellfish ” par excellence , but are also possessed by not a few other

LIFE BY THE SEASHORE.

animals. Thus some worms, like Serpula and Spirorbis, make white limy tubes and shells into which the whole body can be retracted. The acorn-shells, which are often the commonest of all animals on the shore rocks, are Crustacea which secrete a limy shell into which the whole animal can be withdrawn, and which can then be closed to prevent evaporation of moisture. In these cases, however, the animals are completely sedentary, never moving from the place where they have settled down in youth, and from their size and shape offering little opportunity to the waves. It is otherwise with the Molluscs, which frequently possess considerable power of movement, and have, as it were, to consider both the necessity of protection from drought and from the destructive force of the breakers. We are just beginning to understand the significance of the shapes of shells considered from these points "of view, and some of the more obvious adaptations only can be pointed out here.

'Most of the molluscs of the shore have either a shell composed of two valves, like cockles, mussels, and their allies, or have univalved shells like limpets, periwinkles, and whelks (Gasteropods). Among the latter the limpet represents the simplest though perhaps not the most primitive condition. Its shell is simply conical, and protects the dorsal region of the animal only ; but as everyone knows the limpet has extraordinary clinging power. The thick shell prevents loss of water by evaporation, the firm attachment prevents dislodgment by the force of the waves. The majority of the univalved Molluscs on the shore differ from the limpet in possessing a spirally coiled shell, which is often exceedingly thick and dense, and into which the whole animal can be withdrawn. Such forms as periwinkles, whelks, tops, dog-whelks and others do not cling like the limpet, but when alarmed or attacked often drop suddenly from their attachment. As they do so they withdraw completely into their shells, and close the opening behind them by a shutter, or operculum, which exactly fits the orifice (see Fig. 70, p. 244). This done, the animal is completely encased and protected from extremes of temperature. The shell is so dense that the breakers do relatively little harm, even though the animals are rolled about roughly enough. It is believed that the shape and the sculpture of the shell

I are all of importance in giving strength to the shell, and in 1 minimising the danger of rough usage. How successful as \ a protection the shell must be is demonstrated not only by \ the great abundance of periwinkles, whelks, etc., on the i rocks, but also by the way in which they expose themselves to view when the tide ebbs, braving the dangers of frost [^and sun.

— The Bivalve shell seems on the whole less efficient as a means of protection, at least very few Bivalves live on the rocks in the exposed way in which the periwinkles and dog-whelks do. Some, like the mussels, grow in great colonies in sheltered places, very many live buried in sand, not a few burrow in rocks, but most are very liable to wholesale destruction in storms. As a rule the Bivalves have little power of locomotion ; they often spin a mass of silky threads, by means of which they anchor themselves to solid bodies, and which, as in the mussels, may constitute their chief defence against the force of the waves.

Analogous to the habit of shell-making is the process of tube/building, which is carried on by hosts of worms. In most cases the tube consists of an organic substance secreted by the animal, to which are added foreign particles such as grains of sand, or fragments of stone and shell. Among the tube-building worms are the “sand-mason” ( Terebella ), a very common form, Sabellaria, a social worm, which builds sandy tubes, and many others. In many cases these tubes must be looked on as chiefly a means of protection against organic foes, but in other cases they are strong enough to protect the animal from the dangers of its physical environment.

^ By far the most effective method of protection against these dangers is, however, the habit of burrowing. A burrowing animal obtains protection from the waves, save in great storms ; it obtains permanent moisture, a more or less even temperature, and finally is safe from the persecution of most organic foes. The list of benefits is so long that it is no wonder that so many different kinds of animals have acquired burrowing habits. We can mention only a few of them. If you stoop under overhanging ledges of rocks, or turn over weed-incrusted stones, you may often see numerous holes in the rock, from each of which a red star

LIFE BY THE SEASHORE.

protrudes. Touch these stars, and they instantly disappear, ejecting a feeble jet of water as they do so. If by means of hammer and chisel you investigate the rock, you will find that the stars are the breathing-tubes or siphons of a little bivalved Mollusc, called Saxicava, on account of its rockboring habits. The little creature remains permanently within its rocky burrows. When the rock is covered by water it protrudes its red tubes, and through them both feeds and breathes ; when the tide ebbs, or enemies threaten, it withdraws the tubes, and is safe. Another, and in some ways an even more interesting rockboring Mollusc, is Pliolas, of which one species is common in the soft fissile rock called shale by geologists. While walking over stretches of shale you may often notice that it is perforated by numerous round holes. When the rock is covered by water these holes are filled by a brown fringe, with some superficial resemblance to a sea-anemone. At a touch the fringes vanish like a shot. The shale is very soft, and can be readily pulled up in great blocks, when you will find that the holes are the openings of the burrows of PhoJas,

Fia. l. — Pholas cnspata, a ,white Bivalve, with a shell which gapes from under surface, to widely, and is beautifully toothed and

wide gapoof ufe shell? sculptured. In the Firth of Forth, where /, foot ; s, siphon. beds of shale are abundant, the rock is often simply riddled by Plwlas burrows. Other species of the genus burrow in hard rocks, and are then much less easy to extricate.

Far more numerous than the rock-borers are the burrowers in sand, which if it does not form so secure a resting-place as the solid rock is one more easily obtained, and is taken advantage of by many animals. Objection may be taken to the word “many,” in view of the fact that children often dig in the sand for hour after hour, and yet rarely come upon a living creature. But the explanation is simple. Animals which burrow in sand almost invariably live on sand; they can therefore only live in sand which is impregnated with organic particles. Such sand occurs usually in the vicinity

THE GENERAL CHARACTERISTICS OF SHORE ANIMALS.

of rocks or near the mouths of rivers, while in the long stretches of clean sand most frequented by children organic particles are remarkable for their absence, do illustrate the variety of sand-burrowing animals, I may give a list of the spoil taken by a party of which I was a member at some sands in the Firth of Clyde, near Millport. We got first a burrowing sea-anemone ( Peachia ), any number of heart-urchins ( Echinocarclium cordatum) covered with beautiful golden spines, Synajita, a curious pink worm-like creature really allied to sea-urchins, razor-shells ( Solen ), otter-shells ( Lutraria ), old maid shells ( Mya ), all living and active, any number of ringed worms of various kinds, some ribbon-worms, and many sand-eels, and all these occurred together within a very limited area, and were taken in the course of an hour’s digging.

Fig. 2. — Sand-launce or sand-eel ( Ammodytes tobianus). After Day.

One is tempted to say of each set of marine animals that they arc the most interesting of all, but surely there is a special interest about sand-burro wers ! The worms, perhaps, one might pass over, for the common earthworm has familiarised us with the burrowing habit, but how does a sea-urchin get deep down into the sand ? Those mentioned above were found in one locality, living, not in sand, but in a sandy gravel full of stones and shells. The shell or test of the heart-urchin is as fragile as glass, so thin that unless held with care one’s fingers go through it. How does it bore its way among sharp-edged stones without injury 1 So with many of the others, as the spade turns them up a dozen “hows” and “whys” crowd upon one. Digging in the sand may seem a childish pastime enough, but if you choose your sand aright it has many fascinations.

There are many other of the more sedentary shore animals which do not burrow and are not protected by a thick shell. These usually settle down in damp and dark situations where the sun’s rays do not penetrate, or they

LIFE BY THE SEASHORE.

creep under stones and into chinks and fissures of the rocks as the tide ebbs, to seek protection both from sun and wind and from the keen eyes of the birds. It is in search of these that the shore-hunter diligently turns stones and creeps under overhanging rocks, where the weeds drip and the sea-squirts eject their tiny jets of water. Most of these are protected from the force of the waves by the fact that they are attached and sedentary, or by the shape of their bodies which makes it easy for them to lurk in crevices out of harm’s way.

There are still other ways in which shore animals may escape the dangers associated with the ebb and the flow of the tide. Thus they may avoid these dangers by their own activity, following the water as it ehhs seaward, and returning with it when it once more flows landward. These are best represented on the shore by some of the Crustacea — such as prawns, some shrimps, various kinds of lobsters — and by certain fishes. In both cases, however, the power of active swimming is comparatively rare in truly littoral forms, probably because the strong shore currents make it a danger rather than an advantage. Thus, of the shore fishes, the blenny ( Blennius pliolis) remains lurking under stones often quite uncovered by water, the sand-eels ( Ammodytes tobianus) often bury themselves in the sand, where sticklebacks ( Gasterosteus ) are also at times to be found. Among the shore Crustacea, as we shall afterwards see, there is evidence that in the higher forms the power of swimming has been gradually lost, and the animals have been adapted for life at the bottom and on the tidal rocks. This has been accompanied in the crabs by a modification of the dorsal shield or carapace, which has for its object the protection of the gills from the risk of drying up. So carefully are these protected in many crabs that the animals can live for a long period out of water. In some cases, indeed, as in the common shore crab, an exposure to air during a portion of the day seems actually beneficial. While very many Crustacea and a few fishes are thus rather to be reckoned among the forms which lurk passively in hiding when the tide ebbs, there are still a considerable number who are active swimmers, and constitute the “ floating population ” of the rocks. The capture of these can only be hoped for when they are trapped in some rock pool by the ebbing waters.

The above brief account of the way in which animals protect themselves against the dangers of their physical environment may serve as an outline which your experience in actual collecting will later enable you to fill up. We may now look for a little at the ways in which the shore animals protect themselves from their organic foes. In some cases, as we have already seen, the same artifice which protects an animal from the one set of dangers protects it from the other. The fisherman’s lob-worm (see Fig. 10, p. 30) is greatly relished by very many fish ; we can hardly doubt, therefore, that it is, in an ordinary way, protected against these by its burrowing habit. Most tube-worms vanish into their tubes instantly at the least alarm, often merely at a shadow. It is reasonable to conclude that the tube affords a natural protection. It is not very uncommon on the shore to find mutilated whelks, which have apparently had their anterior region bitten off by fish before they had time to withdraw into the shell : a fact which again suggests the protective value of the shell. Facts of this kind might be multiplied indefinitely, but the protective value of hard shells is in the general case sufficiently obvious, and we may pass on to less familiar means of defence.

Many shore animals seem to be protected by their weapons, whether of offence or defence, or by some unpleasant attribute. Thus the great pincers of crabs and lobsters make them dangerous adversaries ; jelly-fish and sea-anemones are protected by their stinging-cells ; sponges are often full of sharp spicules ; many worms have an elaborate armature of bristles ; and so on. The power of self-mutilation, or autotomy, is also widely spread among shore animals, and must often assist their escape. Most of the shore crabs, if seized by a limb, will throw off the limb and escape. Brittle-stars break their rays at the slightest touch, and the separated portion keeps up active movements for some time. Not a few “worms” throw off gills or tentacles or other portions of the body when molested. In this case the separated organs move about even more actively than when attached, and doubtless distract the attention of the enemy. In all cases where autotomy is practised, the