Limnology (2+1)

Unit 5- Biological relations

5.3.1. Relations of organisms to movement of water, surface film relations
Relations of organisms to movements of water
Movements of water, in the various forms, affect aquatic organisms in many ways, directly or indirectly, and often play very important roles in aquatic environments.
a) Effects upon sessile animals
Different growth forms in the same animal are the result of presence or absence of water currents or movements especially in fresh water sponges.
The unbranched colonies are the result of unfavorable conditions, but this can scarcely be credited when both the branched and the unbranched forms occur side by side in the same water and on similar supports. Bryozoa have also been supposed to develop different growth forms in standing and in moving water.
b) Effects upon motile animals
Many motile animals show a definite orientation response to current; i.e., they exhibit either positive or negative rheotropism. Orientation reaction may be accompanied by locomotor activities, so that certain animals will not only head upstream but will swim, either maintaining their orginal position or making progress against the current. Sometimes the response to current depends upon some important event in the life history such as sexual maturity.
Atlantic smelt, established in the upper waters of the Great Lakes, which while essentially a lake inhabiting fish, becomes positively responsive to current at the onset of spawning season and exhibits spawning “runs” at night into certain adjacent inland waters flowing into the Great Lakes.
Water in motion imposes pressure against certain surfaces of the animal, and it has been held that equality or inequality of current pressure on different parts of the body affords the stimulus to orientation of some aquatic animals, which, if true, furnishes an instance of the direct effect of current.
Certain fishes are supposed to orient in response to visual impressions as they float downstream (Clausen et al., 1931) but still other fishes have been thought to orient in response to the rubbing of parts of the body on the bottom as the current make them to floats downstream. The visual theory seems ineffective in those instances of runs at night (smelt) or in very turbid waters.
Certain phenomena such as morphological or physiological, may either be caused by, or correlated with, movements of water and their different velocities. A general correlation exists between the rate of flow and the shape of mussels in eastern Bavaria.
Current demand
Certain aquatic organisms exist permanently only in the presence of appropriate movements of water, and it is now known that current is demanded by some of them. For example, black-fly larvae (Simulium) of all species (one possible exception in Asia) inhabit only in rapidly running water. Wu (1931) has shown, among other things, that these larvae possess an inherent demand for current and that their universal absence from standing waters is due directly to the absence of the necessary current.
Secondarily, current is also related to Simulim larvae in such matters as proper food delivery and respiration.
Resistance to water movement
In general, animals which meet this problem successfully do so by means of one or more of the following features: (1) body form which offers least resistance, such as the streamline or the hemistreamline form; (2) unusually well developed burrowing or clinging habit; and (3) special forms of attachment to fixed, supporting objects.
A few may maintain their position because of unusually effective attachment devices (powerful adhesive suckers of the larvae of Blepharoceridae.
Provisions for clinging and attachment
Among the numerous, special provisions for increased efficiency in maintaining position in the face of strong water movement are the following:
1. Strong, recurved tarsal claws.
2. Exceedingly flat ventral surface.
3. Strongly depressed body.
4. Lateral margins of head and thorax produced in the form of flat margins for increased contact with the supporting object.
5. Legs, when of large size, flattened horizontally and applied by their sides as well as by the tarsi to the supporting object.
6. Special flattening of gills or the modification of the entire gill series to form a ventral attachment disk.
7. Special sucking disks. Examples: blepharocerid larvae; leeches; nymphs of May fly, Ephemerella doddsi.
8. Ventral adhesive pads, often bearing recurved spines. Examples: certain stream inhabiting aquatic Hemiptera and May fly nymphs.
9. Terminal attachment disks. Example: Simulium larvae posterior disk a combination of a row of hooks with a gelatinous secretion originating from the mouth.
10. Threads which anchor the animal directly to the support. Example: thread used by Simulium larvae when shifting position.
11. Threads which anchor case or shelter of animal. Example: certain caddis fly larvae.
12. Shelters, tubes, or cases which protect against the wash of currents and waves. Examples: sand constructed case of caddis fly, Molanna; cases of certain midge larvae; egg capsules of leeches; tubes of tubificid worms.
13. Adhesive secretions. Example: common hydra.
Provisions for burrowing
Burrowing is often accomplished by animals having no special structural provision for that purpose. In such instances, they are merely capable of forcing their way into bottom materials, aided by such features as (1) more or less pointed anterior end; (2) body movements of a penetrating sort; (3) setae; (4) longitudinal contraction and extension of a portion of the body; (5) extensile and protrusible body tubercles; and (6) strongly muscular body walls accompanied by freely moving, soft, internal organs and fluids. By such means as these, some of the softest bodied aquatic animals (Oligochaeta and others) penetrate the hard packed sand of barren, exposed shoals, thus maintaining their position in the presence of the strongest wave action.
Other animals have developed special structural features for effecting partial or almost entire penetration of bottom, such as (1) the flattened, shovel like, anteriorly directed front legs, the posteriorly directed hind legs deppressed to the body and adapted for pushing, and the pointed sloping head of the nymphs of the May flies Hexagenia and Pentagenia, and the dragon flies Gomphus; (2) the long, upturned, mandibular tusks of burrowing May fly nymphs; (3) the muscular foot of clams and snails; (4) the long, spraddling, spider like legs of certain dragon fly nymphs (Macromia), so oriented on the body that they rest full length upon the sand and, wriggling movements, work the sand entirely over them thus gaining a certain anchorage; and (5) the strikingly flattened, shoal inhabiting dragon fly nymphs (Hagenius), which weight themselves down by working sand on top of the thin abdominal margins.
Burrowing by some species may be a direct response to excess light, but the end result of maintaining position remains the same.
Habits facilitating resistance to water movement
Animals, which, lacking special structural developments, manage to maintain position in current or wave-swept areas by reactions. These are exemplified by the habitual seeking of (1) the protected sides of and the interstices between rocks; (2) fissures in bottoms and bottom materials; and (3) the more protected parts of rooted plants.
Influence on construction activities
Construction processes of certain animals can be properly performed only in the presence of water movement. A striking instance is that of the net building caddis fly larvae which can produce their nets only in moving water; in calm water, the attempt results only in a shapeless mass of threads.
Distribution of organisms
Since moving water is an effective transporting agent, movements of water play a very active part in the distribution of many aquatic organisms. Pieces of aquatic plants bearing various eggs, larvae, pupae, and even adults of insects, hydra, Bryozoa, Mollusca, and many others break from their attachments and drift with the water.
Molar agents
Sand, fine gravel, rocks of various sizes, and sometimes even boulders, carried or rolled by the water, become a veritable wearing, grinding, fragmenting machine which constitutes one of the serious menaces to the whole biota of those situations.
Indirect effects of water movement
Water movement is concerned with the life of aquatic organisms in a number of ways, the following being among the most important:
1.Constant shifting of bottom materials on shoals and other shallow waters may prevent the rooting and, therefore, the occupancy of these areas by higher aquatic plants.
2.Erosion or transportation of materials may completely alter the environment, converting it into some very different type for which the organisms are not suitable. The example of the cutting off of a sand-spit beach pool from the body of a lake.
3.Circulation, and in some instances the return to circulation, of essential nutritive substances in the water, both dissolved and suspended.
4. Production and maintenance of turbidity thus affecting the light penetration and certain other relations.
5.Delivery of food to sessile or sedentary animals, particularly when the food is in the nature of suspended, living organisms (plankton) and suspended, finely divided, nonliving materials.
6.Respiratory relations, such as (a) renewal of properly oxygenated water to respiratory surfaces and (b) renewal of dissolved oxygen supply from the air by the surface agitations incident to water movement.
7.Temporary exposure to air, as in seiches which imitate the ebb and flow of a tide and which, if of sufficient magnitude, may expose for a time a whole set of shallow water organisms to evaporation and other serious hazards.
Surface film relations
The surface film serves as a mechanical support for organisms and miscellaneous particulate materials. Both surfaces of the film may function in this way. The term neuston, originally applied to minute organisms, is now commonly extended to include all organisms associated with the surface film. Those related to the upper surface of the film comprise the supraneuston; those related to the lower surface, the infraneuston.
The larger animals commonly associated with the supraneuston are: (1) water striders (Gerridae); (2) broad-shouldered water striders (Veliidae); (3) water measurers (Hydrometridae); (4) hebrids (Hebridae); (5) mesoveliids (Mesoveliidae); (6) whirling beetles (Gyrinidae); (7) springtails (Collembola); and (8) certain spiders.