Field Studies: Fresh Water Stream. 1 Freshwater stream: lower region (Lam Tuen River) The river is slow flowing, with its surface mirror like. The oxygen tension in water is relatively low. Sediments of smaller size can deposit. 2 Fresh water stream: upper region
The river is slow flowing, with its surface mirror like. The oxygen tension in water is relatively low. Sediments of smaller size can deposit.
Upper region of the stream is having a rapid flow of water, river surface is rough with some waves observed among rocks. the oxygen tension in water is relatively high; only the large size particles (pebbles) can deposit, the finer particles are being carried away.
Large particles like stones and pebbles can be seen. This kind of substratum provides a firm support or attaching site for the organisms against the rapid water current. Thus, organisms can be found under rocks and among rock crevices.
Muddy particles can only deposit when water current is sluggish. The organic deposits are mainly of leaf litter brought into stream from the woodland at up-stream. These provide the substrate for bacterial decaying action and add nutrients to the habitat.
Found in wet soil near stream. Stem soft, jointed, greenish, resembles grass. Leaves are tiny teeth-like structures found at nodes of the stem. May reach 1.5 m high. they are eaten by birds and herbivorous animals.
An annual plant at shallow water, with erect stem. Leaves are alternate and tapering at both ends. Inflorescence is slender, thread-like spike of red flowers. The plant serves food for aquatic birds and herbivores.
A spread herb with creeping stems which are sometimes rooted at the nodes. Common on swampy areas. Leaves wavy, 3-7 cm long, with pointed tips. Parallel-veined with sheathing petioles.
A caterpillar can be found on the petal. flowers about 1 cm in diameter, petals are attractively blue. 2 lateral petals are longer than the medium third petal. Flowering in late summer and autumn.
9 Ludwigia adscendens水龍,過塘蛇 (Water Dragon; water primrose)
A perennial herb in shallow slow-flowing water, lower part creeping on mud and upper part floating. Each node with white, fleshy, sac-like structures for breathing and floating. Leaves oval, 1.5 – 5 cm long, with a short petiole and blunt tip. The plant is used to feed pigs.
Flowers are single, bisexual, white to yellow, axillary. 5 oval petals, 1 – 1.2 cm long, 6-8 cm wide. Stamens 10; stigma swollen, with 5 shallow lobes. flowers occur in summer and autumn.
11 Hydrilla verticillate (Water Weed)黑藻
A perennial weed immersing in slow-flowing region. With branching stem, up to 2 m long. Leaves in whorls, each leaf is 5 – 15 mm long, narrow, thin and stalkless. the common food of aquatic herbivores, such as snails.
12 Tubifex tubifex (Tubifex Worm)游絲蚓
They are small worms found at the bottom of stream, hiding in the muddy substratum under stone, most abundant in polluted streams, since they can breathe anaerobically. they form tubes in mud and extend part of their bodies into water. Feed on suspended organic debris.
13 Glossosiphonia sp. (Pond Leech)碧蛭
Aquatic leech usually found on submerged rocks or on plant surface. With anterior and posterior suckers, hermaphroditic. Feeds on aquatic snails by sucking them out of their shells. Also frequently found in polluted water.
14 Compsoneuia sp. (Mayfly nymph)
This is the common flattened form of Mayfly nymph found in Hong Kong. they are aquatic and frequently found crawling on submerged stones. They are easily recognized by the 3 long tail filaments and the lateral abdominal (tracheal) gills for respiration. With 3 pairs of crawling legs, Feeding on organic debris and plankton.
15 Tramea virginia (Dragonfly nymph)蜻蜓若蟲
The nymphs of dragonflies are aquatic & predacious. They normally feed on worms, other insect larvae, tadpoles and even small fishes. They also possess the mask-like labium Can be found crawling on rocks, breathing by the rectal gill basket which is irrigated by water brought in through the anus.
The aquatic nymph of dragonfly grows ins size by several moultings under water. But the last moulting takes place on the emerging leaf of the waterside plant. The adult dragonfly gradually crawls out from the slits on the dorsal thoracic segments. The insect can fly when its wings become dry.
Dragonflies are large robust insects, reaching 8 cm long with wingspan of 13 cm. Wings are broad at the base, held laterally at rest. Compound eyes are huge, occupy most of the head.
Morphologically similar with the Zygonyx, except that it is red in colour. Found along stream sides.
The aquatic nymphs of damselflies are found crawling on river bottom. Breathe by the 3 caudal gills, catch prey by the extended mask-like labium. Beneficial insects since they catch midge larvae, mosquito larvae, etc.
Resembles the dragonflies but more delicately built, around 5 cm in length. The wings are held longitudinally over the body at rest. Two pairs of wings are of same size, with thin bases.
Damselflies feed on mosquitoes, midge and other small flying insects. They eat their prey by holding it with their legs and eat with their biting and chewing mouthparts.
A red species is clinging on the leaf of a grass plant. The pair of compound eyes are situated laterally,
23 Perla spp. (Stonefly) nymph石蠅若蟲
The nymphs are present under stones in water. The nymphal stage may last 2-3 years including 20-30 nymphal instars. Breathing actively by pressing its body up and down, gill tufts can be found at thorax. Feeding on green algae, diatoms or small worms. The last nymphal instar crawls onto emergent rock surface and moults into the winged adult.
24 Velia sp. (Water Cricket: Small Pond Skater)水黽
A small bug found skating rapidly on water surface. With piercing and sucking mouth-parts, it feeds on small insects.
A relatively large water bug found on slow flowing region of stream, also skating rapidly to and fro. They feed on small insects (alive or dead) on the water surface. The nymph resembles adult but with a tiny abdomen, whereas the adult is winged.
A water bug swimming upside down n water which is fiercely predacious on insect larvae, worms, tadpoles, etc.
27 Ganonema sp. (Caddisfly) larva: in leaf case石蠅幼蟲
The larva lives in the case formed by 2 leaves (other forms of cases are the sand-grain tubes, small-stone cases, silken webs, etc.
28 Anopheles spp. (Malaria mosquito)虐蚊
The male feed on plant juices, but the female are able to pierce the flesh of man or mammals into which she injects an anti-coagulant and sucks up blood. Usually hiding among the plants near the stream. Eggs are laid on stagnant water surfaces. This mosquito is famous as a transmitting agent of the Malarial fever which is caused by the parasitic protozoa, Plasmodium vivax. It can be easily distinguished from other mosquitoes, since its legs are holding upwards during rest.
29 Psephenus sp. (Water penny) dorsal view
Water penny is the larval stage of Riffle beetle. It is found on the undersides of rocks and stones. Its dorsal side is blackish brown, providing camouflage on rocks. Its body is thin and soft, pressing tightly on rocks against the rapid water current (positive thigmotaxis).
It is oval in shape, the white gills and 6 waving legs can be seen on the ventral side. It can crawl over the rock surface if necessary. Feeds on organic debris and planktons.
Very similar in appearance with the Cybister (龍蚤) which is a predator. It is a scavenger feeding on decaying vegetable matter. It gets oxygen from the atmosphere by periodic visits to the surface. It is propelled by ear-like hindlegs which are fringed with stiff hair. The brown aquatic larva is of another beetle, Stenocolus sp. (Fam: Ptilodactylidae) whose adult form is not aquatic.
32 Potamon anacoluthon (mountain freshwater crab)
It is a bright coloured crab found at the upper region of stream and in the forest. It is semi-terrestrial, crawls around stream sides, seeking for invertebrate preys like worms and insects.
33 Limnaea sp. (fresh water snail)椎實螺
Usually found gliding on submerged rocks, with a spirally coiled calcareous shell for protection. It feeds on algae and vegetable matter by a toothed radula. Snails are hermaphrodite, and they lay fertilized eggs on rock surface or on water plants, which are white, protected by gelatinous mass.
Although the snail are hermaphroditic and can perform self-fertilization, they are equipped with complicated copulatory processes which enable cross-fertilization. The genital opening is situated behind the head, at the dorsal side of the muscular foot.
35 Rana sp. tadpole蝌蚪 Family: Ranidae蛙科
The larval stage of frog is aquatic, found swimming at the bottom of streams. A newly hatched tadpole feeds on the yolk in its yolk sac and respires via external gills. At the later stage, it feeds on water plants; the external gills shrivel and the internal gills then function.
The young frog after metamorphosis leaves the pond, but still with a stumpy tail which will eventually be reabsorbed. It may require up to four years to reach maturity. It feeds on a variety of invertebrates including worms, slugs and flies.