Samoa Guide, 26. Life in a Samoan stream

The other type of shell is the cap-like Septaria species. It has no visible operculum and therefore the snail cannot retract into its shell. Instead it clamps to rocks with its muscular foot, making it very hard to detach. This is useful when you live in turbulent rapids or on steep walls. Besides these two differences, you may also notice variations in shell color, pattern and texture. They range from light to very dark brown with zigzag, wavy, or straight lines, or none at all. They may be smooth and shiny or grooved and wrinkled. Some even have horns.

A small turret shaped snail (Melanoides sp.) can also be found near the edge of streams in gravel and mud. This species is found throughout the Pacific.

Shrimp. Probably the most versatile freshwater species are the shrimp. American Samoa is home to two families, Atyidae and Palaemonidae, each with 4-5 species. The palaemonid Macrobrachium shrimp are possibly the most well known and can be easily seen in the lower to middle reaches of our streams. They are easy to identify by their large pincers. When wading through pools and shallow runs, watch for these shrimp as they make a quick retreat at your approach. Although shy, they will reappear if you keep still. These scavengers feed on both plant and animal material. They may also dislodge Neritina and Corona snails from rocks, open them up and devour them.

Atyidae shrimp can be found throughout many clean streams. Some are excellent climbers and congregate in the lee of rocks and boulders of fast flowing streams. Others seek a quiet place in the vegetation and roots of plants near a stream edge where flow is slow. These shrimp exhibit distinct feeding behaviors. In fast flowing water they orient themselves in the direction of the current and spread the fine brushes on their front legs to form upturned umbrellas. They use these to filter food particles carried in the current and then push them into their mouth. In slower moving water they use their brushes to pluck food items from plants and gravel.

Stream insects. Although some insects are common inhabitants in streams around the world, they are not well-represented on our small oceanic islands. Few of the terrestrial insects we see in American Samoa have an aquatic stage in their life cycle (some exceptions being a few dragonflies, damselflies, midges, mayflies, and beetles).

Herbivorous fishes, like the small gobies (Stiphodon spp. and Sicyopterus spp.), are usually present in large numbers. They may be seen in quiet riffles and pools grazing on long green algal strands or the slippery brown algae coating the rocks. An interesting feature of the goby is that its pelvic fins form a suction cup that enables the goby to cling to rocks and climb waterfalls. One goby (Stiphodon hydoreibatus) is endemic to Samoa and found nowhere else in the world.

Snails. Stream snails, which all belong to the family Neritidae, are herbivores that graze on the algae growing on stream rocks. About a dozen species are known locally. Two distinct types of neritid snails are present. One is the rounded black/brown shell of the Neritina or Clithon species. The shell of these has little or no spire and an interesting device called an “operculum” that acts like a trap door. This allows the snail to retract into its shell and literally close the door -- a great mechanism to have when you do not want to be eaten.

Streams are the plumbing of our islands. They rely solely on rain, which seeps from steep rainforestclad hills and trickles through the porous volcanic soil carrying with it the products of the land. Small streams join with others to create a mosaic of channels across the islands. Waterfalls, cascades, pools and riffles make each stream unique and influence the kinds of animals found there.

Our freshwater stream animals play an important role in maintaining a healthy stream ecosystem. These species might be overlooked because they are shy and rather blandly colored, which might lead to the assumption that they are not there at all, but that of course is not true. The best way to observe them is to find a spot on a rock and quietly watch until they emerge from their hiding places. Keep in mind that from the animal’s point of view there is a lot going on. Animals are constantly interacting with each other as well as with their environment. The presence of a predator, for example, will have a huge influence on the behavior of its prey. It is a constant game of cat and mouse, eat or be eaten. Habitat type, or where an animal likes to live, is also very important. Certain species prefer pools, others like shallow riffles, and for most species, waterfalls are barriers to their upstream movements although a few hardy species can climb up them.

Fish. Predatory fishes, like the freshwater eel (Angulia spp.), mountain bass (Kulia spp.), and dusky sleeper (Eleotris fusca) tend to dominate the lower reaches of a stream and feed on smaller fish and shrimp. They are only average climbers and cannot make it up the steep slopes of a waterfall. Instead they rely mainly on their powerful swimming and gliding ability to move through small rapids and over large boulders and rocks. Look for eels in crevices and holes. Mountain bass are often seen in pools not far from the ocean. Their silvery sides and flag-like tails make them easy to spot. The dusky sleeper uses stealth and surprise to catch its prey. It props itself on rocks using its fins and turns darker for camouflage when ambushing prey. For this reason this species is not an easy one to spot.