MARINE BIOLOGICAL METHODS Every biological investigation of a marine region begins with a description of the species of plants and animals that occur there. These organisms must be collected and identified, and if there are any that are new to science, formal diagnoses must be made and suitable names given to them. The habits and life histories of the various species are then studied—that is, an ecological survey is made. Collecting methods, as applied to the littoral zone, are very simple : the beach especially towards low water of high spring tides is searched; the sand and mud is raked for burrowing ani mals; the interstitial sand water is filtered so as to obtain micro scopic organisms ; rock pools containing much seaweed are dragged with small conical canvas nets in order to obtain the microcrus tacea that shelter there—and so on. The methods are all obvious.
Collecting on the sea bottom in the shallow water zone requires the use of a sailing boat. Formerly the naturalist's dredge was used but this instrument is now superseded by the fisherman's trawl-net. The dredge was simply a rectangular frame of iron about 3 to 4f t. long and gin. to a foot in breadth. The long edges act as scrapers, a bag of coarse or fine netting is laced to the frame. Two handles attached to the latter are connected with a strong rope and the whole apparatus is dragged from the boat, along the sea-bottom. The dredge is still used when it is desired to collect animals that burrow in the superficial deposits of the bottom or live there attached to stones.
The trawl-net is far more commonly employed for general col lecting. This instrument consists essentially of a wooden beam, 10 t o 3oft. long. At either end is a stirrup-shaped iron which keep the beam about a foot above the bottom. A long bent rope, attached to the irons, sweeps on the bottom. A conical bag of netting is laced to the beam and foot-rope. The whole is dragged on the sand or mud. If there are large stones the trawl-net cannot be used. There are very many forms of this apparatus ; the technique of constructing and using it is difficult, and nowadays it is quite necessary to employ fishermen for the purpose. In deep water large vessels are necessary and steam power, both for pro pelling the ship and hauling up the trawl-net, is quite essential. Trawl-nets, in the hands of capable fishermen, can be employed at any depths, but such collecting operations are laborious and difficult and their descriptions cannot be attempted here.
The organisms living in the sand and mud on the sea bottom cannot adequately be collected by the dredge or trawl. Small grabs are used for this purpose, and these consist essentially of two or more open scoops which close when they touch the bottom, thus lifting up a sample of the deposit, with its included organ isms. Peterson's bottom-sampler is such a large grab which lifts up a definite part, say square metre of the upper layers of the bottom materials. The grab is emptied on deck and the material is washed through sieves of various meshes. The organisms are thus picked out and are then preserved.
Sea bottom deposits are obtained in this way, or by the use of sounding-tubes. The latter dip into the soft bottom oozes and fill up with the material. Suitable valves prevent the ooze from being washed out when the sounding apparatus is hauled up to the surface. The ooze is usually dried and then examined micro scopically for the remains of the shells etc. of the demersal or ganisms. Nektic animals, such as pelagic fishes, crustacea, cephalo pods, etc., which frequent the intermediate strata of water, are captured by pelagic nets. These are instruments of many forms that are attached to the tow-rope at various distances from the bottom. Sometimes drift-nets, trammel-nets, etc. are used. These are large nets buoyed to a surface rope and floating vertically in the water. Fishes etc. strike against them and are enmeshed. Plankton is collected by specially constructed nets that operate on the surface, on the bottom or at any desired depth. There are very many forms of these nets. Traps are employed on the bot tom in shallow water. All these apparatus are variations of the fisherman's lobster pot. They are let down to the bottom and the rope carrying them is buoyed at the surface. Usually they are baited, and occasionally an electric lamp, fed by a cable, has been used as a lure. They are left on the bottom for a day or more and are then hauled. Hooks and lines, baited and lowered to the bottom, are also often used to collect fishes and other bottom animals. Swabs or tangles may also be used to entangle loose animals that have spines or other projecting parts, but these are usually attached to the naturalist's dredge.