Classification, the animal kingdom and the plant and protoctist kingdoms

Sorting marine life: shared features, three domains and the kingdoms of Eukarya

Classification is the sorting of organisms into groups according to the features they share. The more features two organisms have in common, the more closely related they are taken to be, so a classification built on shared features is also a statement about relationships. You need it in marine science because the sea holds everything from a single-celled diatom you cannot see without a microscope to a whale, and without groups there is no way to name them, compare them or key them out.

The largest groups are the three domains: Bacteria, Archaea and Eukarya. Bacteria and Archaea both have cells with no nucleus, and they are separated from one another because their cell chemistry and cell walls are fundamentally different.

Bacteria include the decomposers that break down dead material in sediment and the cyanobacteria that photosynthesise in surface water; archaea include organisms of extreme environments such as the hot vents on the deep sea floor. Eukarya have cells with a true nucleus and membrane-bound organelles, and every organism you can see without a microscope - every seaweed, fish, turtle and whale - belongs to this domain.

Inside Eukarya the syllabus asks for three kingdoms found in marine habitats: animals, plants and protoctists. The features that decide the kingdom are the ones you already know from cell structure: whether the cells have a cell wall and what it is made of, whether there are chloroplasts, whether the organism is single-celled or multicellular, and how it obtains its food. Animals are multicellular with no cell wall, they feed on other organisms, and most move at some stage of the life cycle.

Plants are multicellular with cellulose cell walls and chloroplasts, and they photosynthesise; in the sea the only true plants are the seagrasses. Protoctists are the eukaryotic organisms that are neither: mostly single-celled or simply built, and in the sea they include all the algae - the kelps, the dinoflagellates and the diatoms.

The binomial system: two names for every species

Every species has a scientific name made of two parts, which is what the binomial system means. The first part is the genus, the group of closely related species, and it always starts with a capital letter.

The second part is the species, and it is written entirely in lower case. Both parts are printed in italics, or underlined when you write them by hand: Orcinus orca, the killer whale, has the genus Orcinus and the species orca.

The system is internationally agreed, and that is its whole point. Common names are not reliable: the same animal may be called a killer whale, an orca or a grampus in different countries and different languages, and quite different animals may share one common name such as 'sea bass' or 'crayfish'. A scientific name belongs to one species only and is the same in every language, so a biologist in Japan and a biologist in Chile know they are writing about the same organism.

After the first full mention the genus may be abbreviated to its initial - O. orca - which is a convention worth using in an exam answer.

Because the name carries the genus, it also carries information about relationships. Two species share a genus only if they are closely related, so Delphinus delphis and Delphinus capensis are both common dolphins and are closer to each other than either is to Orcinus orca. The species part often describes the organism - marina means of the sea, as in Zostera marina - but it does not have to, and you will not be asked to translate it.

• Genus first, capital letter; species second, lower case; both italic (or underlined by hand).
• Written after the first full use as an abbreviation of the genus: O. orca, Z. marina.
• One name, one species, agreed internationally, so it is the same in every country and every language.
• The name shows relationship: species sharing a genus are closely related.
• Common names are not usable in science because they vary between languages and regions and are often shared by different species.

Marine vertebrates: the four classes and the three features that separate them

Vertebrates are animals with a backbone and an internal skeleton. The syllabus asks you to place marine vertebrates into four classes - mammals, birds, reptiles and fish - using only three features: the skin covering, the method of fertilisation, and whether gas exchange is by gills or lungs. Learn those three across the four classes and you can classify any marine vertebrate you are shown, including one you have never seen before.

Mammals have hair as their skin covering, even if it is only a few bristles around the blowhole of a whale, and a thick layer of blubber under the skin for insulation. They breathe with lungs, they fertilise internally, and the female gives birth to live young and feeds them on milk. The three marine groups are the cetaceans (whales, dolphins and porpoises), the pinnipeds (seals, sea lions and the walrus, which come ashore to breed and rest) and the sirenians (the dugong and the manatees, the only marine mammals that feed almost entirely on seagrass).

Birds have feathers, breathe with lungs and fertilise internally, and they lay hard-shelled eggs on land, which is why seabirds must return to cliffs and islands to breed. Pelagic birds such as albatrosses, petrels and gannets spend most of their lives over the open ocean and come to land only to nest, while shorebirds such as oystercatchers, curlews and sandpipers feed on the shore and in estuaries. Reptiles have dry scales, breathe with lungs and fertilise internally; sea turtles must come ashore to lay their eggs in the sand, while sea snakes spend their whole lives at sea and give birth to live young in the water.

Fish have scales, take oxygen from the water with gills, and in most bony species fertilise externally, shedding eggs and sperm into the water where they meet. The syllabus divides them into cartilaginous fish, whose skeleton is made of cartilage, such as sharks, rays and skates, and bony fish, whose skeleton is bone, such as cod, herring, tuna and seahorses. Cartilaginous fish are the exception to the general rule about fish, because they mate and fertilise internally and many of them give birth to live young.

The two methods of fertilisation are worth understanding rather than just memorising, because they explain the rest of the biology. Internal fertilisation means the male transfers sperm into the female's body, so the eggs are fertilised inside her and she can produce fewer, larger eggs and protect them; external fertilisation means eggs and sperm are released into the water and meet by chance, which is why a fish that spawns this way releases enormous numbers of eggs to leave a few survivors. That is why a turtle lays a clutch of eggs in a nest in the sand and leaves, while a cod releases millions of eggs into the current and gives them no care at all.

The external features of fish and what each one does

The operculum is the bony flap on each side of a bony fish's head that covers and protects the gills. It is not only a lid: as it opens and closes it pumps water across the gill filaments, so the fish keeps extracting oxygen even when it is not swimming forward. Scales are thin overlapping plates set in the skin, and their function is protection: they shield the body from abrasion, from the teeth of predators and from infection, and the mucus that coats them reduces friction as the fish moves through the water.

A fish has two more sense organs on the outside of the body. The lateral line is a row of small sense organs running along each flank, and it detects vibration and pressure changes in the water, which is how a fish senses a struggling prey animal, the pressure wave of an approaching predator, or the movement of its neighbours in a shoal. The nares are paired openings on the snout that detect chemicals dissolved in the water; they are used for smell and are not connected to the gills, so a fish cannot breathe through them.

There are five kinds of fin, and they matter for control as much as for movement. The caudal (tail) fin provides forward thrust and is used for changes of direction.

The dorsal fin on the back and the anal fin on the underside behind the anus act like keels: they resist rolling (turning about the long axis) and yawing (swinging from side to side), which keeps the fish tracking straight. The paired pectoral fins behind the head control pitching (nose up or nose down) and are used for steering, braking, turning and hovering, while the paired pelvic fins on the belly steady the fish against pitching and rolling.

In a diagram question these are the easiest marks on the paper, because every fin is named by its position. Label them in order: dorsal on the back, caudal at the tail, anal on the underside behind the anus, pectoral as a pair behind the operculum, and pelvic as a pair on the belly below and behind the pectorals. Then add one function to each label - and do not forget that the pectoral and pelvic fins are paired, which is one of the features that separates a fish from an animal such as a starfish, whose arms are arranged in five.

Crustaceans and molluscs: two groups with hard coverings

Crustaceans include crabs, lobsters, prawns, shrimps, krill, barnacles and the copepods. They have bilateral symmetry, which means one plane down the middle divides the body into mirror-image halves.

Their most obvious feature is the hard exoskeleton, made of chitin and often stiffened with calcium carbonate, which covers the whole body and protects and supports it like armour; because it cannot grow, the animal must moult and expand before the new shell hardens. They have compound eyes built of many separate lenses, two pairs of antennae on the head for touch and smell, and an abdomen divided into segments, each carrying a pair of jointed legs - the feature behind the group's name and its sideways scuttle.

Molluscs include limpets, snails, mussels, oysters, scallops, clams, chitons, squid, cuttlefish and octopus. They have bilateral symmetry and an unsegmented soft body, which is the key difference from an annelid: there are no rings or repeated segments anywhere along it. Most have a shell, and the syllabus asks you to notice whether it is external or internal.

A mussel, limpet or snail carries its shell on the outside; a squid or cuttlefish has a reduced shell inside the body; an octopus has lost the shell altogether. The soft body is usually enclosed in a fold of tissue called the mantle, and most molluscs move on a muscular foot - a single flat sole in a limpet, a ring of arms in an octopus.

The two groups are easy to confuse when both are hard-shelled, so use the appendages to separate them. A crab has jointed legs, compound eyes and two pairs of antennae; a mussel has none of these, only a soft unsegmented body inside two shells joined by a hinge. In a comparison question, give the feature they share - bilateral symmetry, a hard covering - and then the features that differ.

Barnacles are the trap in this group. An adult barnacle looks like a small limpet glued to a rock, with plates forming a cone and no visible legs, so it is easy to record as a mollusc; but it is a crustacean, because its larva is a free-swimming nauplius and the adult feeds by extending feathery jointed appendages through the opening in the plates. If a photograph shows a hard cone on a rock and asks for the group, look at the animal rather than the shell.

Cnidarians, echinoderms and annelids: symmetry, stings, tube feet and bristles

Cnidarians are the jellyfish, sea anemones, corals and siphonophores such as the Portuguese man o' war. They have radial symmetry: the body is built around a central axis like a wheel, so any plane through that axis divides it into similar halves, and there is no left or right, front or back. Around the single opening - which serves as both mouth and anus - is a ring of tentacles armed with stinging cells.

Each stinging cell holds a coiled harpoon under pressure; when it is triggered the harpoon fires into prey or an attacker and injects venom, which is how a jellyfish paralyses a fish and how a sea anemone holds whatever drifts against it. The body is only two layers of cells thick with a jelly-like layer between, and there are no organs at all.

Echinoderms are the starfish, sea urchins, brittle stars, feather stars and sea cucumbers. Their symmetry is pentaradial: five similar parts arranged around a central disc, which you can see at once in a five-armed starfish.

Their skin is spiny, stiffened beneath by a skeleton of calcium carbonate plates, and they move and feed on tube feet - hundreds of tiny muscular tubes, each worked by water pressure, that grip rock, pull the animal along and prise apart the two halves of a mussel's shell. A starfish that loses an arm can grow a replacement, which is one reason a bed of mussels can be cleared by a small number of them.

Annelids are the segmented worms: ragworms, lugworms and the bristle worms that live in sediment and under stones. They have bilateral symmetry, a segmented body of many similar rings, and a soft body with no shell and no exoskeleton.

Segmentation is not just a pattern: because each ring has its own muscles and can be shortened or lengthened separately, the worm can push part of its body forward while the rest anchors it, which is how it burrows. Each segment carries setae, short bristles that grip the sediment or the wall of the burrow and stop the worm sliding backwards, and they give the group its common name.

Symmetry is not decoration; it follows from the way an animal lives. A radially symmetrical cnidarian has no front end because it does not travel in a chosen direction - it drifts with the current, or it fixes itself and waits for food to arrive from any side, so a ring of tentacles around the mouth is the efficient arrangement.

A bilaterally symmetrical animal such as a crab, a ragworm or a fish has a front end that meets the environment first, with the sense organs and the mouth concentrated there, and a body that can be steered. Echinoderms are the odd case: they creep slowly over rock in any direction on their tube feet, and five-part symmetry suits an animal that can move equally well whichever way is 'forward'.

The plant kingdom: seagrasses

Seagrasses are the only marine flowering plants, and they belong to the plant kingdom. The genera to recognise are Zostera (eelgrass), Posidonia (Neptune grass), Thalassia (turtle grass) and Halophila.

They are multicellular, their cells have cellulose walls and chloroplasts, and they photosynthesise - which are precisely the features that place an organism in the plant kingdom. They grow rooted in soft sediment in shallow water where enough light reaches the bottom, and their leaves are long ribbons, which is where the name comes from.

The four features the syllabus asks for are leaves, roots, rhizomes and flowers, and each does a different job. The leaves are the organs of photosynthesis: flat, thin and ribbon-like, with a large surface area for absorbing light and no waxy cuticle, because in water gas exchange can take place over the whole leaf surface. The roots do two jobs: they secure the plant in the sediment so it is not washed away by waves and currents, and they obtain minerals such as nitrates and phosphates from the sediment.

The rhizomes are horizontal underground stems that run through the sediment and connect individual plants into one meadow, and they are how seagrasses reproduce asexually: a rhizome grows outwards, sends up a new shoot and the meadow spreads. The flowers are the feature that makes seagrasses plants rather than algae, because flowers are the organs of sexual reproduction.

Seagrass flowers are small and inconspicuous, often hidden between the leaves; pollen is released into the water and reaches another flower, and the seed that follows can start a new plant some distance away. A meadow may therefore be one plant spread by rhizome, a mixture of plants grown from seed, or both.

The comparison with seaweed is worth writing out, because it is the commonest confusion in this section. A seagrass is rooted in the sediment and draws minerals from it, has vascular tissue running through its leaves and rhizomes, and reproduces by flowers and seeds; a seaweed such as kelp has none of these, only a holdfast, a stipe and blades, and it releases spores rather than seeds. Both photosynthesise, both live in the sea, and only one of them is a plant.

The protoctist kingdom: kelp, dinoflagellates and diatoms

Kelp is a macroalga - one of the large seaweeds - and it belongs to the protoctist kingdom, not to the plant kingdom. The genera to know are Laminaria, the kelps of cold rocky coasts, and Macrocystis, giant kelp, which forms underwater forests along the Pacific coast of North America. A kelp has no true root, stem or leaf and no vascular tissue; instead its body is a thallus in three parts, and each part has its own job.

The blades are flat, and they are the surfaces for photosynthesis; being thin and held up in the water, they expose a large area to the light, and like any photosynthetic organism kelp has chloroplasts in its cells. The stipe is the stalk that acts as a stem: flexible and strong, it holds the blades up towards the light, and the blades develop from it.

In some species, including Macrocystis, there are also gas bladders - gas-filled floats at the base of the blades - which hold the blades up near the surface where the light is strongest. The holdfast is the root-like structure that anchors the kelp to rock or another hard substrate, and this is the part students get wrong: a holdfast grips, but it does not absorb minerals the way a root does.

Dinoflagellates and diatoms are the microscopic protoctists, and both are single-celled with chloroplasts for photosynthesis, so both are producers at the base of marine food chains. A dinoflagellate is identified by its two flagella: one lies in a groove around the cell and spins it, the other trails behind and steers, so the cell can swim towards light and nutrients.

Some dinoflagellates are not photosynthetic and live as parasites, and some of the photosynthetic ones multiply so fast that the water turns red - the so-called red tide - and release toxins that kill fish and can make shellfish dangerous to eat. A diatom is identified by its silica skeleton, a case of two halves that fit together like a pill box, often with intricate patterns of pores; the two halves are what you see under a microscope, and when the cells die their cases sink and accumulate in the sediment.

Practical skills: identifying, drawing, measuring and keying out specimens

Identification starts with a hand lens, a light microscope or a photograph. Work through the features in a fixed order - symmetry, then segmentation, then appendages, then shell or skeleton, then surface - and match the specimen against the description of each group rather than guessing from a name. A specimen with five parts and tube feet is an echinoderm whatever its colour; a specimen with two pairs of antennae is a crustacean whatever its size; a specimen that is soft, unsegmented and inside a shell is a mollusc even if it looks nothing like a snail.

When you compare specimens, from cells to whole organisms, record the same features for each one in the same order so the comparison is fair: size, shape, symmetry, whether the body is segmented, what appendages it carries, what covering or shell it has, and whether chloroplasts are visible. Comparing a diatom under the microscope with a mussel in your hand is a comparison of scale as well as of structure, so state the magnification and the actual size of each. That is what turns a list of observations into a comparison that earns marks.

Drawing is marked on accuracy and on convention. Use a sharp pencil, single continuous lines, no shading, the correct proportions and a large drawing; label with straight lines that touch the structure and do not cross, and put a title and the magnification underneath. Magnification comes from magnification = image size / actual size, and the two lengths must be in the same unit before you divide.

Rearranged, actual size = image size / magnification. A diatom drawn 60 mm long when the real one is 0.05 mm has been drawn at 60 / 0.05 = ×1200; a structure 2 mm long drawn 10 mm long has been drawn at ×5.

Dichotomous keys are how specimens are identified when you do not already know the answer. A key is a series of pairs of statements - couplets - in which the two statements are alternatives; you choose the one that is true of your specimen and follow where it leads, either to another couplet or to the name of a group.

Every pair must be based on easily seen external features, and the two statements must be genuinely exclusive: shell present / shell absent is a usable pair, large / small is not, because nobody can say where large stops. A key to the five invertebrate groups can be built in four steps.

• 1a Body with jointed legs and a hard exoskeleton -> crustacean
• 1b No jointed legs -> go to 2
• 2a Tentacles with stinging cells -> cnidarian
• 2b No tentacles with stinging cells -> go to 3
• 3a Body in five similar parts, with spiny skin and tube feet -> echinoderm
• 3b No tube feet -> go to 4
• 4a Body divided into many similar rings, with bristles -> annelid
• 4b Body not divided into rings, soft, usually with a shell -> mollusc
• For magnification, convert both lengths to the same unit first: 1 mm = 1000 µm, so 60 mm = 60 000 µm, and 60 000 / 50 = 1200.