The strandline is the most underrated part of any British beach. Most people walk straight past it on their way to the sand, but that dark ribbon of seaweed, shells and debris at the top of the tide is where the sea leaves its postcards. Winter and early spring are the best months for a look, particularly after a gale has coincided with a spring tide. Give it two hours either side of low water, walk slowly, and look at the tideline rather than along it.

Start with a light touch and a ruler

Beachcombing is far more rewarding if you treat it as fieldwork rather than shopping. Take a small bag, a pair of gardening gloves for anything sharp, and something to measure with — a bank card is a handy 8.5cm gauge. Photograph your finds where they lie before you pick them up, because a pale egg case against wet sand is much easier to identify than one dried out on a kitchen windowsill.

A good habit is to take only one or two specimens of anything common, and to leave the rest. The strandline is not rubbish: it is a habitat. Sandhoppers, seaweed flies and a host of tiny invertebrates live in the rotting weed, and wading birds work the line for food. If you turn over a clump of weed, turn it back.

Mermaid's purses: skate or ray?

These are the egg cases of skates, rays and catsharks, and they are the classic beachcombing trophy. Identification is mostly about shape.

• Skate cases are longer than they are wide, with four stiff, pointed horns at the corners. The larger species produce dramatic cases of 12cm or more; hold one up and you can see the rectangular capsule clearly.
• Ray cases, such as those of the thornback ray, are squarer and more compact, usually around 6–8cm long, with short, blunt horns.
• Cuckoo ray cases are unmistakable: the two horns at the top curve inwards to form a distinct handle shape.
• Catshark cases are small, pale straw-coloured and roughly 5–6cm long, and instead of stiff horns they have long, tangled, curly tendrils at each corner.

If a case feels wet, springy and heavier than it should, hold it up to the light. A dark mass inside with a small yolk attached means there is a live embryo in there. Return it gently below the waterline rather than taking it home.

Whelk egg cases and other eggy oddities

Common whelk egg masses are often mistaken for a lump of polystyrene. They look like a fist-sized ball of pale yellow-brown papery discs, each disc being a single egg capsule, stacked and crumpled together. They dry out to a stiff, brittle cluster. Once the capsules have hatched they are usually light and hollow; masses full of tiny developing whelks are best put back.

You may also find the smaller, chambered egg cases of the necklace shell, which look like a strip of flat, translucent beads, and the delicate coiled cases of some sea slugs. All are worth a photograph, few are worth a pocket.

Sea beans: tropical travellers on a British shore

Drift seeds are the real treasure of British beachcombing, because they have crossed an ocean to reach you. They arrive on the Gulf Stream and are most often found on west-facing coasts, particularly in Cornwall, Wales, western Scotland and Ireland. Look for:

• Sea heart — a flat, dark brown, polished disc, often 3–4cm across, with a faint seam around the edge. It comes from a tropical liana.
• Hamburger bean — rounded and glossy brown with a thick pale or black band around the middle. It genuinely looks like a small burger.
• Nickernut — grey, smooth and marble-sized, sometimes with a faint cross marking.
• Sea purse — a flattened, heart-shaped seed pod, pale and papery when dry.

Sea beans are hard, dense and rattlingly solid. If your find is soft, light and smells of the sea, it is probably a piece of plant debris rather than a true drift seed.

Cuttlebones, sea potatoes and sea glass

The chalky white cuttlebone of the common cuttlefish is a familiar find: oval, boat-shaped, with a pointed end and fine ripples across one face. It is light as a crisp and makes excellent food for garden birds, though it will crumble if you leave it in a pocket.

The sea potato is not a potato but the fragile test of a heart urchin — pale buff, eggshell-thin, with a five-pointed star of holes on top. Handle it like a biscuit. Sea glass is the other reliable favourite: genuine pieces are frosted, with edges rounded and softened by years of tumbling. Sharp, clear, brightly coloured fragments are usually modern and worth binning rather than keeping.

One warning: tiny pale pellets that look like fish eggs are nurdles, the raw plastic feedstock used by industry. They are not mermaid's tears and they do not belong on a beach. Collect them if you can, and mention them to your local beach clean group.

What to leave untouched

Some finds should be reported, not pocketed. Use your eyes, keep your hands in your pockets, and take a photograph with something for scale.

• Anything that looks like ordnance or a sealed canister. Do not move it, do not photograph it with a flash, and call 999 and ask for the coastguard.
• Palm oil or paraffin lumps. These waxy, greyish-white blobs smell faintly of soap or fuel and are highly toxic to dogs. Keep pets away and report them to the local council.
• Live animals. Starfish, crabs, anemones, stranded jellyfish and anything still moving goes back in the water or stays where it is.
• Seals, seabirds and dead marine mammals. Never approach a seal pup, no matter how abandoned it looks, and keep dogs on a lead. Report stranded or dead animals to the coastguard or a local strandings scheme.
• Anything on a nature reserve or Site of Special Scientific Interest. Many sites prohibit removing even empty shells, and local byelaws can apply to the whole beach.

Half the pleasure of beachcombing is the not-knowing. Take a photograph, take a note of the grid reference, and leave the rest of the mystery on the sand for the next low tide.