Searching for intertidal isopods can be quite challenging (though hopefully rewarding!), and this page offers some advice and tips for optimizing your chances of finding them.
Dangers, safety and regulations
The coast is, quite frankly, a hazardous place, and fieldworkers must exercise due caution whilst searching for intertidal isopods. Most importantly, you should never visit the intertidal zone without knowing tide times and local tidal conditions, and should always ensure that there is a safe route back to dry ground once the tide starts to rise. In some places the tide comes in extremely quickly and it is often the case that you will find yourself on a bit of higher ground in the intertidal zone, so there is a very real risk of stranding and drowning if due care isn't taken (especially if your attention is focussed on searching for isopods). Other drowning dangers include waves (especially freak waves which by their nature are unpredictable), deep rockpools and drops into deep water at the tidal edge. In addition, you should be aware of weather conditions, especially since it is usually much colder at the coast, and should dress accordingly to avoid hypothermia (or indeed sun burn, even on cooler days). Getting wet will dramatically increase the chances of hypothermia. Other dangers in the intertidal zone include falling (rocks, especially when covered with seaweed, can be extremely treacherous, and there are many holes and deep crevices in the intertidal zone), getting stuck in mud in estuaries and mudflats (don't underestimate the danger of this!), as well as cuts (e.g. from barnacles and sharp rocks), bites and stings. There are many ways to get injured or even killed in the intertidal zone, so make sure to know tide times and local conditions, dress appropriately, and inform others where you are going (a mobile phone is useful for emergency contacts, but isn't much good if you've fallen in water or if there is no signal). Remember, the most important rule of thumb is that no isopod record is worth injury never mind death!
Fieldworkers should also be aware of any local regulations in place, including protective measures and access restrictions. At all times, fieldworkers should aim to minimise their impact on coastal environments.
When to look for intertidal isopods
Your chances of finding particular intertidal isopod species are especially affected by the state of the tide, but also by season, weather and, in some cases, time of day, though there is much we still don't know for definite. Low tides, especially low spring tides, are particularly useful when searching for marine isopods, as many species are only found well down the intertidal zone (indeed some, such as Stenosoma lancifer, will only be encountered right at the lowest margins of low spring tides). Even for species found further up the intertidal zone, a low tide allows for more searching time, whilst certain species (e.g. Eurydice pulchra) are most easily found as the tide rises. The effects of season and weather are more subtle and are sometimes unpredictable; Idotea and Jaera, for example, seem to be more common intertidally in winter, whilst some species (e.g. Paragnathia formica) may be more easily found in the summer due to their life cycle. It is rarely worth the effort (and indeed unsafe) to look for isopods in stormy weather, though a visit immediately after a storm can produce good finds as various species (including Idotea and Cirolana cranchii) may be driven onshore by stormy weather. Whilst night-time visits to the coast are probably best avoided in most cases, some species (e.g. Cyathura carinata) are more easily found in darkness. Early morning and late evening visits to the shore can often be productive for swimming species such as Idotea.
Where to look for intertidal isopods
Marine isopods can be found on all kinds of coastline, including sandy beaches, rocky shores, harbours, estuaries and saltmarshes. Sheltered rocky shoreline with a shallow gradient, lots of rockpools and a good variety of seaweeds is probably the most isopod-rich habitat. Different kinds of habitat hold different suites of species, and this will be affected further by many factors, including geographical location, sea temperature, local tidal, wave and weather conditions, salinity levels, the nature of the substrate and the plants growing on and around it, and levels of human disturbance. The individual species accounts give details about the habitat preferences for each species, though our knowledge of this is often basic and records which provide detailed environmental information are particularly valuable.
So you've got yourself down to the intertidal zone in the hope of finding some isopods, but how exactly do you find them? Although there's a fair amount of luck involved, here are some of the techniques you can use to maximize your chances of finding intertidal isopods. Regina Wetzer's 2015 paper 'Collecting and preserving marine and freshwater isopods (Crustacea: Peracarida)' (Biodiversity Data Journal 3: e4912) is well worth a look for extra information.
Turning things over
Just like woodlice, many marine isopods like to hide under things, and the easiest way to find many species is to turn stones, seaweed and other bits of debris over and check both the underside (especially) and the ground beneath. Stones are particularly good, and you'll soon get a feel for which ones (and where in the intertidal zone) are likely to be productive for particular species. Common genera under stones include Dynamene, Lekanesphaera, Sphaeroma, Idotea, the ubiquitous (but very small) Jaera, and Janira, but just about anything can turn up. Always make sure to return stones carefully to their original positions, and be careful of injury (including from sharp edges and barnacles).
Free-swimming marine isopods can be netted and nets can be swept through seaweeds to capture species clinging on to them. Netting is a good way to find species in the genera Eurydice, Lekanesphaera, Idotea and Stenosoma, and may sometimes turn up less common species.
This can be an extremely productive technique, though care needs to be taken to minimize your impact on the environment. A good sized bucket (e.g. a 20 litre white fishing bucket with a lid) is essential. Having filled the bucket half or two-thirds full of water, you can rinse smaller stones, Laminaria holdfasts and seaweeds in it, churning and rubbing as you do to dislodge any isopods. The water can then be strained through a fine sieve (a plastic kitchen sieve is perfect), and the contents may then be examined in water in a tray (e.g. a cat litter tray in white or some other pale colour) at leisure. All sorts of isopods can be found this way, including species from the genera Cymodoce, Dynamene, Lekanesphaera, Idotea, Stenosoma, Astacilla, Jaera, Janira and Munna. It is often not possible to wash seaweeds without detaching them, but you should keep this to a minimum and avoid targeting less common species.
Samples of mud, sand, grit and gravel can be gathered (e.g. using your hand, a net or small spade) for examination. It may be useful to wash and sieve the substrate to reveal any isopods, examining the processed material in a tray, or to add fresh water or ethanol to drive specimens out of the substrate. Genera including Gnathia, Paragnathia, Lekanesphaera, Cleantis, Jaera and perhaps others may be found in this way.
Isopods like to live in things as well as on them. A certain amount of excavation (always bearing in mind your impact on the environment) may be necessary to reveal some species. For example, Dynamene bidentata likes to live in cracks and crevices or in empty barnacles, whilst Campecopea hirsuta is most frequently encountered amongst the intertidal lichen Lichina pygmaea or in empty barnacles. Gnathia sp. may be found in cracks and crevices and between layers of soft rock. Other species live in mud (e.g. Paragnathia formica), whilst Gribbles (Limnoria sp.) are found in wood. Some means of getting at these species is required, whether it be a scraper, a small hammer and chisel (make sure to wear eye protection and again minimize damage to the environment), a spade, or a penknife (though the wood Gribbles inhabit can often be almost as hard as rock).
Some marine isopods, especially in the genera Anilocra, Eurydice, Conilera (deep water only, common in lobster pots), Cirolana (usually in deeper water, also common in lobster pots), Dynamene (not infrequent in children's crab nets) and Idotea, will come to bait or light traps. Light traps using waterproof torches or glowsticks can be fashioned from plastic bottles or other containers, as can baited traps, and both can be moored to the shore using strong cord. They should not be left for prolonged periods, as animals (not just isopods) will not be able to exit them easily and may be subject to predation. The species Idotea metallica can be encountered in sealed bait pots that have floated across the Atlantic Ocean from North America to be deposited on western shores in Britain and Ireland, having entered the fine holes in the pots as juveniles and having grown too big on the journey to escape.
Finding parasitic isopods
Finding parasitic isopods depends of course upon finding their hosts. External parasites of fish (especially Anilocra and juvenile Gnathia) may be observed in the environment, or may be present on fish caught using rod and line or in nets. Internal parasites present particular difficulties. Some, such as Bopyrus squillarum, Bopyrina giardi, Gyge branchialis and Pleurocrypta longibranchiata are detectable externally by swellings on the host, whilst others (e.g. Hemioniscus balani and Athelges paguri) can only be discovered by dissection of the host (in the case of Hemioniscus balani by collecting scrapings of their barnacle hosts using a scalpel or paint scraper). In many cases, extracting internal parasites involves killing the host, and this means that their study must be balanced against the potential for damage that collection of members of the host species might do, but it is important to remember that it is impossible to know anything about these very understudied species without careful sampling of the host population.
There can be a temptation to bring far too much equipment to the shore, which you will inevitably regret once you have lugged it around for a couple of hours. In addition to the usual optical equipment (especially a hand lens), the most useful items are a large bucket with a lid (which can also be used to carry other equipment), a net (this can be anything from a cheap goldfish net to a professional one, with results just as likely with the cheap one in many cases), a plastic sieve, a tray for observing samples, some small paintbrushes and a pipette (for picking up isopods), and pots for retaining any specimens. A small spade and a paint scraper are often essential as well. Other useful equipment includes diving gloves (for hand protection, especially from barnacles), sealable plastic bags (for seaweed and wood chip samples, as well as for keeping optical equipment, phones and car keys dry), larger containers for samples of substrate or parasite hosts, a magnifying visor for seeing smaller species, a small hammer and chisel (and eye protection), a knife (for cutting seaweeds and extracting wood chips, etc.), a scalpel (for barnacle removal), preserving fluid in tubes (for instant killing of samples required for identification; note that large isopods will eat smaller ones and Jaera will savage each other, so be careful with storing live specimens), and a small gloss black slate and petri-dish (for photography of live creatures; put some water on the slate, set the petri-dish on top, squeezing out air bubbles, add some water and the creature you wish to photograph, using a flash as necessary to capture the image).