Blue mussel

This is the only handling section in the catalogue where the animal cannot hurt you and the section is still mostly about safety.

A mussel pumps many litres of seawater a day and keeps what is suspended in it. That makes it an excellent monitor of water quality and a poor thing to gather without knowing what that quality is, because two categories of contaminant accumulate and neither is detectable by anybody standing on a beach.

ALGAL BIOTOXINS come from microalgae that bloom seasonally and unpredictably. The mussel is unharmed; the person eating it may not be. Paralytic shellfish poisoning can kill an adult, and the toxins are heat-stable — cooking, freezing, smoking, salting and pickling all leave them intact.

MICROBIOLOGICAL CONTAMINATION comes from sewage and agricultural run-off, and the same filtering concentrates bacteria and viruses.

AND NOTHING YOU CAN OBSERVE DETECTS EITHER. Contaminated mussels look, smell and taste normal. The water may be clear. The bed may have been safe last month.

SO THE ONLY REAL CONTROL IS THE ONE YOU DO BEFORE LEAVING THE HOUSE: establish the current classification and closure status of the ground with the relevant food safety or environmental health authority, which in most countries is a different body from the fisheries authority. Treat unclassified or unmonitored ground as unknown rather than as safe.

THE FOLK TESTS ARE ALL WORTHLESS AGAINST THIS. The R-in-the-month rule predates refrigeration and toxin monitoring and describes spawning condition rather than contamination — harmful blooms occur in months with an R in them. A silver spoon detects nothing. And the best-known rule of all, discard the ones that do not open on cooking, tests whether the animal was alive and says nothing whatever about toxins. Every one of those is worth naming explicitly, because they are repeated constantly and they sound like knowledge.

THE GATHERING ITSELF is simple and deserves restraint. Cut the byssal threads rather than tearing the animal off, take from beds submerged at most tides rather than the top of the shore, take a scatter across a bed rather than clearing a patch, leave the small ones, and replace any rock you turn.

KEEPING THEM: cool, damp and unsealed. A closed bag suffocates them. Discard broken shells and any that stay open when tapped, because those are dead — which is a spoilage judgement, not a safety one.

Blue mussel reproduction is simple and it is the source of one of the most persistent and most misunderstood pieces of seafood folklore, which this section exists to take apart.

THE REPRODUCTION FIRST. Sexes are separate, and eggs and sperm are released directly into the water column — chiefly in spring and early summer, with the timing driven by temperature. Fertilisation is external and effectively a numbers game: gametes must meet by chance in open water, which is why the quantities released are enormous and why spawning within a bed is synchronised. The resulting larvae drift for a period before settling and attaching.

SPAWNING LEAVES THE ANIMAL SPENT. A mussel builds gonad tissue through the preceding months — that tissue is a substantial fraction of what is inside the shell and a substantial fraction of what a person eats — and releases it. What is left afterwards is thin, watery and poor. A mussel in early spring, full of gonad, is a plump good one. The same mussel weeks later is not.

AND THAT OBSERVATION IS WHERE THE OLD SEASONAL RULE COMES FROM. The traditional advice to avoid shellfish in months without an R in them tracks a real pattern — warm-season mussels are frequently spent and disappointing — and it has been passed down as though it were a safety rule. It is not, and the confusion is worth breaking because it is dangerous in both directions.

THE ACTUAL SAFETY QUESTIONS ARE UNRELATED TO THE MONTH. They are: whether the water the mussels grew in is sanitarily classified, since filter feeders concentrate whatever is in the water including sewage-derived pathogens; and whether harmful algal blooms are present, since biotoxins that cause paralytic, diarrhetic and amnesic shellfish poisoning accumulate in the flesh, are not destroyed by cooking, and cannot be detected by looking, smelling or tasting. Blooms are monitored by authorities, they occur when they occur rather than on a calendar, and a month with an R in it offers no protection from either problem.

So: the season is a QUALITY guide, monitored classification and biotoxin advice are the SAFETY answer, and treating the first as the second is exactly the mistake to avoid.

Mussel size is a direct readout of where the animal has been living, and for a gatherer it is the most useful thing about it.

THE MEASUREMENT IS SHELL LENGTH — the longest axis of the shell, from the pointed umbo to the broad rear margin. That is the measurement any minimum size is expressed in, and it is the one to take. It is also the measurement most easily got wrong by eye, because a mussel bed is a jumble of overlapping shells and individual animals are hard to isolate without prising them off.

GROWTH IS SET BY IMMERSION TIME AND BY FOOD, AND BOTH VARY ACROSS A FEW METRES OF SHORE. A mussel is a filter feeder: it can only grow while it is under water with the siphons open. High on the shore, where the bed is exposed for most of the tidal cycle, mussels feed for a few hours a day and stay small — dense beds of small, thick-shelled animals. Low on the shore or fully submerged, feeding time is far longer and the same species reaches several times the size. That is why the largest mussels come from sheltered, food-rich, rarely-exposed positions, which is exactly what this record's exceptional note says.

EXPOSURE ALSO CHANGES THE SHAPE, NOT ONLY THE SIZE. Mussels on wave-hammered open coast are smaller, thicker-shelled and more strongly attached; sheltered-water animals are longer, thinner-shelled and meatier. A gatherer comparing two shores is comparing two growth regimes rather than two populations of different quality.

AGE IS NOT READABLE FROM SIZE HERE. Because growth depends so heavily on position, two mussels of identical size can differ substantially in age, and a small high-shore mussel may be older than a large low-shore one. Shell rings are not a reliable annual count in all populations. This record grades the exceptional figure as unknown-confidence for exactly that reason.

AND THE SIZE QUESTION IS NOT THE SAFETY QUESTION. A large mussel from a closed or unclassified area is not safer than a small one; biotoxin and microbiological risk are properties of the water, not of the animal's dimensions, and nothing about size, appearance or smell indicates them. Whether a bed may be gathered from at all is a matter for the local competent authority.

A mussel cannot move, so everything about its life is decided by where its larva happened to settle — and the vertical distance between two mussels a metre apart on the same rock can matter more than a hundred miles of coastline.

THE HABITAT IS THE INTERTIDAL AND SHALLOW SUBTIDAL, ON HARD SUBSTRATE. Mussels attach with byssus threads — protein fibres secreted by the foot and set like glue — to rock, boulder, pier piling, groyne, breakwater, wreck and to each other. Given nothing hard they will use shell and gravel, binding it into a mat, and on some sheltered shores they build extensive beds over what was previously soft ground. That bed then becomes the habitat for everything else: a mussel bed is a structural habitat in its own right, holding crabs, small fish, worms and predatory whelks and starfish in numbers the bare rock never held.

HEIGHT ON THE SHORE IS THE MASTER VARIABLE. A mussel only feeds when submerged, so the higher the bed the fewer hours of feeding a day, and high-shore mussels are consequently small, thick-shelled and slow-growing while low-shore and sublittoral animals of the same age are much larger. The upper limit of a mussel bed is set by desiccation and heat; the lower limit is set by predation — starfish, crabs and dogwhelks work upward from below and stop where exposure defeats them. So the band you can see on a pier leg is the space between drying out and being eaten.

WAVE EXPOSURE SETS THE CHARACTER. On hammered open coast, mussels are small, heavily attached and densely packed; in sheltered estuaries and sea lochs they are larger, thinner-shelled and more loosely held. Both are ordinary. Exposure also decides who eats them: a bed too rough for starfish is a bed that can extend lower.

SALINITY IS UNUSUALLY WIDE and it is why this animal is everywhere. Blue mussels tolerate substantially reduced salinity, which is why they extend far into estuaries and why they occupy the brackish Baltic across enormous areas — smaller there, in the low-salinity north, but abundant, and forming the principal hard-bottom habitat of a sea that has very little rock exposed.

AND THE FEEDING MODE IS THE REASON THE GROUND MATTERS TO A GATHERER. A mussel filters large volumes of water and concentrates what is in it, so the bed's habitat and its water quality are the same subject. That is a food-safety fact rather than an ecological aside, and the classification of any particular ground is a matter for the local competent authority.

Mussel seasonality is a condition cycle with a well-known piece of folklore attached to it, and separating the two is the useful work.

THE CYCLE IS BUILD, SPAWN, RECOVER. Mussels accumulate reserves through the productive months, ripen gonad, and spawn — frequently more than once across a season, cued by temperature and the availability of phytoplankton. After spawning they are spent: thin, watery and poor. Before it they are full and heavy. That is the whole of the condition story and it is why the same bed produces very different animals at different times.

GROWTH TRACKS THE PLANKTON RATHER THAN THE CALENDAR. Mussels are filter feeders, so growth follows the seasonal production of the water they sit in, which means the growing season begins with the spring bloom and can be extended or curtailed by conditions in a way a date cannot predict.

THE OLD RULE ABOUT MONTHS CONTAINING AN 'R' IS HALF RIGHT AND WORTH ADDRESSING PROPERLY. It broadly avoids the warm-month spawning period when condition is poorest, which is a real observation about quality. It is NOT a safety rule and never was: biotoxin risk and microbiological risk are properties of the water and its classification, they occur in months with and without an R, and no traditional rule of thumb detects them. Treating the rhyme as a safety test is the specific error the record's handling material exists to prevent.

RECRUITMENT IS A SEPARATE SEASONAL EVENT. Larvae spend a period in the plankton before settling onto hard substrate, and settlement success varies enormously between years — a bed is restocked by what the tides deliver rather than by the adults on it.

WINTER STORMS REMOVE BEDS PHYSICALLY, which is a seasonal loss that has nothing to do with the animal's condition: a bed on exposed rock can simply be stripped away.

AND WHETHER ANY BED MAY BE GATHERED FROM AT ALL is decided by classification and monitoring rather than by season, changes, and is a matter for the local competent authority.