History of the Irish Mussels
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Blue mussels (Mytilus edulis), in Irish called 'Diúilicín' have been important as food for humans for thousands of years and mussel harvesting dates back centuries. Blue mussel shells have been found in kitchen middens dated at 6000 B.C. and until the 19th century, blue mussels were harvested from wild beds in most European countries for food, fish bait and as fertilizer.
The first farmed mussels were grown on poles in the 13th century in France, where the technique of erecting wooden ‘bouchots’ (poles driven into the estuary mud) has been credited to a shipwrecked Irishman called Patrick Walton. According to the French gastronomic legend, Walton found himself washed up on the Bay of Aiguillon in the early thirteenth century. The story goes that he erected crude nets between wooden poles hammered into the mud flats to catch migrating seabirds for food. He soon observed that tiny seed mussels were gathering on the poles. They flattened up beautifully, an so more and more poles were erected, as mussel cultivation took over from catching seabirds. Bouchot are still widely used in France, especially on the vast tidal mudflats of the French Atlantic coastline.
Another technique of mussel growing that originated on the rocky shores of Galicia in Spain, are long, heavy, suspended ropes, nowadays often studded with plastic spikes to increase the surface area available for clustering bivalves. This, and the method of bottom grown mussels using bottom culture plots developed in Northern European countries are the two base methods that Ireland adapted to grow the local blue mussel on a broader scale since the 1970s. The rope technique refined in and for New Zealand farmed mussels had a big influence on how mussel farms in Ireland are run today.
While ‘farmed’ cultures are the most common way to harvest mussels nowadays, wild beds are still used for juvenile supply in several countries including Ireland and reliability is obtained through the development of spat collecting techniques. The development of hatchery techniques using polyploid mussels is the most recent culture trend.
Mussels as Indicators
Mussels were among the first animals to be used by researchers for assessing the environmental quality of seawater. Records of this date back as far as 1886.
Data for environmental monitoring with mussels are available from more than 50 nations, in some cases with uninterrupted time-series going back to the 1960s. The reason why Mytilus species are so popular as environmental sentinels is their biological and ecological characteristics which make them virtually ideal for pollution monitoring as judged by the OSPAR commission. Blue mussels are sessile and therefore provide location-specific information, they are medium-sized and so one individual may provide enough tissue material for chemical analysis, they form large mussel beds in shallow waters from where they easily can be collected, and they are hardy creatures and therefore easy to keep in culture, making them suitable for ecotoxicological laboratory exposure studies as well as in situ analysis.
Mussel Trivia
Did you know that the ‘beard’ of the mussel, the byssal threads are made from iron deposits that the mussel extracts from seawater? Byssal threads collected from large mussels and other bivalves used to be dried and spun into cloth known as ‘sea silk’. The resulting fabric was so light, strong and fine that a pair of gloves would fit into a walnut shell and a pair of stockings into a snuffbox. Apparently many Roman and Greek emperors as well as King Tutankhamen wore cloaks made of sea silk.
There is a woman on Sardinia who is said to be the one remaining person still able to spin and weave the golden thread from the sea. Read her story here or watch her video.
Ecology of mussel
Common or blue mussels are bivalve molluscs systematically consisting of a group of (at least) three closely related taxa (a unit to classify species in biology science) of mussels, known as the Mytilus edulis complex. These taxa can hybridise with each other, if they are present at the same locality. They are found on shores throughout the North Atlantic, Mediterranean, North and Baltic Seas. They commonly live in large aggregations, attaching themselves to rocks and each other in the intertidal zone with sticky threads known as byssus (or beard, see drawning above). Size and shape vary widely and the colour varies between the typical deep bluish purple and paler or rayed brown. The shell can grow up to up to 10 cm but is usually much smaller (between 3-6cm). Mytilus edulis reaches maturity when one year old and can live between 10-15 years and even more.
Mytilus edulis filter-feeds mainly on phytoplankton (microscopic single celled algae suspended in the water column) by pumping and filtering large volumes of water over its ciliated gills. This seawater filtration behaviour also makes the mussels to efficiently accumulate pollutant chemicals from the seawater, thereby providing an integrative measure of the concentration and bioavailability of seawater pollutants in situ.
This exact feature can occasionally make mussels and all filter feeders that are consumed by humans unfit or dangerous to eat.
The paralytic or diarrhetic shellfish poisoning is, beside general pollution, one of the best examples. This poisoning can occur when certain toxic phytoplankters (mostly dinoflagellates) appear in high numbers which are being ingested by the mollusc, accumulate in its flesh and reach high enough poison levels that it can harm humans when consuming this mussel. However, this is normally a small risk and happens very rarely. To avoid this risk altogether, it is preferable to eat only farmed mussels since they are cultured and marketed under strict conditions and monitored closely for toxic algae under the Food Safety monitoring scheme (operated by the Marine Institute). See more on this in the sustainability section.
Ecosystem Engineers
Blue mussels are seen as ecosystem engineers as they do not only filter seawater and hence improve it, they also provide essential ecological services such as food and habitat to a multitude of other species e.g. invertebrates (e.g. polychaeta, starfish, dog whelks and crabs), fish, sea birds, and seals. Natural mussel banks and artificial mussel farms boost biodiversity in Irish bays and shores and the only downside that is heard from a tourist perspective is the visual impact on the natural beauty of the Irish landscape/seascape.
Reproduction cycle of the blue mussel
Mussels have separate sexes and reach maturity when one year old. Once the sperm and eggs are fully developed they are released into the water column for fertilization. Although there are about 10,000 sperm per egg, large proportions of eggs deposited by blue mussels are never fertilized and only about 1% of the matured larvae reach adulthood. The majority are eaten by predators before completing metamorphosis.
The reproductive strategy of blue mussels is very interesting and is called 'planktotrophic' in scientific terms. By minimizing nutrients in egg production to the bare minimum they are able to maximize the number of gametes produced. If the adult mussels are stressed during the beginning of gametogenesis, the process is terminated.
When stressed while fresh gametes are present, adult mussels reabsorb gametes. Larvae viability is also affected by the condition of parents: high water temperatures, pollutants and scarcity of food, during gamete production lead to reduced larvae viability. It is thought that this reduction is caused due to the lack of lipid reserves distributed to the eggs.
Larval development can last from 15 to 35 days depending on environmental conditions including salinity and temperature, as well as location.
The first stage of development is the ciliated embryo (see illustration adapted from Kamermans et al. 2013). Within 24-hours after fertilization it forms the 'trochophore' larva. At this point although mobile, the larva is still reliant on the yolk for nutrients. Characterized by a functional mouth and alimentary canal the veliger larva stage also has cilia which are used for filtering food as well as motility. A thin translucent shell is secreted by the shell gland forming the notable D-shaped larva (veliger). The veliger continues to mature and developes distinctive eye spots by the end stage and an elongated foot (pediveliger larva) with a byssal gland formed.
Once the pediveliger larva is fully developed, its foot extends and makes contact with substrate. The initial contact with the substrate is loose. If the substrate is suitable, the larva will metamorphoses into the juvenile form, plantigrade, and attach byssus threads. The mussel will remain in that state until reaching 1-1.5mm in length. This attachment is the prerequisite for the foundation for the blue mussel population. In sheltered environments large masses sometimes form beds which offer shelter and food for other invertebrates.
The main purpose of extemely dense mussel aggregations is still unclear and it is speculated that it might have different purposes under different circumstances.
History of the Irish Cockle
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Cockles have formed part of the Irish diet since the earliest of times. Archaeological remains have been found at early historic sites such as Oughtymore, Co. Derry, Park Cave, Co. Antrim and Potters’ Cave, Co. Antrim. The Irish name for cockle is 'ruacan'. And sure, even though Molly Malone didn't cry for ruacan and rather 'cockles and mussels, alive alive oh', the legendary Molly is the iconic representation of the seafood trade in Dublin in the early 1700s.
Cockles have been picked along the east ocast of Ireland for centuries and is specifically synonymous with Blackrock and Dundalk Bay in Co. Louth. It wasn’t just a hobby but for some a vital activity to get food. Most people hand-picked the bivalve by just walking along the shore at low tide, some used rakes to bring them up to the sandy surface. Written evidence to cockle picking was found from the early 1800s. Indeed, the importance of the humble shellfish is commemorated in the place name Cocklehill and most recently, since 2018, in form of a Cockle Picker sculpture in Blackrock by local artist Michéal McKeown.
In the early 1800s, cockles were also at the centre of controversy in Blackrock when the landlord Lord Clermont imposed a tax on the cockles collected by local people. There was a lot of opposition to this as Lord Clermont was also collecting rents from his tenants. To avoid the cockle tax, the cockle pickers would bury the cockles in the sand as soon as they spotted the bailiff and would return to shore empty handed, only to collect their harvest the next day.
During the Great Potato Famine of 1845, cockles formed the mainstay of the diet for locals along the coastal regions. It is said that people, often widows and their children survived the famine by gathering cockles from the shores, boiling them with a little oatmeal and where available milk to sustain the family for a whole season.
Up to the 1960s cockles were also commonly eaten on Good Friday and during lent.
Cockle games and remedies
Anna Kenny from 'Clifton' in Howth, describes a game called “Hot Cockles” in the Schools' Collection on dúchas.ie.
Any number may play this game. The players sit round in a ring, and one goes round and round the ring and says: "Hot cockles, hot cockles my hands are warm, what shall I do to cool them. I wrote a letter to my sweetheart and on my way I dropped it, some of you have picked it up and put it in your pocket." Then she lets the two stones which are in her hands fall at someone's feet and this person must run after her. If she cannot follow the leader in and out the ring, the others call out "Wild Duck". She must sit down the ring again, and the leader goes round the ring again.
Another source from dúchas.ie says that barnacles and cockles would purify the blood if eaten. John Rutty's 'Essay toward a Natural History of the County Dublin' from 1772 informs us that cockle broth was also used "as a bath to strengthen the limbs of weakly children".
Cockle Trivia
Did you know that the The common English phrase "it warms the cockles of my heart", is used to mean that a feeling of deep-seated contentment has been generated.
Different derivations of this phrase have been proposed, either directly from the perceived heart-shape of a cockleshell, or indirectly from the scientific name for the genus 'Cardium' meaning 'heart' in Latin. Another proposed derivation is from the Latin for the ventricles of the heart, 'cochleae cordis', where the second word is an inflected form of 'cor'- heart, while 'cochlea' is the Latin for snail.
Ecology of cockles
The common cockle (Cerastoderma edule) is an extensively studied species. Its populations have a large geographic range along European Atlantic coasts from Norway to Senegal.
Cockles live buried in the sediment of sandy, sheltered beaches, usually with some fresh water influence and typically using their foot to borrow. Like mussels, they eat by filtering the surrounding water for phytoplankton. The burrowing and filtering feature makes them ecosystem engineers, since they alter their habitat by bioturbation and influencing hydrodynamics.
The density of populations can be extremely high: up to 10.000 animals per square meter have been counted.
Cockles can live up to ten years.
The distinctive rounded shells are bilaterally symmetrical, and heart-shaped when viewed from the end. They typically reach a size of 3.5cm and 5cm, but can grow as big as 6cm in length. Numerous radial, evenly spaced ribs are a common feature of the shell in most genera. More than 205 living species of cockles have been identified and many more fossil forms exist.
The shell of a cockle is able to close completely (i.e., there is no "gape" at any point around the edge). This helps the cockle to protect itself against predators of which there are many, including sea birds, crustaceans, fish and seals.
Reproduction cycle of cockles
As is the case in many bivalves, cockles display gonochorism which means that their sex varies according to their surrounding environmental conditions.
They reach sexual maturation between the first and second year and have high fecundity. Their spawning season is usually from late spring to mid-autumn.
The larvae are part of the zooplankton community (they drift in the water column) for around 30 days, which allows for passive larval dispersal by ocean currents that drive connectivity and gene flow between populations spread along the Northeast Atlantic. Then they settle as spat (seed). The reproduction cycle of cockles is similar to the one of mussels and can been seen in the illustration below.
Commercial value and decline
Cockles are a highly valued commercial species to European fisheries, with capture production peaks reaching over 100,000 tonnes per year in the 1980s and early 1990s. However, it is a fishery in decline, similar to others coinciding with reports of changing fisheries policy, significant overfishing, variable recruitment and mass mortalities. Some of these reported mortalities are a result of climate related events (e.g. high precipitation, storms and heat waves), which are anticipated to increase in frequency in future.
The cockle is also impacted on by a wide range of parasites and pathogens. For example, the recent discovery of the pathogen Marteilia cochillia is causing significant mortalities in cockles in Galicia, Spain.