Growing greenshell mus­sels is a bit like growing a weed, says Coromandel marine farmer Allan Bartrom. “Put a log or boat in the water around here for long enough and it will grow mussels.”

Unlike weeds, greenshell mussels have proved a lucrative—if, lately, an unpredictable—crop for New Zealand’s aquaculturalists.

Last year, they earned the coun­try $52 million in export revenue, with a further $10 million-worth be­ing consumed locally. Just a decade ago, the industry was worth less than $5 million.

The greenshell mussel, Perna canaliculus, is native to New Zea­land, although it has close relatives in South America, South Africa and the tropics.

Several other species of mussel inhabit the New Zealand coastline (see page 120), but Perna is the only one in cultivation. There are several reasons why this is the case. Perna is a large, meaty mussel which grows in deeper water than most other species. It also grows quickly, reaching a marketable size in 10 to 18 months, depending on the loca­tion of the farm: Coromandel mussels grow more quickly than their counterparts in the Marl­borough Sounds.

Most important of all, Perna is the only green mussel in cultivation anywhere in the world. In the com­petitive international shellfish market, with a turnover of more than a million tonnes per year for mus­sels alone, the New Zealand mussel stands out from the crowd, both in colour and size, and is capable of fetching a premium price.

The attractive green shell (now adopted and patented as the official name, replacing the former “green-lipped mussel”) is nicely comple­mented by the creamy white (male) or apricot (female) flesh. The New Zealand industry emphasises these features in the way it presents the product: the largest export category is frozen mussels on the half shell. Other products include smoked, stuffed, crumbed, coated and mari­nated mussels, mussel chowder and mussel “pick ‘n’ dip.”

The live mussel trade is relatively small—about five per cent by vol­ume—but there are hopes that the rapidly growing Asian market may take more live product. At present, Europe is by far the country’s largest export mussel market, followed by Japan, the United States and Aus­tralia.

Twenty-five years ago, mussel farming in New Zealand was un­known. The shellfish were either picked from the rocks or dredged from shallow seabeds. The largest, most succulent specimens—with shells up to 180-200 mm—were dredged, particularly near Coromandel.

Stuart McFarlane was one who fished for these beauties, but he soon realised that the harvest was unsus­tainable. Instead, he took up the idea of farming them.

In 1969 he set up a trial raft at Chamberlin’s Bay, Ponui Island, in the Hauraki Gulf. Ironically, the ex­periment proved too successful: such was the weight of growing mussels that the raft broke in two and sank.

At about the same time, Keith Yealands, a Marlborough grocer, was developing his own plans to farm mussels. Yealands had suffered a heart attack and been ordered by his doctor to find a less stressful job. He had read about Spanish methods of mussel culture and had seen a Victo­ria University study on the subject, and decided that aquaculture was just the sort of outdoor pursuit he needed.

The local harbour board granted him permission to establish a one-acre experimental farm in Queen Charlotte Sound. But all was not smooth sailing.

“I built a huge concrete raft with two double pontoons to hold the growing ropes, collected some seed, and in no time at all had a success­ful crop. But the harbour board wouldn’t let me harvest it. I was al­lowed to take only the legal daily quota of four gallons. The raft got heavier and heavier as the crop grew. I pleaded with the board to let me take more mussels off, but they refused to allow it. Eventually, in rough weather, the raft broke up and went to the bottom with the mus­sels.”

Despite this setback, Yealands had proved he could grow mussels, and in 1972 he gained his first li­cence. The venture was a family enterprise, and, though productive, it was far from profitable. To pay the bills, the family also ran a car-wreck­ing business. “So much for the quiet lifestyle prescribed by the doctor,” muses Yealands.

In the early 1970s, aquaculture was still a very new concept in New Zealand. Oyster farming was just be­coming established, and both the mussel and oyster industries devel­oped slowly. But towards the end of the decade other entrepreneurs be­gan to look at the idea of growing mussels.

Allan Bartrom was one. “We set up an experimental farm in 1979 to determine conditions suitable for cultivating mussels, and then pre­pared a Marine Farm Plan—akin to a Town and Country Plan—setting out where farms might be established. In all, it took five years of work—and we copped a fair amount of flak. Because recreational boaties and mussel farmers are both looking for the same thing—clean, sheltered wa­ters—the conflict of interests was often intense. In the end, only one per cent of the area we proposed was approved for mussel farming.”

Today, there is general acceptance of the presence of the farms both on the Coromandel Peninsula and in the Marlborough Sounds—the main mussel farming region in the coun­try—although a few dissenting voices remain. For them, mussel farms will always be a blot on the seascape.

*

Although mussel farming had its beginnings as a cottage industry through which individuals, families or small partnerships could supple­ment their incomes by selling to the local market, today it is big busi­ness, with major companies involved, and most of the harvest exported. The Yealands family’s own firm, Marlborough Mussel Company, owns 16 farms and man­ages a further 17 in the Sounds. It handles 5500-6000 tonnes of export mussels a year, and Keith Yealands’ contributions to aquaculture have been recognised with an OBE.

Getting from those first disinte­grating rafts to our present total production of 46,000 greenweight tonnes (1992) has involved consid­erable refinement of production methods along the way.

Initially, Spanish raft cultivation techniques were used, with 10-me­tre-square frames growing up to six tonnes of mussels on ropes sus­pended beneath the raft. This system proved inefficient, as mus­sels near the centre of the frames did poorly—probably because the shell­fish around the outer edges took most of the available food particles in the water currents.

Farmers soon began experiment­ing with an adaptation of the Japanese longline system, using bu­oys to suspend parallel lines of growing ropes or “droppers” in the water. The system has been further modified on larger farms, where the droppers are loops in one continu­ous rope. The loop method greatly facilitates harvesting and re-seeding.

From the air, today’s typical mus­sel farm is about three hectares in area, and comprises four parallel rows of large black floats roped to­gether, with the first rows 50 metres offshore and extending out to 200 metres from the coast.

Each row of floats supports a pair of 100-metre-long backbone ropes or longlines which are anchored at each end and aligned parallel to the shore (to allow the best exposure of each mussel to the nutrient-bearing tidal flows). Droppers (either indi­vidual ropes or a continuous series of loops) hang 7-10 metres down into the water, and support 300-350 mussels per metre of rope. The com­bined weight of mussels on each longline can reach 40 metric tonnes in a single growing cycle.

The two backbone ropes in each pair are less than a metre apart, and each set of floats and ropes is sepa­rated from the next by just enough room to allow vessels to pass up and down the farm, harvesting and seed­ing.

Getting tiny juvenile mussels to adhere to the growing ropes was once a laborious, manual task that involved feeding small mussels into long “stockings” through which the growing ropes passed, then lower­ing the stockinged ropes into the water. Young mussels rapidly attach themselves to the ropes, and the stocking rots away, leaving the mus­sels to grow to harvest size.

Some small farming operations still use this method, but most farms are seeded by contractors working from specially adapted vessels. Again, it was the Yealands family who pioneered mechanised seeding, some ten years ago. Now the family business manufactures the biode­gradable stocking which holds the young mussels in place.

Curiously, despite its growth in the last 20 years and its technologi­cal advances, the mussel farming industry is still heavily reliant on the unpredictable arrival each year of mussel spat on Ninety Mile Beach.

Bob Hickman, an aquaculture re­search scientist with the Ministry of Agriculture and Fisheries, Welling­ton, discovered the spat when casually investigating a pile of sea­weed on the beach in 1974.

“The weed was absolutely smoth­ered in minute mussel spat. I was surprised to find it washed ashore in the middle of a long sandy beach, far from the usual rocky areas where mussels live,” he recalls.

The potential of this spat bonanza for the development of the industry was immediately apparent, and be­fore long a handful of locals had set themselves up in business as har­vesters. They simply picked up the seaweed, packed it into plastic buck­ets or bags and freighted it to the mussel farmers. If kept cool and damp, the mussel seed survives for several days out of water.

The only problem with “Kaitaia spat,” as it is called, is its unpredict­able arrival. When it does come, it washes ashore in such large quanti­ties that it is scooped up by the truckload. But months can go by when the sea yields nothing. Har­vesters check the beach daily—especially after a north-east­erly blow—looking for freshly washed up weed. Often, it is only stranded for a single tide before be­ing sucked out again.

“Although very irregular in tim­ing, I think there has only been one period of 12 months in which there hasn’t been at least one stranding of weed,” says Hickman.

In both Coromandel and the Marlborough Sounds, local spat is collected on ropes set out for this purpose, but Kaitaia spat is eagerly taken when it is available. The rea­son is that local spat appears to pro­duce adults which spawn at a different time from those grown from Kaitaia spat, and by using the two seed sources farmers im­prove their chances of being able to harvest all year round.

Spat on the Kaitaia weed can be as small as 0.25 mm or as large as 10 mm, depending on its age. It reseeds most success­fully at 10 mm, though juveniles as large as 50 mm can be attached to growing lines. Some mussel farmers take the spat directly from those who collect it; others buy older spat that has been on-grown by farmers with specialised mus­sel “nurseries.” Mussels are usually 4-6 months old by the time they are seeded on to the “production lines.”

*

It was a cold May morn­ing when Chris Godsiff, operations manager for Sanford South Island, took me to see a harvesting team at work on one of the many marine farms his com­pany contract seeds and harvests in the Marlborough Sounds.

Most farms are inaccessible by road, and isolated from each other by kilometres of water­ways, but Godsiff makes short work of the distances in his 10-metre jet boat, which hurtles along at close to 30 knots.

I step aboard the country’s largest mussel harvester, the Pelorus Trader, which is capa­ble of harvesting 90 tonnes of mussels a day. Growing ropes, like the monstrous necklaces of some huge sea monster, are being winched aboard and stripped of their emerald jewels.

Sorters quickly separate the green mussels from seaweed and un­wanted blue mussels.

Blue mussels, the mainstay of the mussel industry in other parts of the world, are regarded as a pest by New Zealand farmers because they attach themselves to droppers and compete with the greenshells for food and space.

Harvested mussels are loaded into one-tonne bags, which are la­belled with the date of harvest, farm number and harvester. In coded form, this information accompanies the mussels to their final point of retail sale, so that, in the event of problems with quality, the batch can be instantly traced. Ministry of Agri­culture and Fisheries inspectors police the system.

The natural vulnerability of shell­fish to contamination (highlighted by two recent Listeria-related deaths which resulted in a Marlborough mussel processing plant being temporarily shut down) means that the mussel industry must take the issue of quality control very seriously.

The commitment to strict hygiene standards during processing was brought home to me when I visited Marlborough Perna’s plant in Blenheim.

First, I was obliged to re­move shoes, earrings and watch, before stepping over a low barrier and entering a quar­antine area. Here I donned white boots, coat and surgical cap, before fumbling with a knee-operated tap as I washed my hands. A foot bath fol­lowed, and only then could I enter the factory.

The 100-strong staff must pass backwards and forwards over these hurdles every time they enter or leave the factory, changing their footwear and outer clothes on each occasion. The company has even in­stalled its own laundry to ensure that factory clothing is scrupulously clean.

Staff are required to sign declarations confirming they will not work if they have an infectious illness.

Aspects of the processing it­self, such as opening and packing, are strictly separated within the factory. “Our hy­giene standards are higher than those required by most mar­- kets, but we have set them to enable us to export direct to re­tail in Britain,” says Cathy Climo, the company’s quality assurance of­ficer.

This year, Marlborough produc­ers have gained access to the Italian mussel market for the first time. MAF inspector Paul Anderson re­calls the visit of two Italian veterinarians, who were sent by the Italian government to assess condi­tions at our mussel farms when the deal was being negotiated.

“They couldn’t believe that we had no industrial plants along the coastline. They were so impressed with the quality of the water they ate the mussels straight from the farms `sa sashimi’ style, raw, right there and then—something not possible on most overseas mussel farms,” says Anderson.

*

The method bywhich mus­sels are grown may sound deceptively simple—drop  the seeded ropes into the water, pull them up when you’re ready to harvest—but it is not with­out its problems. The molluscs can be knocked off their dropper ropes in rough weather, crowded out by blue mussels or attacked by fish.

Farmers tell of schools of snapper stripping freshly seeded farms, leav­ing buoys that were once well submerged floating light and high with their diminished loads. Little is known about the extent of fish predation, but some farmers believe farms in deeper water are less vul­nerable from bottom-dwelling fish than are shallower farms.

Presuming that they survive to maturity, mussels have a sex life ap­parently designed to frustrate farmers. They can spawn suddenly and unpredictably, rendering their flesh less palatable and “skinny.”

“Mussels which have spawned lose condition and flavour very quickly. If they do it on the farm, they can be left to regain these quali­ties, but they can also spawn on the back of a truck on their way to a pack house, or even on the factory floor,” says Anderson.

However, it is seldom that all farms spawn at once, so in the Sounds, where most harvesting is done under contract, operators can move from farm to farm, taking mus­sels still fit for processing.

These are trifling concerns, how­ever, when compared with the catastrophe which shut down the country’s entire mussel industry in February of this year. Until then, mussel farmers had had a relatively clear run for over a decade, with few disease or parasite problems. Then, around Christmas, an algal bloom, possibly triggered by colder than usual coastal sea temperatures, be­gan to affect shellfish, and cases of poisoning as a result of shellfish consumption began to be reported, first in Northland, and then further south. Rigorous testing of mussels around the country indicated wide­spread presence of toxins in mussel tissues, and MAF banned all mussel harvesting.

Elsewhere in the world, blooms of particular types of algae and dinoflagellates which contain toxins frequently render mussels toxic, and lead to the closure of mussel fisher­ies. The animals become toxic because they filter toxic organisms out of seawater during the course of their regular feeding. (Mussels proc­ess six to eight gallons of seawater per hour.)

Some of the accumulated toxins cause little more than minor food poisoning in humans—a short, un­pleasant bout of vomiting and diarrhoea, and the disease passes. Other toxins, however, can cause pa­ralysis and even death. (See box)

As well as imbibing potentially toxic plankton, mussels and other shellfish can take in and concentrate bacteria and viruses from seawater that may have been contaminated by effluent. Shellfish meat—like other meat—provides a rich substrate for bacterial growth, hence the impor­tance of farming shellfish in clean water, and maintaining strict hy­giene standards during processing and storage.

Research has determined that there is a correlation between rain­fall and bacteriological counts in mussel flesh. With heavy rain comes run-off from countryside surround­ing the mussel farm, carrying potential pollutants to the filter-feeding mussels, and, consequently, to anyone eating them.

Trace metals, herbicides, pesti­cides, and especially bacteria and viruses are the main threats, and all are regularly tested for under a rigor­ous sanitation programme which, among other stipulations, sets rain­fall limits for each farm. In the Marlborough Sounds, four auto­matic gauges at strategic points monitor precipitation, while Coromandel’s six gauges are read manually. When the limit is reached, farms are closed for a set number of days, after which harvest­ing may begin again. Illegal harvesting carries heavy penalties, but to date there have been no in­fringements.

In the Sounds, the focus of mus­sel processing is on export, with nearly all mussels being cooked, while half of Coromandel’s mussel harvest is shipped live to Auckland and sold from seawater tanks in su­permarkets.

Brian Walker of Thames is MAF’s shellfish inspector on the Coromandel Peninsula, and is re­sponsible for the 37 local mussel farms and the processing plants.

“Mussel farming has found an ideal niche at Coromandel,” he says, “because as a primary industry it fits like a hand in a glove. It has brought a vital boost to the local economy, which suffered under the fishing quota management system, and it is also one of the reasons Coromandel township has a Rolls Royce sewer­age treatment plant. Protecting the marine environment was a vital con­sideration in the construction of the plant.”

Today, around 100 Coromandel Peninsula people are directly em­ployed in mussel farming, with many more finding work in servic­ing industries. Coromandel has its own processing factory, with an­other at Whitianga, one in Tauranga and four in Auckland handling Pe­ninsula mussels. Annual production from the 37 farms-32 of which are close to the Coromandel township—is around 10,000 tonnes.

Jim Jessep of Blenheim, president of the New Zealand Marine Farming Association, chairman of the Mussel Industry Council and mussel farmer, has seen the Sounds industry grow to the point where today 355 li­censed marine farms produce 35,000 to 40,000 tonnes of mussels each year. Around 900 people in Marlborough are employed within the industry, which is worth $40 million to the region.

Despite the big money involved, mussel farming is not highly profit­able for farmers themselves. With a return of around two cents a mussel, many are looking to diversify to im­prove their livelihood.

Jessep is one who is keeping his options open. He’s applied for li­cences to farm scallops, crayfish and even seaweed, as well as mussels.

“The whole fishing industry will be totally different in five years’ time,” he says. “It’s worth a billion dollars to the economy now, but the second billion is going to come from aquaculture. The time to diversify is now.”

Not everyone is so upbeat about farming the New Zealand coastline. Environmental groups hold a number of concerns about marine farming, and not infrequently op­pose mussel licence applications.

Assistant conservation director with the Royal Forest and Bird Pro­tection Society, Mark Bellingham, voices some of these concerns: “Rec­reation and landscape values are detrimentally affected by marine farming structures which prevent free passage of boats, and limit ac­cess.

“These structures can detract from the harmony and beauty of the Sounds for tourists and residents alike.”

Conservationists also worry about the long-term effects of mus­sel farms on water quality, arguing for lower mussel densities and bet­ter water circulation.

On this issue, Alastair Macfarland, deputy head of the Fishing Industry Board, believes the presence of mussel farms in an area should be regarded as a powerful guarantee of purity. “Mussels are the ultimate canary,” he says. “If there is a farm in a bay, that’s the best proof of water quality you can have.”

But what effect does the massive biomass of mussels have on local­ised areas of coastline? Does a solid wall of mussels, all filtering away madly, suck out the available nutri­ents arriving with each incoming tide, depriving downstream marine life of a meal? No one knows. Al­though it is well established that the outer echelons of mussels on a farm reach maturity more quickly than those closer to the shore, no serious research on this question has been undertaken.

And what about the mussels and associated encrusting organisms which get knocked off the droppers by storms or during harvesting, and end up on the seabed? The Novises of Port Charles talk of large, well-fed octopus feasting on the free meals from above, and an increase in the number of scallops around their farm, but is there a danger of sessile organisms being smothered? Again, no research has been undertaken.

In the long run, arguments about aesthetics or environmental impact may be overtaken by questions of the viability of farming mussels in some parts of the country. This sum­mer’s toxic scare, while mild by international standards, was enough to make some farmers (especially those in Northland) re-examine their long-term prospects.

Most, though, see a great future for what is arguably the cleanest, and certainly the greenest, mussel in the world.

*

Of horses and fleas . . .

Bivalve molluscs have been around for about 450 million years, but only in the last 65 million years have they asserted their dominance over lamp shells (brachiopods), their main filter-feeding shell rivals of the previous age. Primitive bivalves like today’s dog cockle were symmetrical shellfish with a crescent-shaped gill suspended between a pair of similar-sized shell-closing muscles.

From this stock, the asymmetri­cal mussels evolved. The anterior end, which is pulled down hard against the rock by byssus threads, became narrower and pointed, while the rear of the shell expanded to accommodate a greatly enlarged gill and broad water intake and outlet ports.

The ancestry of mussels can still be seen in the very young spat. As they settle from the plankton to begin life on the shore, the minute shellfish are rounded and sym­metrical. Each one has an active exploring foot to haul itself along as it seeks a suitable site to anchor to.

As the juveniles grow, there is little development of the anterior (front) shell-closing muscle, and in some species it is lost altogether. The gape is controlled by an enlarged posterior muscle, and the shellfish quickly takes on the tear­drop mussel shape.

Once the mussel has settled, the foot switches from being a locomotory organ to something akin to a glue-gun.

The “sole” of the foot is curled inwards to make a groove along the lower surface. From a gland imme­diately in front of the foot, byssus substance (“glue”) is squeezed along the length of the groove. With the groove tightly closed over, the foot lengthens greatly, snaking out of the shell and reaching for the rock. There it presses the tip hard against the rock, causing the end of the byssus substance to be flattened and stuck against the surface.

When the glue has set fast, the groove is opened, exposing the stretched byssus material to sea water and causing it to harden into a strong but slightly elastic thread.

Byssus anchors are particularly important to those mussels which live in exposed places—green, blue, ribbed, bearded and flea mussels. Their interlaced threads are at­tached either to the rock or to the shells of neighbouring mussels, giving communal resistance to the wrenching forces of breaking waves.

Surprisingly, for animals that are battered by surf, mussels have very thin shells—much thinner than those of other molluscs on exposed shores. They compensate by having a very streamlined shape, and are covered with a silky-smooth outer periostracum that is as hard and shiny as a plastic coating. With this shape and surface smoothness the drag force of water rushing past is reduced to a minimum.

Having such thin shells would theoretically make mussels vulner­able to many predators, but there are few that can tolerate the strong wave conditions that ribbed, blue and green mussels thrive in, along open shores.

The reef star Stichaster australis, is a notable exception. Its ten arms, each equipped with hundreds of suctorial feet, cling tightly to wave-washed rocks . . . or sheets of mussels. Where there is sufficient shelter, the white rock whelk, Thais orbita, attacks the larger mussel species, while its smaller relative Lepsiella scobina, the so-called oyster borer (that seldom eats oysters), feasts on little black flea mussels higher up the shore.

Flea mussels are one of the few animals found where rocks abut a sandy beach. In these areas, most encrusting animals and plants are unable to colonise exposed surfaces because of the abrasive scouring by sand particles. In the turmoil of breaking waves and swirling cur­rents, the rocks and any attached animals are subject to nature’s own sand-blasting machine.

Not all mussels live on such exposed surfaces. Some nestle beneath overhangs, under and between stable boulder litter and within the holdfasts of kelp. The plump, nut-like, nesting mussel, Modiolarca impacta, that grows to the size of an olive and is olive-green to brown in colour, is often found in small clumps just below low-tide mark. Gaining some protection by living in confined spaces, these mussels mask their presence still more with a veil of finely interwoven byssus threads which accumulates silt and will eventually obscure the mussels within. “Windows” in the veil allow water to flow in and out.

Date mussels choose an even more confined habitat: they bore into soft rock. Almost true to its generic name Lithophaga (literally “rock-eater”), this mussel slowly nibbles away at the rock by abrad­ing it with its shell edges as it pulls down hard with its byssus threads (possibly assisted by an acid secretion from its mantle edge when boring into limey rock). In reality, it does not eat any of the rock, but uses ciliary currents to carry away the abraded particles.

The coarse, hairy growth cover­ing the posterior end of the bearded mussel, Modiolus areolatus, is no more than the bristles of the thick periostracal coat that covers the hard chalky part of the shell. The long golden beards that are some­times found on green mussels are quite different. They are not part of the mussel shell, but are colonies of an encrusting hydrozoan (related to sea anemones) which has coral-like polyps interconnected within a slender horny skeleton.

Beards frequently grow over 15 cm long and have a hundred or more branched hairs, each one commonly consisting of 300 to 500 polyps. They may therefore consist of up to 50,000 individuals.

Living quite a different life from other mussels is New Zealand’s largest species, the horse or fan mussel, Atrina zelandica. It com­monly grows to 30 cm in length, and especially large specimens may reach half a metre.

Atrina , a worldwide genus, is not tear-drop-shaped like most other mussels, but looks more like a flattened ice-cream cone. Nor does it attach to hard surfaces; instead it lives upright, partially buried in sand or mud.

Lacking the broad tongue-shaped foot of a tuatua or cockle, the horse mussel cannot burrow quickly to escape danger, and can haul itself only slowly into deeper sediments by sending down byssus threads and then pulling on these.

Eventually, the horse mussel establishes a multi-filament spread­ing anchor that is somewhat analo­gous to a land plant’s root system. With enough fine anchor threads deployed, the mussel is able to maintain its position against moderate pulling forces produced by currents along the sea bed or by being tugged by fish.

The shell is never pulled right down into the sand, but is left with a few centimetres protruding, thus keeping its water intake clear of the gill-clogging silt of the sea bed.

The exposed tip of the horse mussel is an ideal settlement site for other marine animals, and these tips may become completely obscured by rich growths of sponges, sea-squirts, tubeworms, barnacles, bryozoans and even the nesting mussel. The encrusting animals probably give some protec­tion to the horse mussel’s exposed posterior shell margins, though the trade-off is that those animals will compete with their host for avail­able plankton food in the water.

In shallow waters, where there is sufficient light, the shell ends also serve as attachment points for seaweeds. If these grow too large, they may become serious survival hazards for their hosts. During a severe storm, large seaweeds can act as an aquatic “sail” and wrench the mussel out of the substrate, leaving it at the mercy of the waves, which invariably wash them up on the shore to die.

One of our most attractive mussels is a fairly recent arrival to the country. Musculista senhousia, a small mud mussel about 3 cm long when fully grown, has a thin olive-green shell patterned with irregular brown zig-zag lines.

In life, these patterns remain hidden, for, like the nesting mussel, the mud mussel covers itself with a cloak of byssus threads, trapping silt and debris.

Mud mussels are usually found in aggregations of thousands of individuals that blanket the sea bed, the whole mass being loosely tied together with silted byssus.

The species, which is common in Asia, was first reported nine years ago in Auckland’s Tamaki Estuary, and has now spread throughout the Auckland region. Along some shores there is now a superabundance of shells—perhaps several million in a 50-metre stretch of beach. Residents of some North Shore beaches are concerned that the character of their shores may be irreversibly changed with the invasion of the mud mussel, and the baggage of silt which accompanies it.

True mussels are confined to salt water but, like most other coun­tries, New Zealand has a few freshwater mussel species as well. Although their shells are inflated at the posterior end, making them mussel-shaped, these shellfish are not closely related to the marine mussels, and are classified in a different order.

The local species belong to the genus Hyridella. They are found throughout the North and South Islands in lakes, streams and rivers, where they live shallowly settled into soft sand or mud.

Biologically, they are extremely interest­ing in that they can maintain their distri­bution upstream in river systems where the water flow is always in one direc­tion: downstream. It would be reasonable to expect that any larvae produced would be flushed out to sea, but freshwatey mussels have an ingenious method of ensuring that this does not occur.

Unlike marine mussels, which release sperm and eggs into the sea at spawning, freshwater mussels brood their larvae inside their mantle cavities until they reach a two-shelled stage. It is then that the mussel plays a biologi­cal trump card.

Each of these special larvae, called a glochidium, is equipped with rows of teeth along the margins of its shells and a very long tentacle that protrudes from within. This tentacle is sensitive to the presence of native fish such as the koaro and giant bully, and with it the juvenile shellfish is able to latch on to a passing fish, clamping its toothed shells on to a fin. The hitch-hiking mussels hold on as the fish swims upstream, finally releasing their grip and sinking to the river bed when they judge they have travelled far enough.