Biology of the New Zealand Greenshell Mussel

Biology of the New Zealand Greenshell Mussel
Kutai, Kuku
NZQA unit standard 16340 version 6
A comprehensive guide to understanding the anatomy, life cycle, and cultivation of Perna canaliculus
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Contents
01
Describe the Greenshell™ Mussel
Classification and naming conventions
02
Anatomical Features of Greenshell™ Mussels
External and internal structures
03
Life Cycle, Feeding and Reproductive Cycle
From larvae to adult mussels
04
Growth of the Greenshell™ Mussel
Factors affecting development
05
Disease
Health concerns and management
1. Describe the Greenshell™ Mussel
Classification
All living creatures, both plant and animal, are classified by a hierarchical, scientific system that classifies them according to their similarities.
Mussels belong to a scientific grouping of animals (Phylum) known as the Mollusca or Molluscs. One of the distinguishing features of this grouping of animals is the presence of a shell (produced by a mantle). It is from this feature that the grouping gets its common name of "Shellfish".
All molluscs are then further subdivided into groupings based on their shared characteristics. This phylum includes seaslugs, bivalves (two shells), univalves (one shell), chitons, & cephalopods (squid, octopus).
Mussels belong to the Class of Bivalves – so called because they have two distinct shells or valves. The bivalves are the second most numerous group of molluscs. Some 10,000 species may be found worldwide and nearly 400 of these can be found in New Zealand waters.
Naming
In order to correctly identify a mussel it is necessary to use the scientific name. The scientific name is consistent throughout the world so that you can be sure exactly what animal is being talked about. Every different animal has its own unique, scientific name.
The scientific name contains two parts : i.e. Genus species
If two animals have the same Genus it indicates that they are very closely related. This part of the name may be shared by close relatives, like a surname. However the two-part name is always unique.
The scientific name often uses Latin words, which generally have some meaning relevant to the species being named. For example words may be used which represent the country where the species is found or describe a distinguishing feature. Sometimes the species may be named after the person who discovered it.
Common names however are simply the names by which the animal is "commonly" known and can vary from country to country and even between people. Each animal may be known by several different common names.
Cultured Mussel Species Worldwide
Table 1. Some cultured mussel species from different parts of the world
New Zealand Mussel Species
There are 16 species of mussel in New Zealand, but the two most well known are the two edible types the blue mussel (Mytilus galloprovincialis) and the Greenshell mussel (Perna canaliculus).
Some of the more common mussel species in NZ are listed over the page including both their scientific and common names.
Pictures of each species, distinguishing features and habitats are also given to enable the Greenshell mussel to be compared and readily identified.
Common New Zealand Mussel Species
Table 2. Distinguishing features and habitat of common New Zealand mussel species
Species Overview
Perna canaliculus - Green-lipped Mussel, Greenshell™ Mussel, Kutai, Kuku, Green Mussel
Mytilus galloprovincialis - Blue Mussel, Black Mussel, Mediterranean Mussel
Modiolarca impacta - Nesting Mussel, Small Brown Mussel
Aulacomya maorianus - Ribbed Mussel
Xenostrobus pulex - Little Black Mussel
Distinguishing Features of Common NZ Mussels
Greenshell Mussel (Perna canaliculus)
Colour
Green to black dependent on water depth / uv exposure and age. Juveniles are bright green. Has distinctive green lip (hence its name) along the inside shell margin.
Shape
Glossy smooth shell. Shell is slightly more rectangular shaped with more prominent angles than Mytilus.
Size
Up to 240 mm in height
Other Distinguishing Features
Has no anterior adductor muscle.
Habitat
Prefers the warmer northern waters but tolerant of a wide range of temperatures and salinities. Found in the low intertidal area and subtidal to over 50m. Prefers sub tidal Larvae prefer to settle on fine filamentous substrates such as algae, or in culture special fibrous collection ropes. May form dense beds up to 100m2 Attaches to wharf piles, rock faces, culture ropes etc Also found amongst algae beds and in deeper water over mud or sand bottoms.
Geographical Distribution
Unique to NZ Widely found throughout the whole of NZ (including the Chatham & Kermedec Islands) but most common in central and northern NZ
Aquaculture Notes
Principal Species farmed in NZ The Main areas where mussels are farmed are Marlborough/Golden Bay, Coromandel/Thames, Stewart Island. Though farms have recently been established outside of these areas.
Blue Mussel (Mytilus galloprovincialis)
Colour
Blue to Black. Usually with a chalky white appearance around the anterior or hinge area. When cultured shell appears glossy blue/black and is quite fragile
Shape
Glossy smooth shell with growth lines only. Dorsal and Ventral surfaces are more rounded than Perna.
Size
Up to 100 mm in height
Other Distinguishing Features
Has an anterior adductor muscle. Posterior adductor muscle is smaller than in Perna.
Habitat
Found naturally in the high intertidal range. Attaches to hard surfaces such as wharf piles, rock faces, culture ropes etc. Not as tolerant of very strong wave action.
Geographical Distribution
More common in the south of NZ. Abundant in Cook Straight. Occasional isolated populations in northern districts.
Aquaculture Notes
One of the principal species farmed overseas. (The main species of blue mussel farmed overseas is Mytilus edulis)
Other Species
Nesting Mussel
Modiolarca impacta
Olive brown to black, oval shaped with opposing ridges. Up to 50 mm. Wrapped in byssal threads. Found on kelp holdfasts and under rocks.
Ribbed Mussel
Aulacomya maorianus
Sienna to purplish-black with deep longitudinal ridges. Up to 85 mm. Found with blue mussels on low tidal rocks in Cook Strait and South NZ.
Little Black Mussel
Xenostrobus pulex
Blue-Black, small ovate smooth shell up to 30 mm. Very common in high intertidal zones in densely packed colonies. Prefers heavy wave action.
2. Anatomical Features of Greenshell™ Mussels
2.1 External Features of Mussels – location, identification and function
Bivalves are flattened from side to side (laterally compressed) so that the valves are positioned to the left and right of the body. In other molluscs such as oysters or scallops, where the shells are different, the left valve is usually the cupped valve, which contains the body.
The mussel shell is typically pointed at the umbo, which is found at the front or anterior of the animal.
The valves are hinged by a flexible ligament (hinge), around which develops a series of small ridges or teeth to ensure a snug fit when closed, and to prevent any sideways displacement of the valves. The hinge is located on the dorsal (back) surface of the animal. The ligament, which is constructed of horny conchiolin, is internal and tends to spring the valves apart. Hence in a dead or relaxed state mussel shells are naturally open.
The shell is rounded at the rear or posterior end. The shape and nature of the shell are quite variable depending on the environment in which the mussel has been grown. Rings of growth may be evident in the shell similar to rings on trees which are caused by changes or checks in growth. In aquaculture these checks are often caused by management practices such as transporting spat or reseeding. Factors which can affect the growth of mussels are covered in section 4 of this resource.
The long axis of the mussel (often referred to as length) is actually the animals height. The shape of the shell enables mussels to attach to surfaces in crowded groups while still gaining access to the water to take in food and get rid of wastes.
Often the byssus (beard) or strong attachment threads are obvious hanging outside the shell on the anterior ventral surface opposite the hinge. The mussel uses these threads to attach itself to the surface and can release its hold on these strands, and secrete new ones at any time enabling it to move, or alter position. The amount of byssus produced varies dependent upon the environment the mussel is living in (eg the strength of current it has to resist).
Shell Structure
The shells main purpose is to protect the soft body and it is composed of three layers
Table 3. Layers of the Mussel Shell
Periostracium
Outer layer
Composed mostly of protein (organic) Protective external layer relatively thin. This layer gives the green colour to perna, but can become almost black when exposed to sunlight, in the top layers of the water.
Prismatic Layers
Middle layer
The thickest layer Is often chalky in nature Made mostly of inorganic calcium carbonate (90%) in a crystalline structure (Calcite or aragonite) mixed with small amount of protein substance called conchiolin
Nacreous Layer
Inner layer
Thin, often shiny or lustrous - the pearl forming layer. Very hard Also made of calcium carbonate but here the crystals are in sheets which reflect and bend light giving a shimmery appearance.
2.2 Internal Features of Mussels
Location, identification & function
Internal Organs
3. Life Cycle, Feeding and Reproductive Cycle
3.1 Life Cycle
Mussels are broadcast spawners. This means that they simply release their eggs and sperm into the water. The eggs of perna are about 0.05 mm in size and are fertilized in the open water by sperm which have been released by male mussels.
The larvae of mussels are free swimming and have a planktonic stage which lasts around 4 to 6 weeks before they are ready to settle. The planktonic stage enables the larvae to disperse to new areas, and mussel larvae may be moved several hundred km by the currents. During this stage the larvae go through 2 changes before metamorphosing into a miniature adult mussel which is called the spat.
Within 24-48 hours the fertilized egg has developed into a D shaped larva so called because of its straight-hinged shape. These larvae have some ability to move vertically in the water column using an organ called the Velum which is covered in small cilia. However they are mostly at the mercy of ocean currents. The velum is also used for feeding.
The larva continues to grow and changes shape developing an umbro (point to shell). The veliger larva then develops two pigmented eyespots and a functional foot (at which stage it is called a pediveliger and is getting ready to settle).
The pediveliger uses its foot to search for settlement surfaces, and its velum to move between potential settlement sites. The larvae look for a suitable filamentous substrate such as seaweed or culture rope. They then attach themselves to the surface by secreting byssal threads.
Immediately after settlement the larvae metamorphose into the adult form, loosing the velum and becoming bottom dwellers. At this stage they are known as spat.
Early on the spat can still move if the site they have chosen doesn't suit, by releasing the byssal threads and using mucus threads which act like parachutes enabling them to drift in the water currents. This enables them to recruit into adult beds. They loose this ability to drift when they are around 6mm in height.
Larvae of the blue mussel Mytilus tend to settle close to the water surface due to their preference for intertidal sites, whilst those of the green mussel tend to be more densely distributed, deeper in the water column.
Juvenile mussels continue to grow, and if conditions are right will usually become sexually mature (adults) in the first year of growth.
3.2 Feeding Method
Mussels are filter feeders, which means that they take in (inhale) seawater from the environment and "filter" out particles present in the water, which are then bound, sorted, and moved to the mouth for consumption. The food particles mussels eat are phytoplankton (algae).
Larval Feeding
In mussel larvae the velum acts as the sieving apparatus. The velum has a band of long hairs (cilia) which collect suspended food particles from the water. These particles are then moved towards the mouth by the beating of shorter cilia contained in a special "food groove" which leads to the mouth.
The velum is also able to absorb dissolved nutrients directly from the water.
These cilia are able to sort and select food of the correct particle size (In newly hatched larvae particles of 2-6 m are preferred).
Once the particles enter the stomach further sorting occurs and inert particles (not food) can be passed directly to the intestine for removal. Food particles are torn open and digested by the rotation of a crystalline style as in adult mussels.
As the larvae grow the sizes of phytoplankton they can accept increase. Pediveligers are capable of feeding on phytoplankton of 15-20m in size.
Once the larva settles and undergo metamorphosis, it looses the velum and assumes the adult form (grows gills etc).
Post Settlement Feeding
In post settlement mussels the gills act as the sieves to remove the particles from the water. Adult mussels are fairly non-selective feeders eating phytoplankton of various sizes. The gills are also able to absorb dissolved nutrients directly from the water.
Covering the gills is a layer of small hairs or cilia which move water and food particles by beating together rhythmically in waves. Water is moved across the gills and out the exhalant chamber behind the adductor muscle. (Details the water flow across the gills are shown in the diagram 3.)
From this water the gills strain or filter potential food particles of the correct size from the water and capture these in a layer of mucus which is then moved to special food transport pathways and conveyed to the labial palps by other gill cilia.
The labial palps further sort the particles trapped in the mucus into food of the right size, and waste. Food particles are then placed in the mouth (waste particles are rejected as psuedofaeces).
As with larval mussels, once food enters the stomach it can be sorted further and non-food items bypass the digestion process and are directed straight into the intestine.
Mussels are capable of pumping large volumes of water and can increase or decrease this rate dependent upon the concentration of the food in the water around them. An adult mussel typically filters 6-9 litres of water per hour whilst a 12 month old mussel is capable of moving 2-3 litres of water per hour. Rates for individual mussels as high as 350 litres per day have been recorded.
The gills are extremely efficient at filtering this water and can sort down to 1μm in particle size! This means that mussels also efficiently strain out and concentrate any bacteria or contaminants that are present in the water. Hence shellfish can be sources of bacterial infection to organisms which eat them (eg humans) and programs such as the Sanitation program are in place to prevent health risks to consumers.
3.3 Reproductive Cycle
Sexual maturity in mussels is often reached by the first year of age, though this very dependent upon growth rates achieved. In Marlborough culture, 14-24 months to maturity is usual. Mussels tend to mature and spawn in cycles which are controlled largely by environmental factors, predominantly water temperature and food availability. The development of reproductive tissue only begins at temperatures greater than 11oC.
Greenshell mussels tend to spawn mainly between spring and autumn. However, within a population of mussels there may be adults at a variety of different stages of gonad development year round, and spawning condition may be maintained for several months. Not all mussels spawn at the same time. Spawning peaks occur in Aug/Sep & Mar/Apr.
There are five stages which identify the stages of mussel gonad development:
Gonad Development Stages
In mussels the gonad or reproductive tissues form both in the inner mantle tissue and in the mesosoma. In immature or resting mussels this tissue is undeveloped, of consistent texture and appears creamy coloured. Hence, male and female mussels cannot be identified when immature or resting. Both the size and shape of the gonads change as the animal matures.
As the gonads begin to develop spaces form called follicles. The cells which line these follicles produce the gametes (eggs and sperm). In females the eggs progressively grow in size until in mature females they are packed in so tightly they form polygon shapes. In males the sperm form laminae (layers) around the follicles which are dense and compact in mature males.
During spawning the gonads empty, until in the spent animal only residual gametes and collapsing follicles are found.
Females tend to spawn out completely relatively quickly whereas males tend to spawn gamete more slowly over an extended period and then reabsorb what remains.
In summer the spawning is prolonged and does not appear to be followed by a major resting phase, most mussels enter straight into redevelopment.
Hence the time marketable mussels are most likely to be limited is after the winter spawning event (less often in summer). This is particularly the case in Coromandel where mussels recondition rapidly in summer.
The density of the follicles and their size is a good indicator of condition.
The female gonad or ovary produces eggs (ova) and as these form, the ovary develops an orange/pink colour. A female mussel may produce up to 100 million eggs in a season.
The male gonad or testes produces sperm and the testes are creamy white in colour.
When mussels are close to peak condition the mesosoma becomes large and swollen, and discharge tubules may appear like veins across the surface of the gonads. At this time when ready to spawn, they may release sperm or eggs when touched.
Close-up of ripe male gonad. (Buchanan 1999)
Once spawned the gonad becomes almost transparent in some cases, though there may be some remnants of eggs and sperm remaining. Mussels in this stage are referred to as Spent and are not suitable for harvesting.
The sexes are separate in green mussels though in blue mussels there may be some hermaphrodites which have both patches of white and orange and produce both female eggs and male sperm.
Reproductive Cycle Details
Table 4. Reproductive Cycle in Greenshell Mussels
4. Growth of the Greenshell™ Mussel
4.1 Growth
The growth rate of the larval stages of mussels is mostly dependent on food availability, temperature and also salinity. Normally the larval phase lasts between 4 and 6 weeks.
Following settlement growth rates vary widely, and again are dependent on food availability and water temperature at the settlement site. These factors can vary widely from site to site, between seasons and between subsequent years.
For example in culture, mussels of target harvest size (80-115 mm) are typically achieved in 14-24 months, following reseeding in Marlborough, and 7-14 months in warmer Coromandel waters.
Method of Shell Production
Mussels grow by adding new shell along the edges of the existing shells. This new shell is produced by the outer mantle. The outer lining of the mantle lobe thickens the shell whereas the marginal mantle folds add new shell.
As the growth rate changes, rings (like growth rings in a tree) are formed. Periods of slow growth result in strong or concentrated lines which are evident as ridges (or'rings') on the shell. These "checks" in growth are often most obvious following reseeding.
4.2 Factors which Affect Growth
The main factors affecting mussel growth in culture are stocking density, food supply (quantity, quality, & replenishment) and temperature of the water.
Stocking Density
The stocking density is a measure of the quantity of mussels occupying a known volume of water (both the size and number of mussels are important). Correct stocking densities are critical to produce quality product. Stocking densities must be matched to individual site characteristics and particularly the food supply or productivity of the farm. Too high stocking density will result in a variable sized crop with poor mussel condition particularly from the centre of the farm. Too low and full potential of the area is not realized causing an increase in the cost of production.
Critical factors in controlling stocking density are the number of mussels per m of dropper, the spacing between droppers and the balance between these two factors. The stocking density must also be relevant to the size of the mussels being seeded (eg spat seeding vrs final seeding).
The effects of stocking density may be compounded by other effects such as the decrease in the current flow which may occur through the farm when droppers are placed closer together. High densities of mussels per meter may result in increased competition between mussels for available food and hence a greater size variation. Farmers may also vary the total farm stocking by altering the layout of the farm area such as the distance between the long lines (number of longlines) or the length of the long lines.
The suitability of a chosen stocking density or farm layout regime may also vary dependent on both seasonal and annual variations affecting food supply throughout the farm, water replacement rates and productivity of the area.
Optimal stocking densities vary dependent on the individual farm but examples of variations in the two main factors include;
Number of mussels per m of dropper
(may range from 130-180 per m in Marlborough 200-250 per m common in Coromandel)
Number of droppers per m of longline
(600 - 1000 mm spacings common in Marlborough & Coromandel)
Food Availability / Phytoplankton Levels
Optimal growth is achieved when food availability is not limiting but is matched to total farm stocking rate / temperature so that there is sufficient food for all mussels to grow.
The production of phytoplankton in the water varies seasonally and annually dependent upon:
Light levels (ie day length)
Nutrients (eg freshwater runoff, upwellings, storms, nutrient cycling)
Temperature
These three things are required for phytoplankton to grow and multiply and result in constant changes in phytoplankton abundance.
As mussels are filter feeders, higher concentrations of phytoplankton in the water will require lower filtration rates to intake the maximum volume of food and enhance growth. Mussels can alter their filtration rates based on the concentration of plankton in the water.
Seasonal variations in plankton may also be considered when deciding on stocking strategies for different farms (eg rotational cropping, suitable seeding size).
Higher levels of phytoplankton in the water will generate faster growth rates.
Environmental Factors Affecting Growth
Food Quality (Type of Phytoplankton)
Mussels are selective filter feeders and therefore the level of suspended matter in the water (such as measured by a sechii disk) or the levels of total phytoplankton in the water (e.g. chlorophyll level) are not necessarily good indicators of mussel growth.
Some of the suspended matter in the water column may not be food (such as suspended sediments after heavy storms or rainfall) and some of the phytoplankton species may be of relatively poor nutrition (of the wrong size or poorly digestible). Algal blooms may not be favourable to good growth if the species, which is most abundant, is nutritionally deficient or poorly digested by mussels.
Best conditions for growth occur when there is a low proportion of non-food particles in the water and a good mixture of algal types giving a nutritionally varied and adequate diet.
Water Flow or Replacement Rates
Mussels are constantly removing phytoplankton from the water. In a farm situation this results in decreasing concentrations of plankton in the water, which is moving past the mussels. It is important that this water is mixed and or replaced with "ungrazed" water in order to ensure that the concentration of food in the water the mussels are feeding on is always sufficient for growth.
Even in areas with low tidal currents high wind exposure can result in good water mixing and cycling of nutrients, allowing good phytoplankton growth and replacement of phytoplankton concentrations around the mussels.
Farm layout regimes and management techniques can have important impacts on water flows through the farm and hence food exchange. In culture problems such as "dropper twisting" result in reduced food supply & water flow around portions of the crop affecting growth rates in some mussels and reducing crop consistency. The effects of twisting may be reduced by appropriate management techniques (eg untwisting by divers, staggered hanging techniques, spacer bars, reduced dropper depth).
For optimum growth therefore it is essential that there are good levels of water exchange or currents throughout the whole farm. This ensures that depleted water is constantly replaced (eg increased algae concentrations, consistent dissolved oxygen).
Water Quality
Temperature
Mussels tolerate a wide temperature range (from 5 - 27oC) though they tend to be more abundant in warmer northern waters. They will only mature and spawn in temperatures above 11oC.
Water temperature has an impact on growth rates, both directly through its effect on the metabolism of the mussel and indirectly through its affect on phytoplankton growth (ie food supply). As water temperatures increase so do filtering rates of mussels and their metabolic rate (and hence the potential for a higher growth rate).
Other factors however, such as food availability interact with temperature to determine the actual growth rates achieved.
Dissolved Oxygen
Mussels, like other animals require oxygen to survive. But due to the fact that they are stationary, they have relatively low oxygen requirements. This oxygen is absorbed from the water by the gills.
In most cases oxygen levels in marine areas are well above the levels required by mussels for growth. Occasionally however it is possible there may be incidences where limited areas suffer from a high biological oxygen demand (such as in a pollution event or die off from an algal bloom) and deaths may occur as a result.
Normal coastal oxygen levels, in areas with no bloom or pollution problems, provide optimal conditions for growth.
Additional Growth Factors
Salinity
Normal coastal marine salinities are between 30-35 ppt and are best for optimum growth.
Greenshell mussels have been shown to be tolerant to a wide range of water salinities and in laboratory experiments have survived in salinities as low as 25 ppt.
Short periods of time where salinities drop lower than this can also be tolerated but are likely to cause reduced growth while the low salinity water is present. Salinities may drop as a result of high levels of freshwater inflow (such as occurs in the inner Pelorus Sound) though most often these lower salinity waters are confined to the top few meters of the water column.
There have been occasional instances of mortalities amongst Greenshell mussels growing in beach areas following extended periods of high fresh water inflow.
Suspended Solids / Weather Patterns
Because shellfish initially sieve food from the water by size rather than identity of the particle, they are very efficient at removing all particles from the water, and can waste a lot of energy in dirty waters removing and transporting sediments and other non-food items. This wasted effort can slow down the growth of the mussel.
Stormy weather can affect water quality by mixing phytoplankton, suspending sediments, or adding sediments of land origin to the water column. The higher the amount of non food items in the water the more energy needs to be expended filtering out and ejecting (psuedofaeces or faeces) non-food particles.
Low proportions of non-food particles in the water are therefore optimal and likely to increase the potential growth of the mussel.
Toxic Algae
Mussels eat algae and hence blooms can often be beneficial to growth. However some types of algae are toxic to shellfish and can reduce growth or cause mortalities. Non-toxic algae can also affect growth if they reach very high levels in the water and / or die off (creating an oxygen shortage in the water).
Other Factors Affecting Growth
Handling (Stripping/Reseeding)
In aquaculture some stripping and reseeding is necessary to ensure good crop density and hence maximize growth. However, reseeded mussels must expend energy both repositioning and producing new byssal (beard) growth for attachment. This results in a reduced growth for a period of time which is often evident as a check line on the shell.
Minimal handling is best to get best results for growth.
Exposure
Greenshell mussel spat are physiologically unable to survive exposure to air unlike some of the other mussel species and as such the Greenshell mussel is uncommon above the mid shore, and is most common subtidally.
Optimum growth occurs subtidally as mussels, which are exposed during tidal cycles, have both reduced access to feed in the water and a reduced ability to uptake oxygen.
Water Depth
Greenshell mussels prefer Subtidal habitats and can exist to depths over 50m. However the food supply of mussels requires light to grow and hence where currents are not sufficient to ensure good mixing of the water column food availability may be limited at greater depths.
The optimum farming depths avoid the surface water layers (which attract blue mussel settlement), avoid droppers laying on the bottom (poor water circulation, food supply) and where the water is deep avoid depths where food supplies may be limiting (or where dropper depth may encourage dropper twisting).
Predator & Fouling Control
Particularly important for the spat stages of growth are
Competitors (e.g. blue mussels) which remove food particles from the water.
Foulers (e.g. seasquirts) which can smother, lower water movement and again reduce the ability of mussels to obtain food from the water.
Predators (e.g. spotties, snapper)
Optimal conditions for growth utilize farm management methods to minimize these factors. (eg clean spat catch, careful selection of spat holding areas, suitable farm sites & dropper depths, rotation of spat sites).
5. Disease
Viruses, bacteria, fungi, protozoa, and parasites can cause infectious diseases.
Viruses, bacteria and protozoa are microorganisms, so called because individuals are single cells, which are too tiny to be seen with the naked eye. Viruses are less than 0.3 micrometers in size and actually live inside the living cells of the host who is infected. Infectious diseases can be passed directly from one organism to another (eg mussels to mussel, oyster to mussel).
Non-infectious diseases cannot be passed from one organism to another, but are caused by external factors such as nutrition (eg poor plankton availability or type), environment (eg toxic algae blooms, smothering by competitors) or physical trauma (eg predator damage, reseeding shell damage).
Diseases Present In New Zealand
To date, New Zealand appears relatively free of diseases affecting Greenshell mussels.
Although there are several natural parasites of the Greenshell mussel, most of these have low infection rates and do not cause either significant mortalities or concern to the aquaculture industry. Some parasites found in mussels include pea crabs, copepods, mudworms and flatworms.
Digestive epithelial virosis, a viral disease has however caused high mortalities in post settlement mussels, and resulted in some problems for the aquaculture industry.
Mussels may also be affected by Algal blooms, although is most cases, the main concern in mussel culture is that algal toxins may be concentrated in the flesh making them a human health risk and thus unmarketable for periods of time. Algal blooms do however also have the potential to affect mussel growth or cause mortalities.
The shellfish toxin testing program which is in place to protect humans from consuming toxic shellfish, also warns farmers of bloom conditions by checking water samples for both algae numbers and types.
Below is a list of some diseases present in New Zealand that have the potential to affect Greenshell mussels.
Algal Blooms
APX (Apicomplexan Parasite X)
Digestive epithelial virosis
Flatworm infestation
Mudworm infestation
Information of each of these diseases, their gross signs, the causative agents, and any treatments or preventions are contained in the following handbook:
Diggles, B,K; Hine, P,M; Handley, S; Boustead, N.C. (2002). A handbook of diseases of importance to aquaculture in New Zealand. NIWA Science and Technology Series No. 49. 200pp.
For convenience, extracts from this handbook on the following four diseases are provided in Appendix A of this resource, courtesy of NIWA.
Algal blooms
Digestive epithelial virosis
Flatworm infestation
Mudworm infestation
See Diggles et al, 2002 (and Appendix A) for more background on other diseases, including gross signs, causative agents, treatment and prevention.
References
Buchanan,S.J. (1999). Spat production of the Greenshell mussel Perna canaliculus in New Zealand. Ph.D. thesis, University of Auckland, Auckland, New Zealand.
Diggles, B,K; Hine, P,M; Handley, S; Boustead, N.C. (2002). A handbook of diseases of importance to aquaculture in New Zealand. NIWA Science and Technology Series No. 49. 200pp.
Gosling,E. (1992) Editor. The Mussel Mytilus; Ecology, physiology, genetics and culture. Developments in Aquaculture and Fisheries Science 25, Published by Elsevier.
Gunson, D. (1993). A Guide to the New Zealand Seashore. Penguin Books (NZ) Ltd
Hayden,B.J. (1994). What do we know about Greenshell mussel spat Part I. Seafood NZ Oct 94, 45-47.
Hayden,B.J. (1994). What do we know about Greenshell mussel spat Part II. Seafood NZ Nov 94, 34-35.
Jeffs,A.G.; Holland,R.C., Hooker,S.H. and Hayden,B.J. (1999). Overview and bibliography of research on the Greenshell mussel, Perna canaliculus, from New Zealand waters. Journal of Shellfish Research 18(2):347-360.
Jenkins, R.J. (1985). Mussel Cultivation in the Marlborough Sound (New Zealand). 2nd edition, revised by Meridyth-Young,J. & Waugh,G.D.NZ Fishing Industry Board.
Powell, A.W.B. (1979). New Zealand Mollusca marine, land and freshwater shells. William Collins Publishers Ltd, Auckland.
Pownall, Glen (1979). New Zealand Shells and Shellfish. Seven Seas Publishing Pty Limited, Wellington.
Quayle, D.B. and Newkirk,G.F. (1989). Advances in World Aquaculture , Volume 1. Farming Bivalve Molluscs; Methods for Study and Development. The World Aquaculture Society.