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.
Diagram 5. The Stages in a Mussel Life Cycle
a. Pictures courtesy of Cawthron Institute b. Pictures taken from Jenkins (1985)
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).
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 mm 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 15-20mm in size.
Once the larva settles and undergo metamorphosis, it looses the velum and assumes the adult form (grows gills etc).
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.
Diagram 6. Food particle sorting by 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:
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.
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.
Table 4. Reproductive Cycle in Greenshell Mussels