by Maureen Bentley (Huddersfield)
Although my family and I live quite a distance from the coast, we are avid rockpoolers and lovers of marine life. We keep a number of aquaria housing both coldwater and tropical marines.
Being an inland aquarist does have its problems, the worst in our experience has been supplying sufficient live foods, especially for difficult to keep tropicals, which refuse dried or frozen foods.
In 1993, I began to research methods of culturing various natural foodstuffs available from our shores, in particular the mysid shrimp, which is eaten regularly by many fish.
A scientist at the Gulf Research laboratory in the USA was kind enough to send photocopies of the various culture methods used in the laboratories, and some useful information on the breeding habits etc. of mysids. Due to his help I have been able to set up a successful culture which yields and average of 200 young per day.
Mysidacea, or Opossum Shrimps, are found world-wide. They are shrimp-like organisms found in a variety of habitats. At least 460 species have been recorded, although identification is difficult. Close study of the tail region can determine each species. Adult mysids seldom exceed 30 mm in length. The average life span is 12 months; the females have a large brood patch in which they carry developing young.
From May to mid-August Neomysis integar can be found at the heads of certain British estuaries where they breed. This species prefers reduced salinities and is the easiest to culture.
For collecting mysids, a large net is required, which should be about 30 cm square, with very fine mesh, with the holes under 1 mm. The mysids can be stored in polystyrene containers.
At low tide look for a shallow, sandy, weedy area. Mysids prefer calm waters and can be easily spotted on a sunny day, resting on the sand or swimming amongst the weed. However, we have found that the best time for collection is at dusk when they are more active as mating is usually nocturnal.
The National Rivers Authority has no jurisdiction on mysid collection. Many estuaries are National Nature Reserves and permission may be needed.
THE MAIN CULTURE TANK
Mysids are used in the USA for toxicity testing, as they are very sensitive to toxic substances, therefore a well established tank with zero readings of nitrite, ammonia etc. The main culture tank should be large with undergravel and external biological filters. Aeration should be medium to heavy. A protein Skimmer is essential. The substrate is a layer of crushed shells with calcium plus, topped up with a fine layer of coral sand. A 'Gravel Tidy' can be installed to separate the layer of coral sand.
A salinity of 50% of natural seawater, about 1.7% is recommended. At 180C this is measured on the hydrometer at a specific gravity of 1.016. This is just off the scale of the normal bulbous aquarium hydrometers. The temperature should be kept at a constant 180C. If synthetic salt is used, it should be aged for at least one month before the mysids are introduced. Lighting is by two fluorescent Grolux tubes with a photoperiod of 14 hours per day. A pH of between 8.0 and 7.8 should be maintained. Reproduction ceases at a pH below 7.4.
Mysids reproduce at all times of the year. During the summer months a large female will produce an average of 40 young per brood, with a gestation period of 16 days. In winter the brood size is drastically reduced to an average of 5 young with a gestation period of 3 to 4 weeks. Breeding is at a minimum in December and January.
The eggs are fertilised by the male depositing spermatozoa into the brood pouch of the female. The juveniles are released just before the female's moult and mating occurs soon after.
HARVESTING THE YOUNG
If large numbers of juvenile mysids are needed, the diagram shows the best way of daily harvesting. The water returns to the tank through two siphons, made off plastic tubing with two small funnels on the end in the large tank. The mouths of the funnels are covered by 7 mm mesh netting which is fastened to the funnel with rubber bands. The netting decreases the number of adults, which are sucked into the siphon. Two siphons are used because one is insufficient to return the volume of water pumped by the filter. The water volume in the two aquaria is adjusted so that if one or both siphons fail, all the water can be pumped into the large tank without overflowing it, and, if the pump fails, water will siphon into the small tank without overflowing it.
The siphons empty into a trap in the small tank which is made of a cylinder
of 0.333 mm mesh cloth which is glued on with silicon rubber cement. The
young mysids pass through the siphons into the trap where they can be harvested.
When the system is operating, the water level in the large tank is about 6 cm higher than that in the smaller tank. The funnels are positioned so that they open in the quieter water, away from the filter pump discharge hose. The newly discharged young tend to congregate in the quieter water.
The system is operated when the lights are out and the young are harvested each morning. The young not needed immediately for food are transferred to a smaller tank where they are grown to the required size.
Small Scale Operation
If not many young mysids are required daily, 15 to 20 large gravid females can be placed in a small glass tank with a small undergravel filter in operation, with the water circulated using a small capacity diaphragm air pump. When the young have been released the females can be returned to the main tank and the young can be kept for short periods in the small tank.
Young mysids left to grow in the main tank, but many will be eaten by the cannibalistic adults, (especially the males).
In their natural habitat, mysids are carnivores and scavengers. They feed mainly on zooplankton, but are capable of carrying off and chewing large pieces of meat and seaweed.
In captivity they will eat a variety of foods. The main diet should be 24 hour old Artemia nauplii, or copepods, with occasional supplements of marine flake, and mussel or cockle cut up into tiny pieces. They are particularly fond of brine shrimp flake. Newly released juveniles will need newly hatched Artemia and rotifers until they are a few days old. The culture is fed twice daily.
The amount of food need every feeding is rather difficult to establish as this will depend on the size of the culture. A feeding frenzy follows the introduction of live food. This is expected to last for 15 minutes after which all the live food will have been consumed. After a feeding of Artemia the mysids, usually transparent, should have orange stomachs. Signs of insufficient feeding are demonstrated when the adult males cannibalise the younger mysids in large numbers.
I am still searching for more information on this subject, but sick or dying mysids show signs of erratic swimming and a 'whitening' of the tail area.
If you introduce natural seawater, there is always a danger that plankton and parasites will be introduced at the same time. The plankton can provide additional food, but dying populations may cause oxygen depletion. In the USA a hydroid colony was inadvertently introduced. Hermit Crabs were added to the tank to clear up the problem. A few of the small Hermit Crabs found on the shore in the British summer would make an amusing addition to the tank.
STOCKING AND DENSITY
When stocking the culture, introduce the mysids gradually. The density should be between 20 and 40 adults per gallon. Overstocking will result in fighting between the males, and the deaths of the gravid females. This also applies to the containers used for transportation. Large numbers of mysids jumping out of the water and getting stranded on the glass sides is a good indication that the tank is overstocked.
It is advisable to treat freshly collected mysids with a bactericide suitable for invertebrates.
Mysids can be difficult to find during the winter months, due to the decrease in the number of young born. Searching for them in the cold conditions is not very pleasant. A few can still be found in estuaries and rock pools.
In the north-west and north-east coast of England we have discovered a few locations where swarms can be found in the winter months. It is not clear why these areas are preferred. If any other members have discovered other areas where winter swarms are to be discovered I would be very pleased to hear from them.
The mesh cloth for the nets can be obtained from specialist haberdasheries.
many of the large nets suitable for pond fish have
1 mm netting, and can be obtained from aquarium retailers and garden centres.