Publication Extract

The following extract is the Teachers' Notes, Activity 7, pages 27 and 28, from the BES Publication, "Brine Shrimp Ecology"

Exploding Populations

Teaching objectives

Students must by now be able to tell the sexes apart. In three or four weeks this experiment should produce some very telling results on the clear relationship between food supply and population growth. In every bottle the experiment may not show a clear distinction but with the three replicates there should be a clear mean difference. The students are asked to make their own hypothesis. They are also asked to make their own table of results and devise their own graphs. Students will need to watch for hatching events and be vigilant observers.

Background

The experiment requires some addition of tank substrate. This is needed to provide the continued release of algal nutrients for three groups of experimental populations equally. The Liquizell should ensure good survival of hatched nauplii and a strong differential food supply. As long as the 0:1:3 differential between the three sets of experiments is maintained the result should be clear. The experimental design means that you will have three replicates of each of the three conditions. It would be advantageous to use females and males that have only recently reached sexual maturity. If you can arrange to use a fairly recently hatched set of shrimps for this experiment that would be good. If one, or both, adults die before the eggs are laid then replace the dead adult with another of the same sex. Students could show you their pairs for you to check.

Forward Planning

Young adult shrimps would be an advantage at the outset. It is suggested that these be hatched just two to three weeks before the practical. They should then be able to be sexed but not yet necessarily paired up. This investigation should have some long term source of heat and light if done in the winter. A south facing window-sill will be fine in the summer term otherwise.

It is suggested that nine bottles is the minimum number for a whole class. The experiment, as written here, need only therefore be done once by the whole class.

Apparatus

• nine plastic bottles (or beakers), 330 cm² are ideal
• salt water (a concentration of 30-35 g per litre - this needs to be made up first). Allow nine litres for the whole experiment (there are 15 litres in a typical plastic bucket)
• teat pipettes, or dessert spoons or sieves
• nine gummed labels to identify each bottle (or beaker)
• substrate from the main brine shrimp tank
• bench lamps - to provide a source of heating and lighting for the shrimps during the month, or term, that the experiment is running
• liquid fertiliser and Liquizell. One bottle will easily supply a whole class for this experiment. (see suppliers sheet). If you have no Liquizell, yeast suspension is a less good, but real, alternative. With yeast, oxygenation is needed as the algae may not provide oxygen adequately.

Risk assessment

No hazards are envisaged.

Notes on Procedure

1. To do this whole experiment as a full class experiment will mean that there is one bottle to be looked after for n/9 students. Two parallel experiments would mean that one or two students could deal with each bottle. There will be a lot of counting to do if the bottles are large and the experiment runs for several weeks.
2. It is important to ensure that there are a mating and viable pair in each bottle. You might check on this after the initial class.

Class findings

A data table will be needed with at least ten columns. The optimal design might be as shown below.

Extension

The difference between arithmetic and geometric progressions should be clear to the most able. You might suggest that they work out theoretically the rate of population increase (r) from the data at the end of the Challenge.

Questions/answers

1. The purpose of the experiment is, in part, to show the constraints on the holding tank's own carrying capacity. It will not have grown proportionally in population as much as even the control bottle.
2. Each bottle tends to behave slightly differently. The larger the bottle ecosystems the more steady they are likely to be.
3. Studies on the Great Salk Lake Utah (Cuellar 1990) indicate that the fequency of females rises during the boom of summer populations and that the males become predominant at the close of the season. Statistically significant departures from a 50:50 ratio would suggest this strategy was one for optimising egg production in the good times. The mechanism for this imbalance between the sexes is not understood.

Copyright: BES Brine Shrimp Ecology Project: Homerton College Cambridge