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Zooplankton of Wellington Harbour, New Zealand

Materials and Methods

page 2

Materials and Methods

Plankton Sampling

Plankton samples for analysis were obtained with a 36 gauge extra heavy grit gauze two foot diameter cone net, with a 2lb "Agee" preserving jar tied into the collar. The sampling gear was towed behind a launch using from 50 fathoms to 60 fathoms of 8lb three-quarter inch hemp. While taking a sample, the towing speed which varied between ¾ knot and 1½ knot was adjusted to correct for current, wave, and wind resistance to both the gear and the launch. This allowed the net to filter at depths between one fathom and three fathoms for the greater part of the ten to twenty minute towing period. By turning the launch in a tight circle once during each tow, the speed of the net in relation to the water was reduced allowing the net to sink and filter at depths of five or six fathoms for about five minutes.

On completion of every tow the plankton adhering to the mesh on the inside of the net was washed into the jar with rinses of sea water. The bulk of the sample was preserved in 5% formalin solution in the field, while a selected sub-sample was kept for the study of living material. The date, time of day, cloud and wind conditions, state of tide, and surface water temperature, were all recorded for each plankton tow.

Analysis of Samples

The volume of plankton in each sample was recorded in a graduated measuring cylinder after settling for ten minutes. Where the duration of a tow was greater or less than the standard of 20 minutes the total volume of plankton was adjusted by simple proportion. From the number of samples in each month the mean monthly volume was estimated.

For comparison of samples with wide variations in volume the following sub-sampling technique was employed to obtain a constant representative volume (l0cc) of each sample.

Samples were diluted to approximately two litres with fresh water and well stirred. Sub-samples of 50cc were then removed and allowed to settle in a 50cc measuring cylinder. The excess water was siphoned off and successive sub-samples were added to the measuring cylinder to give exactly 10cc of settled plankton. This 10cc sub-sample was diluted to approximately 100cc with water and poured into a very large, shallow Petri dish which was placed on top of a circular piece of black paper divided into ten equal sectors with white ink. Three randomly selected sectors were counted for each of the species considered in the analysis. If the results of these three counts were erratic, further sectors were counted. When the total volume of a sample greatly exceeded 10cc, a further 10cc sub-sample was taken and treated in the above manner.

The number of individuals of each species in a 10cc sub-sample was thus obtained by simple proportion from the number counted in three or more of the sectors, and the result was adjusted to the total volume of the sample by simple proportion.

The counted and estimated animals were then divided into the following five major elements: Pleurobrachia pileus (O. F. Müller); Obelia geniculata (L.); Octophialucium funerarium (Quoy and Gaimard) together with Phialella quadrata (Forbes); Salpidae; and Crustacea. These five elements representing either individual species or groups of species were found to have considerable seasonal variation in numbers. This is reflected in their respective contributions to the total volume of the plankton samples (Text-fig. 8).

The species constituting the bulk of the volume together with the species selected as seasonal indicators were used to compile the annual plankton calendar (Text-fig. 7).

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Laboratory Rearing of Crustacean Larvae

Unidentified larval Crustacea were reared further in the laboratory. Two or three larvae were placed in a covered finger bowl filled with approximately 200cc of fresh sea water. The larvae were fed every second day with teased particles from the mantle and gonads of the marine mussel Perna canaliculus (Gmelin). To reduce invasions of protozoans, fungi, and bacteria to a minimum, the amount of mussel supplied at each feeding was no more than required by the species, the amount being learned by experience. The early juvenile stages of several brachyuran species consumed their own weight of mussel in only three feedings over a six day period. Anomuran larvae were found to be considerably less voracious, taking up to seven or eight feedings to consume a similar quantity. The sea water was drained and renewed on non-feeding days. These frequent water changes eliminated the need for aeration and ensured the regular removal of faecal pellets and any unconsumed food material.

Throughout 1961 and 1962 the experiments were without temperature control and temperatures fluctuated between 16°C. and 23°C. Under these conditions the mortality rate was extremely high, but limited results were obtained. In April, 1963 an insulated refrigerator trough 30 inches in length, 18 inches in width, and eight inches deep was made available for rearing experiments. Finger bowls containing larvae were placed in the trough and the temperature was held between 11°C. and 13°C. Within this temperature range the rate of larval and juvenile ecdyses was comparatively rapid, and harmful infections of fungi, protozoans, and bacteria were kept at a low level. Lower temperatures greatly increased the intermoult period of the larvae and temperatures in excess of 15°C. increased the mortality rate to a level similar to that of experiments without temperature control.

Very encouraging results were obtained with this refrigerating apparatus, and the use of mussel as food was found to be generally successful in the rearing of larval and juvenile Brachyura, larval Stomatopoda, and many later stage anomuran larvae. This feeding method gave little success with early stage anomuran zoeae.

Rearing Larvae from Adult Crustacea

Decapod crustacean larvae found in the plankton were in some cases identified by comparison with larvae hatched from eggs of ovigerous females obtained from beach collections. Females carrying eggs about to hatch showing larval eye spots were kept in a small sea-water aquarium until the eggs hatched. The hatched larvae were kept alive for as long as possible by the described laboratory rearing method.

Illustrations

Drawings of entire specimens were made on squared paper with the aid of a wide field "Spencer" binocular microscope fitted with a squared micrometer in one eyepiece. Coloured chromatophores are indicated by the code shown in Text-fig. 1.

Text-fig. 1.—Key to chromatophores.

Text-fig. 1.—Key to chromatophores.