9. INTERNAL ANATOMY AND PHYSIOLOGY
269
Thysanoessa raschii in darkened tanks increases the length of their
lives under laboratory conditions. Baker (1963) caught euphausiids in
an Isaacs-Kidd midwater trawl in hauls of 30-45 min duration a t
speeds of 14-2 knots and transferred the liveliest individuals to tanks
at 7"-10"C and 12"-15"C. The species kept in the tanks were Meganyctiphanes norvegica, Thysanopoda tricuspidata, Euphausia brevis, E .
krohnii, E . hemigibba, Nematobrachion sexspinosum, N . boopis, Nematoscelis spp. The maximum survival time was 15 days for Meganyctiphanes norvegica, all other species living for lesser periods. Much greater
success has attended the maintenance of Euphausia pacijica, E . eximia,
and Nematoscelis dificilis (Lasker and Theilacker, 1965). The nets were
never towed for longer than 15 min at depth, and were hauled slowly
to the surface. The animals were placed in buckets of sea water a t
10°C, at a density of not more than five per litre. The ship then steamed
for 3-4 hr homeward to the laboratory where the euphausiids were
placed singly in " white, translucent polystyrene containers of one litre
capacity " and covered with black polyethylene sheeting. Several
thousand newly hatched nauplii of Artemia salina were placed in each
bucket; single Euphausia pacijica or E . eximia were able to ingest as
many as 70-95 nauplii per day when a total of only 100 nauplii were
available. The intermoult period of these euphausiids shortened from
5 days, when fed on algae, to 3-4 days when fed on nauplii. The water
in the buckets was maintained a t temperatures between 14.8 and
16.4OC for 3 months and but for an initial mortality during the first
2 days after capture, survival was good.
The ease with which these euphausiids can be maintained under
experimental conditions has enabled Lasker and his colleagues to
investigate the moulting cycles (Lasker, 1964, 1966; Jerde and Lasker,
1966). He first studied the moulting of Euphausia pacijica in the
laboratory ashore and found that it moulted, on average, once every
5 days at experimental temperatures ranging from 9" to 14°C. One
specimen lived for 50 days in the laboratory and moulted eleven times
in that period. Jerde and Lasker extended this work on E . pacijica to
several species. They successfully maintained forty E . pacijica, nine
E. eximia, three Thysanoessa spinifera, three Nyctiphanes simplex, and
four unidentified euphausiids all of which moulted two or three times.
The mean frequency of moulting of these fifty-nine animals is shown in
Fig. 103 where it is seen that most moult every 5 or 6 days. The changes
in length of the intermoult periods of Euphuusia pacijica were related
to the changes in the temperature of the water in the Scripps Institution
of Oceanography's experimental aquarium where the animals were
kept. Lasker studied the changes in the length of the intermoult period
269
Thysanoessa raschii in darkened tanks increases the length of their
lives under laboratory conditions. Baker (1963) caught euphausiids in
an Isaacs-Kidd midwater trawl in hauls of 30-45 min duration a t
speeds of 14-2 knots and transferred the liveliest individuals to tanks
at 7"-10"C and 12"-15"C. The species kept in the tanks were Meganyctiphanes norvegica, Thysanopoda tricuspidata, Euphausia brevis, E .
krohnii, E . hemigibba, Nematobrachion sexspinosum, N . boopis, Nematoscelis spp. The maximum survival time was 15 days for Meganyctiphanes norvegica, all other species living for lesser periods. Much greater
success has attended the maintenance of Euphausia pacijica, E . eximia,
and Nematoscelis dificilis (Lasker and Theilacker, 1965). The nets were
never towed for longer than 15 min at depth, and were hauled slowly
to the surface. The animals were placed in buckets of sea water a t
10°C, at a density of not more than five per litre. The ship then steamed
for 3-4 hr homeward to the laboratory where the euphausiids were
placed singly in " white, translucent polystyrene containers of one litre
capacity " and covered with black polyethylene sheeting. Several
thousand newly hatched nauplii of Artemia salina were placed in each
bucket; single Euphausia pacijica or E . eximia were able to ingest as
many as 70-95 nauplii per day when a total of only 100 nauplii were
available. The intermoult period of these euphausiids shortened from
5 days, when fed on algae, to 3-4 days when fed on nauplii. The water
in the buckets was maintained a t temperatures between 14.8 and
16.4OC for 3 months and but for an initial mortality during the first
2 days after capture, survival was good.
The ease with which these euphausiids can be maintained under
experimental conditions has enabled Lasker and his colleagues to
investigate the moulting cycles (Lasker, 1964, 1966; Jerde and Lasker,
1966). He first studied the moulting of Euphausia pacijica in the
laboratory ashore and found that it moulted, on average, once every
5 days at experimental temperatures ranging from 9" to 14°C. One
specimen lived for 50 days in the laboratory and moulted eleven times
in that period. Jerde and Lasker extended this work on E . pacijica to
several species. They successfully maintained forty E . pacijica, nine
E. eximia, three Thysanoessa spinifera, three Nyctiphanes simplex, and
four unidentified euphausiids all of which moulted two or three times.
The mean frequency of moulting of these fifty-nine animals is shown in
Fig. 103 where it is seen that most moult every 5 or 6 days. The changes
in length of the intermoult periods of Euphuusia pacijica were related
to the changes in the temperature of the water in the Scripps Institution
of Oceanography's experimental aquarium where the animals were
kept. Lasker studied the changes in the length of the intermoult period