148
THE BIOLOGY O F EUPHAUSIIDS
animals which have not yet matured sexually, that is have not laid eggs
or produced spermatophores while the I-group are animals which have
bred once and are therefore more than one year old.
The depth a t which these two species live in the Firth of Clyde is
the same except that there is a tendency for T . raschii t o have its centre
of vertical distribution a few metres above that of Meganyctiphanes
norvegica but the difference between the two species, if indeed it exists,
is very small. The winter observations were made in November, the
first haul being taken a t noon, the remaining hauls a t four-hourly
intervals through to noon the next day. Meganyctiphanes norvegica
occurred only in the lowermost net at noon on the first day but was
present in the two lowermost nets a t noon on the second day (Fig. 55).
This is related to cloud cover ; the sky was clear and the sun bright on
the first day but at noon on the second day heavy cloud and rain
obscured the sun. Thus, lower light intensities cause the animals to rise
higher in the water column. Similarly, dull weather conditions prevailed a t noon on the two days when Thysanoessa raschii was sampled.
I n November, dusk occurred about 1600 hours and an upward movement of both species had begun by then and continued until midnight.
There is no evidence in the data of a “ midnight sinking )’ nor, indeed,
of a dawn rise ”. A “ midnight sinking ’) is probably caused when
the organisms approach the surface but stop moving upwards when
darkness falls, whether they have reached the surface or not; after
dark the stimulus for upward movement (probably the upward movement of a layer of constant light intensity-an isolume) has disappeared
and the organisms then simply maintain themselves near the surface
but, because they are denser than water, tend to sink, and so produce
the phenomenon of a “midnight sinking ”. Cushing (1951) has
reviewed the occurrence of this effect in species of crustacean plankton
in general and points out that such an effect is present in Esterly’s
(1914b) data on the euphausiids, Euphausia pacijka and Nyctiphanes
simplex. There is also an indication of a sinking of Meganyctiphanes
norvegica away from the surface in the data of Jespersen (1944). Lewis
(1954) examined the vertical migration of many species but was only
able to describe those of Euphausia tenera and Stylocheiron carinatum
in detail because of lack of numbers of animals of other species.
Stylocheiron carinatum sinks during the night but the results are only
suggestive of the same feature in the distribution of Euphausia tenera.
As Cushing points out : “ If the midnight sinking were a reality then it
would be consistent to find a rise towards the surface a t dawn. Light
intensity increases slowly and then, with the sunrise, rapidly ; it is in
the first part of this increase that a rise to the surface might be expected,
THE BIOLOGY O F EUPHAUSIIDS
animals which have not yet matured sexually, that is have not laid eggs
or produced spermatophores while the I-group are animals which have
bred once and are therefore more than one year old.
The depth a t which these two species live in the Firth of Clyde is
the same except that there is a tendency for T . raschii t o have its centre
of vertical distribution a few metres above that of Meganyctiphanes
norvegica but the difference between the two species, if indeed it exists,
is very small. The winter observations were made in November, the
first haul being taken a t noon, the remaining hauls a t four-hourly
intervals through to noon the next day. Meganyctiphanes norvegica
occurred only in the lowermost net at noon on the first day but was
present in the two lowermost nets a t noon on the second day (Fig. 55).
This is related to cloud cover ; the sky was clear and the sun bright on
the first day but at noon on the second day heavy cloud and rain
obscured the sun. Thus, lower light intensities cause the animals to rise
higher in the water column. Similarly, dull weather conditions prevailed a t noon on the two days when Thysanoessa raschii was sampled.
I n November, dusk occurred about 1600 hours and an upward movement of both species had begun by then and continued until midnight.
There is no evidence in the data of a “ midnight sinking )’ nor, indeed,
of a dawn rise ”. A “ midnight sinking ’) is probably caused when
the organisms approach the surface but stop moving upwards when
darkness falls, whether they have reached the surface or not; after
dark the stimulus for upward movement (probably the upward movement of a layer of constant light intensity-an isolume) has disappeared
and the organisms then simply maintain themselves near the surface
but, because they are denser than water, tend to sink, and so produce
the phenomenon of a “midnight sinking ”. Cushing (1951) has
reviewed the occurrence of this effect in species of crustacean plankton
in general and points out that such an effect is present in Esterly’s
(1914b) data on the euphausiids, Euphausia pacijka and Nyctiphanes
simplex. There is also an indication of a sinking of Meganyctiphanes
norvegica away from the surface in the data of Jespersen (1944). Lewis
(1954) examined the vertical migration of many species but was only
able to describe those of Euphausia tenera and Stylocheiron carinatum
in detail because of lack of numbers of animals of other species.
Stylocheiron carinatum sinks during the night but the results are only
suggestive of the same feature in the distribution of Euphausia tenera.
As Cushing points out : “ If the midnight sinking were a reality then it
would be consistent to find a rise towards the surface a t dawn. Light
intensity increases slowly and then, with the sunrise, rapidly ; it is in
the first part of this increase that a rise to the surface might be expected,