166
T H E BIOLOGY OF EUPHAUSIIDS
Consequently, low temperatures seem to restrict the downward movement of the deeper portions of the populations.
Lewis examined the relationships between isolumes and isotherms
and the vertical distribution of the euphausiids during the night. He
found no positive correlations between isotherms and night levels of
occurrence of euphausiids but there were positive correlations between
the depth of the
isolume a t midnight and the 50% night level in
Euphausia species and Xtylocheiron species (Table VI). He concludes,
therefore, that light intensity alone controls the vertical distribution of
these animals a t night.
The effect of low temperatures on the lower limits of populations of
euphausiids in the water column does go some way towards explaining
the geographical variations found in the vertical distribution of species,
as Lewis points out. I n fact, there is strong evidence to suggest that,
where temperature is not limiting, the vertical distribution is controlled
solely by levels of subsurface illumination but if water of low temperature is present in the column then the animals do not descend into it
even if light conditions there are near optimal. Lewis quotes his and
previous workers' estimations of the vertical distribution of certain
species of euphausiids in different geographical regions (Table VII). He
examines the temperature profiles of the water columns in the various
geographical regions and compares the temperature a t the depth of the
l o e 2 isolume in summer in the Atlantic, charted by Moore (1952), to
the temperatures which lie a t this depth in other areas.
isolume in the Florida Straits is li'OC, in the eastern Atlantic outside
the Bay of Biscay it is between 10" and 1l0C, in the western Atlantic
within the line of the Gulf Stream it is less than lO"C, and in the
Mediterranean it is between 13" and 14OC. The higher temperatures
prevalent in the lower regions of the water column in the Florida Straits
would thus allow the euphausiids to migrate to deeper levels than in the
other geographical areas (Table VII).
Having now examined the evidence that levels of illumination in the
sea control the vertical distribution of these organisms we must
consider briefly what appears to be a partial contradiction to this which
so far remains unexplained. A light lure, in the form of a 1000 watt
light, hung overboard from a ship can attract euphausiids to it. Mazoue
(1931) found that Euphausia lcrohnii and Thysanoessa gregaria were
attracted in small numbers and Fisher et al. (1953) report that fishermen
in the Port of Monaco use light lures to catch swarms of Meganyctiphanes
norvegica ; these swarms appear in the harbour at irregular intervals for
no known reason. Nyctiphanes couchii and Meganyctiphanes norvegica
collected around lights used by Blaxter and Parrish (1958) to test the
The
T H E BIOLOGY OF EUPHAUSIIDS
Consequently, low temperatures seem to restrict the downward movement of the deeper portions of the populations.
Lewis examined the relationships between isolumes and isotherms
and the vertical distribution of the euphausiids during the night. He
found no positive correlations between isotherms and night levels of
occurrence of euphausiids but there were positive correlations between
the depth of the
isolume a t midnight and the 50% night level in
Euphausia species and Xtylocheiron species (Table VI). He concludes,
therefore, that light intensity alone controls the vertical distribution of
these animals a t night.
The effect of low temperatures on the lower limits of populations of
euphausiids in the water column does go some way towards explaining
the geographical variations found in the vertical distribution of species,
as Lewis points out. I n fact, there is strong evidence to suggest that,
where temperature is not limiting, the vertical distribution is controlled
solely by levels of subsurface illumination but if water of low temperature is present in the column then the animals do not descend into it
even if light conditions there are near optimal. Lewis quotes his and
previous workers' estimations of the vertical distribution of certain
species of euphausiids in different geographical regions (Table VII). He
examines the temperature profiles of the water columns in the various
geographical regions and compares the temperature a t the depth of the
l o e 2 isolume in summer in the Atlantic, charted by Moore (1952), to
the temperatures which lie a t this depth in other areas.
isolume in the Florida Straits is li'OC, in the eastern Atlantic outside
the Bay of Biscay it is between 10" and 1l0C, in the western Atlantic
within the line of the Gulf Stream it is less than lO"C, and in the
Mediterranean it is between 13" and 14OC. The higher temperatures
prevalent in the lower regions of the water column in the Florida Straits
would thus allow the euphausiids to migrate to deeper levels than in the
other geographical areas (Table VII).
Having now examined the evidence that levels of illumination in the
sea control the vertical distribution of these organisms we must
consider briefly what appears to be a partial contradiction to this which
so far remains unexplained. A light lure, in the form of a 1000 watt
light, hung overboard from a ship can attract euphausiids to it. Mazoue
(1931) found that Euphausia lcrohnii and Thysanoessa gregaria were
attracted in small numbers and Fisher et al. (1953) report that fishermen
in the Port of Monaco use light lures to catch swarms of Meganyctiphanes
norvegica ; these swarms appear in the harbour at irregular intervals for
no known reason. Nyctiphanes couchii and Meganyctiphanes norvegica
collected around lights used by Blaxter and Parrish (1958) to test the
The