Diversity and Vertical Migration of Euphausiids (Straits of Messina)
139
can be utilized in a comparison of distribution in
both areas.
During the daily cycle Euphausia hemigibba,
in the Ionian basin, showed dear migratory
behaviour. In the morning. this species was near
the surface. The few caught individuals showed
that the greatest part of the population started
migrating down at daybreak, reaching at midday
the maximum density for adults between 400
and 500 m, while the juveniles occurred up to 100
m. At IS.00 h they shifted upward reaching a
peak of abundance between 100 and 200 m, while
in the night time they concentrated in the upper
200m.
Also Nematoscelis megalops had clear migratory behaviour. In the morning. its vertical distribution pattern showed that the maximum density was between 0 and 250 m, with the remaining
population down to 1000 m. At midday, the greatest concentration of individuals was at 500 m
depth and a lesser percentage of juvenile population remained near the surface down to 100 m. At
18.00 h species began assembling in surface layers between 0 and 200 m though some individuals were caught down to 2000 m. This condition
remained the same also at night time. Patterns of
vertical distribution of Euphausia krohni showed
this species had higher abundance near the surface (0-200 m) during the evening and at 6.00 h,
whereas at 12.00 h this species occupied a wide
depth range down to 600 m.
The E. hemigibba, in South Tyrrhenian Sea,
showed a more different day/night vertical
migration than in Ionian Sea. In the morning it
was found between 200 and 600 m. while its vertical distribution became sharper at midday
when it assembled between 400 and 600 m. In the
afternoon and in the evening, the population
migrated upward, occurring in the 0-500 m strata, while at 24.00 h it showed a preference for the
surface layers between 0 and 100 m.
Nematoscelis megalops didn't migrate, but
had a wide homogeneous distribution. between
200 and 1000 m during daytime, while in the
evening there was a slight migration upward,
which was dearer at night with a maximum of
density between 200 and 300 m. Stylocheiron
abbrelliatum didn't make day/night migrations
but always occurred in the same bathymetric
range down to 750 m, with its maximum concentration between 0 and 100 m, especially in the
evening and at 24.00 h. Thysanopoda aequalis
had a vertical profile that showed a separation of
adults and juveniles. During daylight, adults were
in the strata between 400 and 700 In, while juveniles occurred approximately in the upper 700 m
layer to the surface; then they started to migrate,
concentrating in late morning down to 500 m.
adults instead occupied lower strata down to
1000 m. Population began again to reverse
migration during afternoon to evening, inhabiting the surface layer down to 200 m.
Discussion
The number and species composition confirmed
what was reported by Guglielmo et al. (1995a) in
the Straits of Messina except that Thysanoessa
gregaria was considered an indicator species of
Atlantic waters (Casanova 1974). Its absence in
our samples could demonstrate that, during the
sampling, Atlantic water influence in the study
area was slight. Another reason could be ascribed
to the fact that BIONESS didn't sample near the
surface which is a typical habitat of this species.
The number of euphausiids were lower during
the day, indicating that avoidance of the sampler
may be a problem (Sameoto 1980), while at }4 the
lowest abundances could be due to the water turbulences of that zone which may negatively influence the sampling.
Lower diversity index value in the Ionian
(1.51) than in Tyrrhenian Seas (1.84) and a similarity index of 30.5%, were probably due to different hydrological conditions which seemed to
influence the faunistic composition and the
dominance of certain species; this is similar to
observations made by Casanova (1968) and
Wiebe and D'Abramo (1972) at stations located
dose to the Straits of Messina area. Firstly, evident differences, showed that abundance values
in an area didn't correspond with a similar area
in the other basin. Thysanopoda aequalis,
Euphausia hemigibba and Stylocheiron abbrelliatum were more abundant in the South
Tyrrhenian Sea (about 24%) than in Ionian Sea
where they reached a percentage of 3%, while
Nematoscelis megalops and Euphausia krohni
were more abundant in the Ionian Sea than
in Tyrrhenian Sea with percentages of about
38% and 3.6%, respectively. By analysing these
values and the duster graphs we can deduce that
sets of species could be considered as characteristic of a single basin: N. megalops and E. krohni,
in Ionian-Straits of Messina (cluster n,
139
can be utilized in a comparison of distribution in
both areas.
During the daily cycle Euphausia hemigibba,
in the Ionian basin, showed dear migratory
behaviour. In the morning. this species was near
the surface. The few caught individuals showed
that the greatest part of the population started
migrating down at daybreak, reaching at midday
the maximum density for adults between 400
and 500 m, while the juveniles occurred up to 100
m. At IS.00 h they shifted upward reaching a
peak of abundance between 100 and 200 m, while
in the night time they concentrated in the upper
200m.
Also Nematoscelis megalops had clear migratory behaviour. In the morning. its vertical distribution pattern showed that the maximum density was between 0 and 250 m, with the remaining
population down to 1000 m. At midday, the greatest concentration of individuals was at 500 m
depth and a lesser percentage of juvenile population remained near the surface down to 100 m. At
18.00 h species began assembling in surface layers between 0 and 200 m though some individuals were caught down to 2000 m. This condition
remained the same also at night time. Patterns of
vertical distribution of Euphausia krohni showed
this species had higher abundance near the surface (0-200 m) during the evening and at 6.00 h,
whereas at 12.00 h this species occupied a wide
depth range down to 600 m.
The E. hemigibba, in South Tyrrhenian Sea,
showed a more different day/night vertical
migration than in Ionian Sea. In the morning it
was found between 200 and 600 m. while its vertical distribution became sharper at midday
when it assembled between 400 and 600 m. In the
afternoon and in the evening, the population
migrated upward, occurring in the 0-500 m strata, while at 24.00 h it showed a preference for the
surface layers between 0 and 100 m.
Nematoscelis megalops didn't migrate, but
had a wide homogeneous distribution. between
200 and 1000 m during daytime, while in the
evening there was a slight migration upward,
which was dearer at night with a maximum of
density between 200 and 300 m. Stylocheiron
abbrelliatum didn't make day/night migrations
but always occurred in the same bathymetric
range down to 750 m, with its maximum concentration between 0 and 100 m, especially in the
evening and at 24.00 h. Thysanopoda aequalis
had a vertical profile that showed a separation of
adults and juveniles. During daylight, adults were
in the strata between 400 and 700 In, while juveniles occurred approximately in the upper 700 m
layer to the surface; then they started to migrate,
concentrating in late morning down to 500 m.
adults instead occupied lower strata down to
1000 m. Population began again to reverse
migration during afternoon to evening, inhabiting the surface layer down to 200 m.
Discussion
The number and species composition confirmed
what was reported by Guglielmo et al. (1995a) in
the Straits of Messina except that Thysanoessa
gregaria was considered an indicator species of
Atlantic waters (Casanova 1974). Its absence in
our samples could demonstrate that, during the
sampling, Atlantic water influence in the study
area was slight. Another reason could be ascribed
to the fact that BIONESS didn't sample near the
surface which is a typical habitat of this species.
The number of euphausiids were lower during
the day, indicating that avoidance of the sampler
may be a problem (Sameoto 1980), while at }4 the
lowest abundances could be due to the water turbulences of that zone which may negatively influence the sampling.
Lower diversity index value in the Ionian
(1.51) than in Tyrrhenian Seas (1.84) and a similarity index of 30.5%, were probably due to different hydrological conditions which seemed to
influence the faunistic composition and the
dominance of certain species; this is similar to
observations made by Casanova (1968) and
Wiebe and D'Abramo (1972) at stations located
dose to the Straits of Messina area. Firstly, evident differences, showed that abundance values
in an area didn't correspond with a similar area
in the other basin. Thysanopoda aequalis,
Euphausia hemigibba and Stylocheiron abbrelliatum were more abundant in the South
Tyrrhenian Sea (about 24%) than in Ionian Sea
where they reached a percentage of 3%, while
Nematoscelis megalops and Euphausia krohni
were more abundant in the Ionian Sea than
in Tyrrhenian Sea with percentages of about
38% and 3.6%, respectively. By analysing these
values and the duster graphs we can deduce that
sets of species could be considered as characteristic of a single basin: N. megalops and E. krohni,
in Ionian-Straits of Messina (cluster n,