¨ M
. Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101 87
During the last years there has been extensive research on mysid ecology in the northern Baltic Sea Rudstam and Hansson, 1990; Salemaa et al., 1990; Kauppila, 1994;
¨ Nordstrom, 1997; Viherluoto, 1998; Viitasalo and Rautio, 1998; Gorokhova, 1999. This
called for a review on what is known about mysid vision in this area, with the two species of M
. relicta separated, and the newcomer of the nineties, the pontocaspian neoimmigrant H
. anomala included. The aim of this review is to show to what extent the eyes of mysids from the Finnish
coastal area are adapted to the light conditions prevailing in their normal habitats. The S
l recorded from the eyes of the different species are compared with the spectrum of downwelling light in their respective habitats.
2. Material and methods
2.1. The species 2.1.1. M. mixta and M. relicta
M . mixta dominates over the two partially sympatric sibling species of M. relicta sp. I
¨ ¨ ¨
¨ ¨ ¨
and sp. II, Vainola, 1986; Vainola and Vainio, 1998 in the western parts of the Gulf of Finland and in the northern Baltic proper Salemaa et al., 1986. In the Bothnian Sea the
M . mixta and the M. relicta group species occur in about equal numbers. M. relicta is
the more euryhaline of the two, and tolerates the lower salinity waters of the Bothnian Bay. All species require benthic contact Salemaa et al., 1990 and are oxygen-
demanding, thus avoiding anoxic bottoms. Both feed on phyto- and zooplankton; M .
mixta mainly on zooplankton Rudstam, 1988; Viherluoto, 1998. One generation is produced annually. In all three species, most of the population undertakes nocturnal
vertical migrations, part remaining close to the bottom also at night. M . mixta avoids
24
light levels exceeding 10 lux estimated value, Rudstam et al., 1989; Rudstam and
Hansson, 1990. M . relicta is also found in many Finnish lakes, where it also undertakes
nocturnal vertical migrations during the summer months Hakala, 1978. Samples of populations conventionally identified as M
. relicta were collected from two Baltic Sea localities using a bottom sledge ending in a plastic bag. From the deep
¨ ¨ ¨ max. 86 m, dark-water humic lake, Lake Paajarvi the animals were collected with a
vertically towed net from about 60 m. The Baltic material was later found to include two ¨
¨ ¨ species, inseparable by anatomical means, provisionally named sp. I and sp. II Vainola,
¨ ¨ ¨
1986; Vainola and Vainio, 1998. After collection the animals were stored in darkness at the temperature prevailing at the sampling locality.
¨ The Baltic material was collected in 1972 Lindstrom, 1976 and 1991 from
¨ ¨
¨ Tvarminne Storfjard in the outer archipelago near Tvarminne Zoological Station. The
1991 sample was electrophoretically identified as sp. II of the M . relicta group R.
¨ ¨ ¨
Vainola, pers. commun.. Samples of the population from the Pojoviken Bay a fjord in the Gulf of Finland were collected in 1983 and 1985, and the population in Lake
¨ ¨ ¨ ¨
Paajarvi in 1982, 1983 and 1987 Lindstrom and Nilsson, 1984, 1988. These two ¨
¨ ¨ populations belong to the M
. relicta group sp. I Vainola, 1986.
¨ 88
M . Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101
2.1.2. N. integer, P. flexuosus and P. inermis N
. integer and P. flexuosus were collected by a hand-held net from the sheltered ¨
littoral zone in Tvarminne. P . inermis was collected in the same way in an exposed
sandy and rocky shore habitat. The animals were stored at the temperature prevailing at the sampling locality in darkness or at a low light light:dark cycle.
N . integer is a genuinely brackish water species, which is common in the Baltic Sea
˚ Segerstrale, 1945. It occurs in great shoals in the sea grass meadows and occasionally
pelagically further out at open sea. Adult individuals are found down to 40 m in late summer. It is omnivorous, but prefers to feed on zooplankton Rudstam et al., 1986;
Uitto et al., 1995. It has two generations a year. The species is a large component of fish diets Mauchline, 1980. P
. flexuosus occurs in small shoals among Fucus vesiculosus, in inshore habitats, and so does P
. inermis, which, however, does not shoal. 2.1.3. H. anomala
In 1992, H . anomala was seen for the first time in the Baltic Sea by a diver in the
South-West archipelago of Finland Salemaa and Hietalahti, 1993. It occurs in swarms at depths of 2–12 m in rocky crevices and in cavities of boulder shores but from 1996
has been found several times in shallow water close to the shore. H . anomala was
collected by divers in 1993. The animals were stored in darkness at the temperature prevailing at the sampling site.
2.2. Electroretinogram ERG technique S
l and light tolerance of the eye was measured using the ERG technique. Only the main features will be dealt with here as the method is described elsewhere Donner,
¨ ¨
¨ 1971; Lindstrom and Nilsson, 1983, 1988; Nilsson and Lindstrom, 1983; Lindstrom et
al., 1988. A hole was punched through the corneal cuticle of one eye with a microneedle. Since the eyes contain between 600 and 2000 ommatidia, only a few are
destroyed by the procedure. The electrode, a glass micropipette filled with a saline solution 1 M NaCl and with a tip diameter of about 10 mm, was lowered about 50 mm
into the eye. The neutral electrode was a AgAgCl wire in contact with the Ringer solution brackish water. After it had become evident that the eyes of the lake mysid
¨ were easily damaged by strong light Lindstrom and Nilsson, 1984, the animals were
collected at night and the preparations were performed in infrared IR light, using IR ¨
image converters Lindstrom and Nilsson, 1983, 1988. Recordings performed earlier M
. relicta sp. II, N. integer and M. mixta were repeated using the IR technique. Always the same dorsal region of the eye was aimed for when the electrode was inserted Fig. 2
¨ in Lindstrom and Nilsson, 1988. The preparations were left to adapt for 1–2 h before
the experiments commenced. Storage time before preparation did not affect the results. Neither were there any differences between adults and juveniles.
The eye was exposed to 600 ms light flashes of 14 different wavelengths ranging from 406 to 673 nm in steps of ¯20 nm Schott double-interference filters, half-band width
7–16 nm. In H . anomala and P. inermis also 393 nm was used. The responses were
recorded on the oscilloscope. Calculation of the S l curves was based on the relative
quantum intensities needed for each wavelength to evoke responses of equal amplitude
¨ M
. Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101 89
in the eye. Assuming the top of the S l curve to be symmetrical in shape in analogy
with the absorption curves of rhodopsin in solution, I used the best fit of Dartnall’s nomogram to the blue and red flanks of the curve to determine the position of the
S l
. During the experiments, slow sensitivity changes sometimes occurred. These
max
were corrected for by returning to the same wavelength about every 10 min. The resulting time-sensitivity curve was used for correcting all wavelengths to a fixed
moment of the experiment. The strange peak present in most curves at 549 nm is an artefact caused by a calibration error, later corrected.
2.3. Light measurements The UWL spectra of the different water bodies were recorded at several occasions
from 1984 on, independently of animal sampling using a QSM 2500 submersible
22
˚ quantum spectrometer Techtum, Umea, Sweden in quanta per m
per second and nm
22 21
21
qu m s
nm . Also the integrated quantum fluxes from 400 to 750 nm were
22 21
recorded qu m s
. Measurements were performed at 1, 3, 5, 7, 10, 15, 20, 25 and 30 m depth or down to the limit of the meter’s light detection. In the Baltic Sea the
monitoring continued at irregular intervals for 12 months in 1989–1990. The UWL was measured around noon during moments of clear sky. The position of the spectral peak
did not change at any of the three localities during the year, only the total amount of incident light. The spectra became increasingly narrower with depth, with unchanged
wavelengths of maximum transmission. As the pelagial mysids spend the day close to the bottom, where it was too dark for the light meter even in daytime, I have chosen to
present the spectral distributions of light at the different localities at depths at which the quantal fluxes were about equal.
3. Results and discussion
