¨ 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
3.1. Light measurements ¨
Lindstrom and Nilsson 1988 reported the light spectra at 5 m depth to have maxima at about 555–575 nm in the open Baltic Sea, at 565–585 nm in the Pojoviken Bay and
¨ ¨ ¨ between 600 and 700 nm in Lake Paajarvi. The light intensity, measured at a depth of 5
m, differed as much as one log unit between each of the three localities, the open sea ¨ ¨ ¨
having the highest and Lake Paajarvi the lowest transparency. In the present in- vestigation the spectral transmission maxima have been stated more precisely at 565,
575 and 670 nm using data recorded from depths where the peaks are well discernible. It is notable that the spectra shown for about equal total quantal flux Fig. 1 are recorded
at different depths, 20 m, 10 m and 5 m respectively.
The Baltic Sea was classified as water type coastal 3 by Jerlov 1976, with maximum transmission at 550 nm. However, at the sampling locality closer to the shore the light
transmission of the water shifts to longer wavelengths because the water discharge from land contains humic substances. The Secchi depth changes considerably depending on
¨ 90
M . Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101
Fig. 1. Spectral distribution of light at A 20 m depth open circles, Baltic Sea; B 10 m dots, Pojoviken ¨ ¨ ¨
Bay Baltic Sea; and C 5 m crosses, Lake Paajarvi.
the plankton situation, but may during favourable conditions reach 8 m Feb. 1, 1996. The Pojoviken Bay is a bay of the Baltic Sea, separated from it by a shallow sill. It has
still lower transparency, and the transmission maximum at longer wavelengths depend
˚ on discharge of humic substances from land and the Svarta river, discharging at the end
of the bay. Maximum Secchi depth was 5.25 m, and the colour value was 35–45 mg Pt
21
l Aug. 23, 1969, Niemi, 1973. The colour of the water of the mesohumic Lake
21
¨ ¨ ¨ ¨
Paajarvi, maximum depth 86 m, varies between 40 and 60 mg Pt l Ruuhijarvi, 1974
and the Secchi depth between 1.8 and 2.4 m Ilpo Hakala, pers. commun.. 3.2. Sl
All the mysid species dealt with here responded to light stimuli by a corneal negative
¨ M
. Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101 91
on-response followed by a decay to baseline. The overall shape of the S l curves was
somewhat broader than would have been expected from the nomograms by Dartnall 1953, constructed on the basis of visual pigments in solution. The shape of the curves
resembled absorption curves for porphyropsin Bridges, 1967. Porphyropsin was looked for by high performance liquid chromatography in the eye of M
. relicta from Lake ¨ ¨ ¨
¨ Paajarvi, but was not found Lindstrom et al., 1988. A broad shape of the curve does
not necessarily indicate presence of a second pigment in the eye. Eye structures may absorb light selectively, thereby depressing the peak of the curve. Even self-screening
may occur to some extent Goldsmith, 1978. Proximal screening pigment migration changes the S
l of the Australian fresh water crayfish Cherax destructor on a circadian scale Bryceson, 1986.
Probably N . integer, P. flexuosus, P. inermis, M. mixta and M. relicta sp. I and sp. II
possess only one visual pigment, which rules out colour vision. For all species, the S
l is at shorter wavelengths than the peak irradiance at the corresponding locality.
max
Only H . anomala has a Sl curve indicating a sensitivity increase at the far blue end of
the spectrum. In future experiments it will be of great interest to extend the ERG recordings into the UV.
3.3. Pelagic mysids The three different populations of M
. relicta examined had different Sl curves Fig. 2. The animals captured in the open sea sp. II had a S
l between 505 nm
max
¨ Lindstrom, 1976 and 520 nm, the animals from Pojoviken Bay sp. I had their S
l
max
¨ at 550 nm, and the lake animals sp. I at 570 nm Lindstrom and Nilsson, 1984, 1988.
The visual pigments of M . relicta sp. I were measured spectrophotometrically by
Dontsov et al. 1999, who found minima of bleaching difference spectra at 550 and 580 nm for the sea and lake population respectively.
3.3.1. M. relicta sp. I. In M
. relicta sp. I, it was possible to compare the position of the Sl of two
max
populations of the same species living in different water bodies, with different optical ¨
properties. The eyes of both populations responded to near-IR light Lindstrom and Meyer-Rochow, 1987. The S
l at 570 nm in the lake population is among the most
max
red-shifted spectral sensitivities known. The eye responded with relatively high signal amplitudes to IR light used for dark-adapted preparations of the isopod Cirolana
¨ borealis, for which the light was completely invisible Lindstrom and Nilsson, 1983.
It is supposed that the two M . relicta sp. I populations became separated during the
¨ ¨ ¨ ¨
last glacial age about 9000 years ago when Lake Paajarvi was formed Ruuhijarvi, 1974. Their S
l at 550 and 570 nm indicate a maybe still active evolutionary
max
adaptation of the visual pigment towards the spectral peak of the downwelling light. The ¨ ¨ ¨
S l
at 570 nm in M . relicta sp. I from Lake Paajarvi has already almost reached the
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maximum wavelength of downwelling light in the Pojoviken Bay, 575 nm, which the bay population has not arrived at yet. This shows a plasticity of visual pigment
evolution. The cause for the further and faster development of the S l
in the lake
max
population might be found in stronger selection pressure, because of the much lower transparency of the lake water. The shift of the S
l towards longer wavelengths has
max
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M . Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101
¨ ¨ ¨ Fig. 2. Relative spectral sensitivity of A M
. relicta. sp. I from Lake Paajarvi N511; B sp. I from the Pojoviken Bay Baltic Sea N 511; C sp. II from the open sea N 53. In Figs. 2–4, curves are vertically
displaced for clarity.
increased the ability of the eye to absorb the weak reddish light prevailing in the lake. In deep water, a close match between the S
l and the light transmission maximum is of
max
utmost importance for being able to see prey organisms, and to avoid predators Lythgoe, 1968, 1972, 1979. The relatively great difference between the 570 nm
pigment and the transmission maximum of 670 nm may depend on an inability of the pigment to develop further towards longer wavelengths. During summer days, the
¨ ¨ ¨ densest concentration of M
. relicta in Lake Paajarvi is found at about 15–30 m depth. ¨
During the night it migrates closer to the surface Hakala, 1978; Gronholm, 1980. It may be pointed out that in the lake the QSM quantum spectrometer was unable to record
¨ light deeper than about 7 m in daytime Lindstrom and Nilsson, 1988. The persistent
¨ M
. Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101 93
nocturnal vertical migration of sp. I thus indicates a high sensitivity of the eye, because the lower limit for light penetration determined by the 1 value is only 4 m for the
wavelength band of maximum penetration, 600–700 nm Elomaa, 1977, and the S
l is at 570 nm. Factors which may affect the vertical distribution of the mysids
max
are, except from high temperature and light, the presence of predating fish and the ¨
vertical migration of plankton prey organisms Gronholm, 1980. 3.3.2. M. relicta sp. II
The S l
of M . relicta sp. II at 505–520 nm, Fig. 2 far from the spectral peak of
max
light transmission, 565 nm, is in accordance with the results by Gal et al. 1999 who found only one single pigment with a S
l of 520 nm in M
. relicta from Cayuga
max
Lake, at a water peak transmission at 563 nm. Beeton 1959 recorded the activity spectrum of M
. relicta and found a maximum at about 515 nm and shortening reaction times towards the near-UV end of the spectrum. He interpreted this as evidence that M
. relicta would have two visual pigments. Neither in the present investigation, nor in the
study by Gal et al. 1999, were there any indications of a second visual pigment in M .
relicta . The latter authors discussed possible causes for Beeton’s results. No direct
comparisons with the Finnish mysids can, however, be drawn as the North American ¨
¨ ¨ mysid belongs to M
. relicta sp. IV Vainola et al., 1994. 3.3.3. M. mixta
M . mixta is common in deeper water, further out from the shore where the water is
clearer and more transparent. M . mixta is originally an Atlantic species later adapted to
brackish water Salemaa et al., 1986. In the North Atlantic, the maximum transmission is at about 500 nm Jerlov, 1976. This may explain the displacement of the eye’s
S l
495–510 nm from the maximum wavelengths of downwelling light in the
max
Baltic Sea Fig. 3. A displacement of the visual pigment’s absorption maximum from the maximum
wavelength of downwelling light will decrease the amount of light available for vision, and in vertical migration the preferred light intensities if there are such will
consequently be found at different depths in water bodies of different light transmission properties. The situation may be comparable to the vertical zonation of the mysid
Boreomysis megalops in experimental conditions after an exposure to strong light that caused pathological changes in the eye. Light exposed animals chose a position much
closer to the surface during their nocturnal pelagic phase Attramadal et al., 1985. A similar observation was made by Meyer-Rochow 1988 on blinded rock lobsters,
Panulirus longipes.
Both M . mixta and M. relicta are pelagic at night, and M. mixta is known to migrate
closer to the surface than M . relicta sp. II Salemaa et al., 1986. This ecological niche
separation might be an effect of the slightly different visual capacities of the two species. The problem of comparing the visual sensitivities or action spectra of animals with
different S l
in a spectrally defined environment is attacked by Gal et al. 1999,
max
who for M . relicta introduced the unit ‘‘mylux’’ which is comparable to the human Lux
concept, but is unique for every different S l, and subsequently for every species or
population too.
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M . Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101
Fig. 3. Relative spectral sensitivity of A Hemimysis anomala N 54; B Neomysis integer N 54; and C M
. mixta N56. In H. anomala the last wavelength used was 690 nm instead of 673 nm.
3.4. Littoral mysids In shallow water the light spectrum is very broad and almost any position of the
S l
would be acceptable for maintaining high visual sensitivity. N . integer, P.
max
flexuosus and P . inermis all had Sl
making them well adapted to the available light
max
in the phytal meadows where light of mostly longer wavelengths is absorbed by the chlorophyll of macroalgae and higher plants, shifting the transmitted spectrum towards
shorter wavelengths. P . flexuosus had a Sl
at 505–515 nm and P . inermis at
max
520–530 nm Fig. 4. The opportunistic species N . integer, which is common pelagically
as well as in the littoral, had S l
at 525–535 nm Fig. 3. The S l
offset from
max max
the wavelengths of maximum irradiance may increase contrast sensitivity in all three
¨ M
. Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101 95
Fig. 4. Relative spectral sensitivity of A Praunus inermis N 54; and B Praunus flexuosus N 53. The peak at 549 nm, present in all curves in Figs. 1–3 is an artefact depending on a small calibration error, later
corrected.
species, according to the ‘‘Contrast Hypothesis’’ which assumes that in shallow water a receptor with maximum sensitivity offset from the maximum transmission of the UWL
is the most efficient contrast detector Lythgoe, 1968, 1972, 1979. All three species depend on zooplankton as food to a considerable extent, particularly P
. flexuosus. Under certain circumstances P
. flexuosus shows true prey selection Viitasalo and Rautio, 1998 which emphasises their use of vision in predation.
3.5. The neoimmigrant H. anomala H
. anomala is a newcomer to the northern Baltic Sea. It was observed for the first
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M . Lindstrom J. Exp. Mar. Biol. Ecol. 246 2000 85 –101
time in the late summer of 1992 Salemaa and Hietalahti, 1993. The animal avoids direct light, and stays during the day in dense swarms under stones and in rocky crevices
at a depth of 2–12 m. Thereby they resemble their Mediterranean relatives, Hemimysis speluncola and Hemimysis margalefi, which both dwell in caves, the latter in a
completely dark submarine cave Alcaraz et al., 1986. The former species migrates daily to the exterior during the night for feeding Riera et al., 1991; Coma et al., 1997.
The S
l of H
. anomala is the most blue-shifted of any recorded in this investigation.
max
The position of the maximum around 500 nm Fig. 3 indicates that the species is adapted to a water body with light transmission properties different from that of the
Baltic coastal water, which has a transmission maximum around 565 nm Fig. 1. A decreased light detection at depths in the Baltic archipelago can be predicted by the fact
that the eye of H
. anomala at the wavelength of maximum transmission ¯565 nm has a performance of only about 53, compared to the wavelength of theoretical maximum
performance, about 490 nm. H . anomala is found in the Caspian Sea and the Black Sea,
from where it is supposed to have invaded the Baltic Sea through man-made waterways. Unfortunately no data are available for the light transmission spectrum in the Caspian or
Black Seas. The sensitivity increase at the deep blue end of the spectrum may indicate the presence of a second, blue- or — maybe — UV-sensitive, pigment. The big
sensitivity variation at 393 nm may indicate presence or low concentration of a second, blue-sensitive pigment, depending on the prehistory of the specimen. A rhodopsin visual
pigment can become reisomerised by light absorbed by the photoproduct, the meta- rhodopsin Donner et al., 1994. If the necessary light spectrum for the process is not
available, the pigment concentration will be low until new pigment is formed. If H
. anomala has got two pigments, it might possess colour vision. In this context it may be
noted that the chromatophore pigments of H . anomala are red, which gives the animal a
reddish appearance in contrast to the other species in which the chromatophore pigments are black. The sensitivity increase may also depend on a single pigment’s small
absorption increase towards the UV, as shown by Gal et al. 1999 for M . relicta in
Cayuga Lake. The position of the S
l at the blue end of the spectrum may give the species a
max
small, temporal visual sensitivity advantage during precuspular hours. At sunset and sunrise the light spectrum of the top 2–3 m of the water column changes into a two-peak
spectrum with a blue maximum at about 480–490 nm and another maximum in red, due ¨
to the Chappuis effect Munz and McFarland, 1973; Lindstrom and Nilsson, 1983.
4. Concluding remarks
