SOME ASPECTS OF PHOTORECEPTION AND VISION IN FISHES
195
into the fish eye. Direct measurements have demonstrated the existence
of high oxygen tensions in the vitreous humour of many marine species ;
tensions ranged from 250 to 800 mm Hg (about 1.5 to 6 times that in
the blood) in benthic and pelagic forms, all of which had well-dovoloped
chorioidal glands. In two fast-swimtning pclagic fishes, bluofish and
remoras, tatisions up to 1320 mm Hg wcre rneasured. Fishes lacking or
having but small retia had low tensions -of oxygen in the vitrcous
humour. The chorioidal gland is well developed in active predatory
teleosts ; possibly its function of supplying oxygen to the retina is to be
correlated with visual acuity, density of receptor elements and thickness
of the retina (Wittenberg and Wittenberg, 1962). It is possible that the
darkening associated with pseudobranch removal is the result of
depreRsed retinal activity.
IX. ECOLOGICAL AND BEHAVIOURAL STUDIES
Breder (1959) has presented an extensive review, containing much
original information, of the responses of fishes to light. The following
discussion is concerned especially with experiments and observations
dealing with quantitative factors and the operation of photoreceptors.
Fishes living habitually in lighted environments make colour
changes; the receptors for these responses are the pineal organ and
paired eyes (see Section 11). Thcse responses can take place very
quickly, for example in fishes living among coral reef (Longlcy and
Hildebrand, 1941). Waring ( 1942) has presented an evolutionary
scheme in which slow chromatic responses, mediated by hormones
(in elesmobranchs), have been superseded by fast responses mediated
by the nervous Rystem (in tcleosts). I':(~H were thought to occupy a
t ransitional fit age, with rcd at i vcl y d o w rc>q I( IIIHCH ; rrialrLncq1 t 1 oron ww(:
innervated, but the nervous Hyetem played no part in the rcspotises of
intact animals. This is highly unlikely because Bateson (18904
observed long ago that white spots in the conger eel come and go very
suddenly.
A related visual reaction is shown by many fishes which will swim
only over a matching (or contrasting) background. Breder (1959) haa
described how schools of clupeids, swimming over light sand, refused
to paas over dark beds of weeds. Field observations indicate that this
kind of reaction is correlated with the degree of pigmentation of the
fish. Analogous behaviour is shown by various !%hes that are more
repelled by vertical nets reflecting light of long wavelengths than by
those reflecting short waves, i.e. they pass in increasing numbers
through nets coloured as follows : red, orange rind yellow, green and bluo.
The responses are governed by visibility and contraat againnt tmck
14
195
into the fish eye. Direct measurements have demonstrated the existence
of high oxygen tensions in the vitreous humour of many marine species ;
tensions ranged from 250 to 800 mm Hg (about 1.5 to 6 times that in
the blood) in benthic and pelagic forms, all of which had well-dovoloped
chorioidal glands. In two fast-swimtning pclagic fishes, bluofish and
remoras, tatisions up to 1320 mm Hg wcre rneasured. Fishes lacking or
having but small retia had low tensions -of oxygen in the vitrcous
humour. The chorioidal gland is well developed in active predatory
teleosts ; possibly its function of supplying oxygen to the retina is to be
correlated with visual acuity, density of receptor elements and thickness
of the retina (Wittenberg and Wittenberg, 1962). It is possible that the
darkening associated with pseudobranch removal is the result of
depreRsed retinal activity.
IX. ECOLOGICAL AND BEHAVIOURAL STUDIES
Breder (1959) has presented an extensive review, containing much
original information, of the responses of fishes to light. The following
discussion is concerned especially with experiments and observations
dealing with quantitative factors and the operation of photoreceptors.
Fishes living habitually in lighted environments make colour
changes; the receptors for these responses are the pineal organ and
paired eyes (see Section 11). Thcse responses can take place very
quickly, for example in fishes living among coral reef (Longlcy and
Hildebrand, 1941). Waring ( 1942) has presented an evolutionary
scheme in which slow chromatic responses, mediated by hormones
(in elesmobranchs), have been superseded by fast responses mediated
by the nervous Rystem (in tcleosts). I':(~H were thought to occupy a
t ransitional fit age, with rcd at i vcl y d o w rc>q I( IIIHCH ; rrialrLncq1 t 1 oron ww(:
innervated, but the nervous Hyetem played no part in the rcspotises of
intact animals. This is highly unlikely because Bateson (18904
observed long ago that white spots in the conger eel come and go very
suddenly.
A related visual reaction is shown by many fishes which will swim
only over a matching (or contrasting) background. Breder (1959) haa
described how schools of clupeids, swimming over light sand, refused
to paas over dark beds of weeds. Field observations indicate that this
kind of reaction is correlated with the degree of pigmentation of the
fish. Analogous behaviour is shown by various !%hes that are more
repelled by vertical nets reflecting light of long wavelengths than by
those reflecting short waves, i.e. they pass in increasing numbers
through nets coloured as follows : red, orange rind yellow, green and bluo.
The responses are governed by visibility and contraat againnt tmck
14