This research was conducted by Robert Huber and Moira J. van
Staaden in collaboration with Les S. Kaufman (Boston Univ.), and
Karel F. Liem (Harvard Univ.)
The species assemblages of cichlids in the 3 largest African Great
Lakes are among the richest concentrations of vertebrate species
on earth. The faunas are broadly similar in terms of trophic diversity,
species richness, rates of endemism and taxonomic composition,
yet are historically independent of each other. Hence, they offer
a true and unique evolutionary experiment to test hypotheses concerning
the mutual dependencies of ecology and brain morphology.
We examined the brains of 189 species of cichlids from the three
large lakes Victoria, Tanganyika and Malawi. A first paper demonstrated
that patterns of evolutionary change in cichlid brain morphology
are similar across taxonomic boundaries as well as across the
three lakes (van Staaden et al., 1995). Here we report a close
relationship between the relative sizes of various brain structures
and variables related to the utilization of habitat and prey.
Causality is difficult to assign in this context; nonetheless,
prey size and agility, turibidity levels, depth, and substrate
complexity are all highly predictive of variation in brain structure.
Areas associated with primary sensory functions such as vision
and taste, relate significantly to differences in feeding habits.
Turbidity and depth are closely associated with differences in
eye size and large eyes are associated with species that pick
plankton from the water column. Piscivorous taxa, and others that
utilize motile prey, are characterized by a well developed optic
tectum and a large cerebellum compared to species that prey on
molluscs or plants. Structures relating to taste are well developed
in species feeding on benthos over muddy or sandy substrates.
The data militated against the existence of compensatory changes
in brain structure. Thus enhanced development of a particular
function is generally not accompanied by a parallel reduction
of structures related to other modalities. Although genetic and
environmental influences during ontogeny of the brain cannot be
isolated, this study provides a rich source of hypotheses concerning
the way the nervous system functions under various environmental
conditions and how it has responded to natural selection.
| check out the brains of some representative cichlids | ||
|---|---|---|
| Xenotilapia
ornatipinnis shallow sand sand-sifting insectivore |
Maravichromis
lateristriga shallow vegetation sand-sifting insectivore |
Chilotilapia
rhodesii shallow mud oral-crushing molluscivore |
| Simichromis
diagramma shallow rock grazer/picker |
Labeotropheus
fuelleborni shallow rock grazer |
Protomelas
similis shallow vegetation higher plants |
| Maravichromis
orthognathus pelagic paedophage |
Bathybates
minor pelagic piscivore |
Haplochromis
nyereri shallow rock zooplanktivore |
This research was conducted in collaboration with M. Kent Rylander
(Texas Tech Univ.)
The size of seven neural structures was compared in 51 species
of Notropis, Pteronotropis, Cyprinella, Luxilus,
Lythrurus, and Hybopsis, and related to the turbidity
of the species' habitat. This last parameter was assessed for
each species by personal communication with 42 ichthyologists.
To control for size differences among species, all analyses were
performed on the residuals from a regression of each character
on standard length. Principal components analysis (PCA) of the
residuals produced four significant PC-axes that together explained
65% of the total variation represented in the original variables.
The size of brain structures concerned with vision, olfaction,
and gustation was correlated with habitat turbidity. Two-way Analyses
of Covariance (ANCOVAs) revealed significant differences between
species in the size of all structures. Sexual dimorphism was exhibited
by olfactory bulb and cerebellum, and significant two-way interactions
(species vs. sex) were detected for the telencephalon, optic lobes,
cerebellum, vagal lobe, and the eye. Cluster analysis indicated
that neither similar turbidity preference nor shared phylogeny
is alone sufficient to explain the observed differences in brain
morphology.
Significant differences in stratification and size of the visual
layers of the optic tectum were found between three clear-water
minnows (Notropis amabilis, N. boops, Cyprinella
venustus) and three turbid-water minnows (N. atherinoides,
N. bairdi, and C. lutrensis). Correlations among
a variety of neural structures suggested the importance of stratum
marginale (SM), stratum opticum (SO), and stratum fibrosum et
griseum superficiale (SFGS), stratum griseum centrale (SGC) and
stratum periventriculare (SPV) in vision, of stratum album centrale
(SAC) and SGC for olfaction, and of SPV for the processing of
acoustico-lateral information.
Quantitative analysis of the optic nerve of minnows using light-
and electron microscopy demonstrated that anatomical characteristics
of the visual system are closely related to habitat turbidity.
Species in the genera Notropis and Cyprinella inhabiting
predominantly clear water had larger eyes and almost twice as
many optic nerve fibers compared to minnows of turbid habitats.
No differences were detected in the thickness of myelination,
the axon diameter profile, or the number of optic nerve fibers
per retinal area, indicating that the relative number of fibers,
as well as their anatomical characteristics, are similar in all
species and independent of habitat turbidity. It is therefore
hypothesized that quantitative differences in the number of visual
elements available for sampling and processing in the retina,
optic nerve, and optic tectum are sufficient to account for presumed
differences in visual performance.
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last modified: 9/17/98
