Our Research: Using a combination of behavioral neuroanatomical, molecular and ecological techniques, we are investigating how habitat structure and mating system shape brain structure and function in a model evolutionary system: the highly visual cichlids of Lake Tanganyika. Cichlid fishes are renown for undergoing the most explosive vertebrate species radiation known, for diverse habitat preferences, and for diverse social behaviors (cooperative breeding, territoriality, lek formation, harems, monogamy and bower building). This system enables us to make fine ecological, behavioral, and neuronal comparisons, providing an unparalleled opportunity to understand how environmental and social pressures shape brain structure and function. 

Our Research Fish : Gallery and information

How does Habitat Complexity and Social Behavior Affect Visual Ability?
BACKGROUND
    Do the visual demands in particular habitats confer different visual abilities?  We tested the visual ability of fish with distinct habitat preferences and of  fish with contrasting breeding behavior.  Visual acuity was measured by rotating a square-wave grating around a subject and monitoring their response.

How does Habitat Complexity affect Memory?

Spatial Novelty
We are using a spatial novelty assay to compare the spatial memory of sand dwelling fish and rock dwelling fish. This assay is a habituation/dishabituation paradigm used in a number of animals and even humans. Previous research has shown that the hippocampus is required for spatial novelty discrimination. The advantage is that the response to spatial novelty is unlearned and therefore does not require training.

Goal: To test spatial memory across species using a habituation/dishabituation paradigm.

Hypothesis: Rock-dwelling species will exhibit a finer capacity for discrimination of spatial changes. Null hypothesis: there is no difference in species’ discrimination.

Methods:

- Four objects are arranged in the arena, 2 natural (rocks or sand formations) and 2 unnatural (PVC tubes).
 - 40 min. acclimation in a clear container allowing the fish to view the entire setup.
 - Habituation consists of three 30 min. sessions in which the fish is allowed to swim freely and explore the experimental arena.
 - Dishabituation consists of a single 30 min. session in which one of the objects has been displaced to a new zone (i.e. object moved from Zone 1 to Zone 2).

The trials are video taped and later a tracking program is used to identify the time spent near each object.

Preliminary results show the rock dwelling fish, A. leptura, going to where the object used to be. The sand dwelling fish, X. flavipinnis, spent the same amount of time in the zone where the object used to be as it did in the zone where the object was moved to.

This research was done in collaboration with Dr. Heike Neumeister of Albert Einstein College of Medicine and Les Kaufman of Boston University

Habituation Setup

The black lines are superimposed on the video to mark the zones for analysis. The white lines represent the fish's path during the trial.

Dishabituation Trial

Note how the sand crater from Zone1 has been moved to Zone 2.

Results for A. leptura: Blue bars represent the time spent in each zone during the final habituation period. Black bars represent the time spent in each zone during the dishabituation period.

We compared the gross brain morphology of closely related species of Tanganyikan Cichlids living in diverse habitats. We investigated whether there were any differences in brain structure between species that lived in different habitats, or had different social organizations or strategies.

Methods
 - We used at least 5 samples per species and used only reproductive males
 - We took digital pictures of brains under a dissecting microscope.
 - Brain structures were measured from the digital photos and the volumes were calculated.
 - Measurements were normalized by brain mass.

Results
As habitat complexity increases:
 - Telencephalon size significantly increases (r²=0.81)
 - Cerebellum size increases (trend r²=0.62)
 - Dorsal medulla decreases (trend r²=0.50)
Social Comparison:
 - Monogamous fish have larger telencephalons than polygamous fish

Research was done in collaboration with the Hofmann Laboratory at Harvard University

How does Habitat Complexity and Social Behavior affect the Brain?
Part 2: Internal Structure

Dl
We sectioned the brains of the rock dwelling monogamous A. leptura and the sand dwelling X. flavipinnis at 50u and stained them with cresyl violet. Cresyl violet stains the ribosomal matter in the cells purple and allows us to discern the regions of the brain.We measured an area of the brains called the lateral division of the area dorsalis or simply Dl (area highlighted in red on the brains to the right). Dl is believed to be the homologue to the mammalian hippocampus, which is implicated in spatial memory. We calculated the ratio of Dl volume to the volume of the telencephalon and we found that Dl is significantly larger in the rock dwelling fish. We are currently working on another pair of sister species within the Ectodini clade where one species, X. papilio, lives in rock habitats and the other, X. boulengeri, lives in sand. In addition we will compare sister species of marine gobies.

Dm
We sectioned the brains of the monogamous X. flavipinnis and polygamous X. ochrogenys at 25μ and stained them with cresyl violet. We measured the area called the medial division of the area dorsalis or simply Dm (see figures). Dm is believed to be the homologue to the amygdala, which is implicated in social recognition. We calculated the ratio of Dm volume to the telencephalon volume and found that the Dm was significantly larger in the monogamous fish than in the polygamous fish.