1. Studies with Pantodon buchholzi, the African butterfly fish

Pantodon

A. Studies of the visual system of Pantodon

Pantodon is a lovely little fish that lives just below the water’s surface in Nigeria. As a consequence of its optical ecology, its eye views into air and water simultaneously. We are currently studying two specific behaviors, feeding and the shadow reflex, and relating them to its brain organization and the visual electrophysiology of cells at different loci of its visual pathway. In particular, we have found a diencephalic nucleus (nucleus RostroLateralis) that is concerned only with vision in air. This nucleus receives direct visual input from the ipsilateral ventral retina and indirect visual input from cells of the optic tectum (bilaterally) that are postsynaptic to the ventral retina. This nucleus is found in less than a dozen of the >20,000 species of fish.

The Eye of Pantodon

Interestingly, it is found in Anableps anableps, another and unrelated species that simultaneously sees in air and water.

 

    The Eye of Pantodon

The eye of Pantodon is unusual because its visual field is closely reflected in eye and retinal structure. The fundus of the eye is bisected by a modified falciform process that extends from the nasal pole almost to the temporal pole. Blood vessels emerge perpendicular to the falciform process to run on the surface of the retina. A small region of continuity extends from ventrotemporal retina to dorsotemporal retina at the temporal pole of the ora serrata. The embryonic fissure extends from the nasal pole to the optic disc. I am currently studying the non-symmetrical postembryonic addition of cells in the retina as a probe to understand the morphological pressures that might account for the extension of the falciform process into the temporal hemiretina. The ratio of temporal / nasal  >>3. 

Visual FieldThe visual field is tripartite. The ventral hemiretina views into the air. The water surface area through which all aerial rays pass is known as Snell’s window. The dorsal retina views into the water column and the region between (which has an unusual retinal structure, see below) views the aquatic environment as seen in a reflection from the aquatic side of the air-water interface. The retina reflects this tripartite visual field in the following manner: The falciform process bisects the retina in the nasal hemiretina (left) and sits on the surface of the temporal retina. The black tissue is due to highly melanized chorioid epithelial cells that penetrate the retina via the optic nerve at the disk. Cones are wider and larger in the aquatic viewing retina (right,top) than the aerial viewing retina (right, bottom).

 

Retina

Pantodon feeds monocularly and it only feeds on targets at the water surface. [In the numerous years of observing this fish, I have seen only 1 example of feeding within the water column.] It only captures a target from within 1 cm of an eye. It is fairly clear that the sensory information involved with feeding is a combination of vision and lateral line with visual input under bright illumination acting in an inhibitory manner. Either vision alone or lateral line alone are sufficient for feeding purposes, but the combination in low illumination provides a more favorable sensory environment.

Although the fish will jump at targets in the air, it does not jump at the targets. It jumps at the image of the target in Snell’s window. The figure below demonstrates that the fish does not jump at the hanging cricket in air. Rather, as you can see  below, the fish jumps at the image of the cricket  in Snell’s window.

Each frame is a composite of a  top view (above) and a side view (below) taken simultaneously (with the aid of an overhead mirror). The small arrows in frame 4 identify the hanging cricket. The large arrows in figure 4 identify the vector of movement. As can be seen in frame 5, the fish (albeit blurred), is jumping at a target adjacent to its eye (ie., within Snell’s window).

B.   Neurology relevant for feeding behavior

1)  Since feeding only occurs from stimulation of the ventral retina, we searched for a neurological correlate or neural element that can be found in the visual pathway concerned with the superior visual field. In the optic tectum, we found a distinctive neuron (below) whose dendrites appear to integrate visual and lateral line input. Since these cells are predominately (>85%)  found in the tectual representation of the aerial visual field, we feel that these cells signal a potential feeding target. Interestingly and as a corollary, we also studied the distribution of these cells in a fish (goldfish) that feeds throughout its visual field and sure enough, the cells are found throughout the tectum. We are currently searching through the tectum of other fish with uniquely peculiar feeding habits for the tectal distribution of these cells.

 

Since feeding only occurs from stimulation of the ventral retina, we searched for a neurological correlate or neural element that can be found in the visual pathway concerned with the superior visual field. In the optic tectum, we found a distinctive neuron (below) whose dendrites appear to integrate visual and lateral line input. Since these cells are predominately (>85%)  found in the tectual representation of the aerial visual field, we feel that these cells signal a potential feeding target. Interestingly and as a corollary, we also studied the distribution of these cells in a fish (goldfish) that feeds throughout its visual field and sure enough, the cells are found throughout the tectum. We are currently searching through the tectum of other fish with uniquely peculiar feeding habits for the tectal distribution of these cells.

2) Nucleus RL is afferent to Nucleus Interpeduncularis in the midbrain

3) Conclusion: The ventral retina > Nu RL > Nu IP is not consistent with a somatic sensory pathway. It is consistent with a primitive model of the Dorsal Diencephalic Conducting Pathway.  This is a mammalian descending limbic-like pathway and by inference, the circuit in Pantodon may represent information to upregulate  motivation.

 

2. Birds, birds and more birds.

    (newly published)   more to come

3. Stochastic Resonance and the Jamming Avoidance Reflex (with Prof. Sunil Shende, Dept. of Computer Science, Rutgers-Camden)

    It is true. We have evidence that with white noise, Apternotus changes its  EOD frequency to stimuli that do not evoke a JAR.    more to come

 4. Studies of the anterior thalamus of Rana pipiens (with Profs. Ed Gruberg, Temple University, Biology and Elizabeth Dudkin (Penn State, Biology)

    (newly published)    more to come