Current research: double cones

"Even more mysterious are the 'double cones' - and the puzzle they present is particularly irritating to the curious investigator because they are so very widespread among vertebrates" Walls 1942.

a. The mysterious function of the most abundant daytime photoreceptor: What do double cones do?

Vertebrate retinae are often called 'duplex' because they consist of two types of photoreceptor cells, rods and cones.

In general, cones are used for vision in daytime and rods for vision at night. Cones can be further subdivided into two types based on anatomy and/or physiology: single cones and double cones.

Double cones are found in most vertebrate groups, with the exception of placental mammals (including humans), sharks and catfish, and are frequently the most abundant cone type in their retinae. In many reef fish, for example, double cones make up over 75% of the retina. Their function in any animal is, however, still as 'mysterious' as it was in 1942. Using a model system of reef fish we aim to clarify the function or functions of double cones in fish and other vertebrates.


Examples of three different types of photoreceptor mosaic from reef fish. The first retina (left) is dominated by rods, whereas the other two are dominated by double cones as typically seen in fish.

Sixty years after the exasperation expressed by Walls, the function or functions of double cones still remains elusive. Solving the problem of double cone function would represent a significant advance in our understanding of the vertebrate retina, from both a functional and evolutionary perspective. Discovering why they are so important in fish will also allow us to understand why placental mammals have lost them, what has taken their place functionally in mammals and why the other large fish group, the elasmobranchs (sharks and allies) lack double cones.


	The retinae of reef fish are dominated by double cones. Why is this the case when they are not evident in the other large marine fish group (elasmobranchs) or placental mammals such as humans?

Our aim will be achieved by testing the following hypotheses:

- Is there a single function for double cones in all species, e.g. luminance vision, or do double cones have different functions in different species, e.g. luminance vision in fish A as opposed to colour vision in fish B?
- Do double cones have multiple / parallel functions within the same animal, e.g. colour discrimination and luminance vision in fish C, and if so how does the neural wiring function to achieve this?
- Are the spatial arrays or 'mosaics', of double cones related to function , e.g. polarisation sensitivity or luminance vision?
- Are the specific functions of double cones determined by / adapted to the visual environment? Different eye regions frequently possess different types and densities of double cone mosaic suggesting this is likely but nothing is proven.
- Does the nature of photoreceptor coupling, horizontal cell connections, ganglion cell density and other post-photoreceptor processing vary with double and single cone complement and the spatial organization of double cones in the retina.

To read more about the project click here.