Caridina species vary greatly in terms of colouration, from being clear with visible organs to having fully opaque bodies in a range of colours. This is due to the presence or lack of pigments and pigment complexes, and their respective distributions.
Pigments and proteins
Pigments are chemicals that absorb certain wavelengths of visible light and transmit the remaining wavelengths. This results in us interpreting the light (and thus the object) as coloured. Different pigments produce different colours and tones due to their structures.
A pigment found in an organism is called a biopigment. The most common biopigments in Caridina shrimp are carotenoids, which appear to be obtained through diet. These pigments can form complexes with proteins, altering the colour or tone produced, and these complexes are known as carotenoproteins. When these complexes are broken, the colour and tone of the original carotenoid is expressed instead. This is demonstrated by how shrimp turn red when cooked: the complexes between the pigment and protein are broken due to the high temperature and the reddish colour of the carotenoid replaces the natural colours of the shrimp produced by the carotenoproteins .
In Caridina shrimp the biopigments are found either in the cuticle, specifically the exocuticle, or in specialised cells called chromatophores. The former are responsible for colouration of the exoskeleton, whereas the latter are responsible for colouration within the body tissue.
Filing those chromatophores
Chromatophores are categorised by their respective colour of pigment, simply by stating the colour e.g a red chromatophore. There are four main categories of chromatophores by this convention:
- Black chromatophores, containing ommochromes, which absorb all wavelengths of light
- Red chromatophores, generally containing astaxanthin
- Yellow chromatophores, generally containing lutein
- White chromatophores, contain pigments that reflect all wavelengths of light, such as pterines or certain flavins
The colour and colour intensity we see with the naked eye is determined by a number factors. At a molecular level, the type of pigment and its complexes with proteins determines the light absorbed and reflected. Then there is the number of pigments, the combination of pigments and their distribution within the chromatophore. Then there is the combination of chromatophores and their density within tissue. It is a similar case for the pigmentation of the exocuticle. On top of these, there are even more factors, such as sclerotisation of the cuticle which gives an amber hue, or the wavelengths present in the light they are exposed to.
The colouration of shrimp is far more complex than one might assume, and even this article is merely scratching the surface. However, good knowledge of how colouration works in shrimp allows the aquarist to enhance the colour of their shrimp in harmless ways.
More on that later…