Chemoluminescence

The natural language of ljudoy and esaanla, chemoluminescence is a light-based language, create by chemical reactions channeled through quartz growths or specialised cells in the body.
Chemoluminescence in esaanla is more correctly referred to as bioluminescence. Esaanla skin contains layers of chromatophores and iridophores, allowing them to create complex and even moving patterns across their whole body. There are 100 common colours and patterns, multiplied by variations in skin texture, plus regional and intra-family dialects, and slang.
Chemoluminescence in ljudoy is generated by manipulating chemical reactions directly. Although they cannot create the same complex or moving patterns as esaanla, ljudoy are capable of an umlimited range of colours with variations in tone and brightness, though these may not be easily distinguishable outside of their immediate social circle.
The two languages are distinct but related and, with practice, one can be understood by a speaker of the other. Although neither language can convey complex ideas, this mutual intelligibility lowers the barrier for conversation between two species that often struggle to adapt their natural speech patterns to inter-species communication. For verbal adults, both bio- and chemo-luninescence are mostly used to emphasise aural speech or general mood, or to signal sarcasm or innuendo.
Both ljudoy's chemoluminescence and esaanla's bioluminesence are instinctive and - in the case of esaanla - fossil evidence suggests that it predate the species' evolution of a spoken language. Infants of both species use luminescence to communicate with their carers long before they develop speech, and childcare responsibilities make adults more likely to luminsece as they speak. Age and experience allow for the creation of more complex patterns and nuanced colour. Artists of both species make extensive use of light and colour, and esaanla and ljudoy poetry, drama, dance, storytelling, and even jokes usually require an understanding of luminescence.