The Morthanveld, described in Matter, are a spiniform aquatic species, who are described as a high-level involved society, meaning that they are part of the 'Optimae', those highest tier of civilisations in power and sophistication to which the Culture itself is counted.
The Morthanveld are described as milky-coloured spheres of approximately a metre in diameter, with hundreds of spines, which are quite flexible and some serve manipulatory functions. They can also be adorned with rings and other decorations and their colour-change is a form of body-language, and squirts of water molecules (assumed to be coded via chemicals) also serve as a form of communication.
There is relatively little description of the society of the Morthanveld in the Banks novels. They are described as being technologically and otherwise on a similar level as the Culture, and also control vast spheres of the galaxy, including being the mentors of various lesser galactic races such as the Nariscene. They have a rudimentary remaining money system, but according to the Culture, are approaching a stage in their development when they will turn into a similar post-scarcity society - a potential cusp which the Culture has chosen to not endanger by interventionary moves which it might attempt with less advanced societies. The Morthanveld have vast numerical superiority over The Culture, as a single Morthanveld "Nestworld" - consisting of a complex, recursive arrangement of transparent tubes within tubes, all revolving around a small central star - is said to be home to more Morthanveld citizens than there are Culture citizens anywhere