Tonight's performance sees that unlikeliest of pairings - wildlife documentaries and the music of Yo La Tengo - blossom under the auspices of London's Royal Festival Hall. But then, the documentaries of Jacques Painlevés, with their gentle humanisation of almost alien species of aquatic life, would provide a wonderfully skewed starting point for any kind of musical accompaniment, alt-rock or otherwise. Likening the armour of a crab to that of a medieval knight, for example, is simple but rather charming. This, in its strangely pretty and surprisingly violent way, forms the ideal base for an accompaniment from one of the most sonically versatile three-pieces to come out of America since fish were invented.

Partially improvised, Yo La Tengo's soundtracks to these small documentaries are reassuringly tempered with their inimitable sensitivity. Rather than doing the obvious and smacking a cymbal every time the scene changes, they carefully create a plopping and slippery series of gentle assaults and, for good measure, one disgustingly oppressive and psychedelic noise explosion for some extreme close-ups of algae or something. The best bits are when the tension between the two mediums is cranked and released, particularly effective with the shocking scenes of a male seahorse being buffeted around by the offspring firing like bullets from his belly. A simply-plonked keyboard accompaniment, some shakey egg percussion and the absolutely correct atmosphere created.

More and more, Yo La Tengo appear to be invincible. Their artistic decisions are complimentary to both themselves and their audiences, and they continue to be intelligent enough not to let themselves become the main focus this evening. They are not lit, save for desklamps, and they scuttle carefully between instruments only when necessary. Even Ira Kaplan's usually central guitar-paddling is less so this evening, more of an atmospheric tool than an exercise in worthy histrionics. It's such an obvious idea, this whole soundtracking thing. Come in, sit down, get taken underwater, feel enchanted and frightened, humoured and lifted, then go home in time for cocoa and bed at a reasonable hour. All on a Sunday. In essence, a charming evening.