Theonella laena n. sp. (Porifera: Tetractinellida: Theonellidae) is a bright orange sponge thinly encrusting
on colonies of living and dead Tenagodus cf. sundaensis Dharma, 2011 (Mollusca:Gastropoda:Siliquariidae) found at 40m depth in the Singapore Strait. The sponge has an intricate aquiferous system built around openings found along the length of the tubular shells of the sessile gastropod, where water expelled by the latter flows into the sponge incurrent canal system. Incurrent water enters the two closely associated organisms through both the gastropod shell aperture and ostia of the sponge located externally but the excurrent water exits only through the oscula of the sponge. The new sponge species is distinguished from its congeners in having acanthose microrhabds (13.6–27 × 2.5–4.2 mm) with bifid spine terminuses, which together form a thin layer on the sponge surface. The entwined tubes of Tenagodus were almost entirely encrusted with the sponge except at the aperture openings of the gastropod. This is also the first report of a Theonella species associated with slit-wormsnails.