Considering only occupied sponges, the mean number of residents per sponge was 1.27 ± 0.60 (mean ± S.D). The number of residents per sponge was related to ‘‘number of sponge tubes’’ (df = 1; F = 59.26; P 0.0001), ‘‘maximum tube length’’ (df = 1; F = 8.47; P = 0.004), and the interaction between ‘‘number of sponge tubes’’ and ‘‘maximum tube length’’ (df = 1; F = 17.48; P 0.0001) (ANCOVA: r2 = 0.41). The interaction between the number of tubes and maximum tube length indicates that the effect of one sponge metric on the number of residents varies with the other sponge metric. Specifically, the effect of maximum tube length on the number of residents is small when the number of tubes is small, and vice versa. In other words, the number of residents was highest in sponges with many, large tubes.