Complex sentence: It commonly inhabits shells of "Cerithium litteratum" but free-living empty shells are in short supply, and on some occasions, the hermit crab takes up abode in the calcified tube of the vermetid gastropod "Spiroglyphus irregularis".
Split into simpler sentences: It commonly inhabits shells of "Cerithium litteratum", but empty shells are sometimes in short supply. On some occasions the hermit crab takes up abode in the calcified tube of the vermetid gastropod "Spiroglyphus irregularis".