**2.1 Collection of materials**

Individual *H. dofleinii* were obtained from shallow subtidal seagrass banks in Moreton Bay, Queensland, at a depth of about 1-2 metres at low tide, close to the western side of Stradbroke Island (153 26.4' E 27 25.13' S to 27 25.68' S), and were held for up to 5 days prior to use in filtered, recirculating seawater tanks at 21.5 – 22 C. The identification of the animals was based on morphology, spicule shape and size and 18S-RNA sequencing (Peng & Skewes, unpublished data).

#### **2.2 Sample preparation**

To collect expelled Cuvierian tubules, *H. dofleinii* individuals were held and gently stimulated underwater until tubules were expelled. Immediately after expulsion, a tubule was individually collected using polytetrafluoroethylene-tipped forceps, and was allowed

to drain briefly (<20 sec), but not by squeezing as had been proposed by others (Zahn et al., 1973). Intact tubules prior to expulsion were obtained by dissection of animals that had been euthanised by freezing at minus 20 °C.
