We surveyed a 500 m long by 125 m wide study area on the reef at Curlew Caye, Belize by SCUBA to map and collect tissue samples from all Elactinus lori individuals, at depths from 10 m to 25 m. We began by fully censusing the focal study area: we marked the location of all host sponges (Aplysina fistularis) and fish (E. lori). GPS data were collected with a Garmin GPSMAP 76Cx unit in an underwater housing. Waypoints are accurate within 5 m. At each sponge we recorded: depth (m, using dive computers), number of tubes per sponge, length of largest sponge tube (nearest cm, using a tape measure). We also counted the number of fish per sponge, and noted which life history stage they fell into: resident ≥ 18 mm standard length (SL) or settler (<18 mm SL).
Next, we collected tissue samples from all residents (n = 425) and settlers (n = 194). Individuals were caught with slurp guns. We took a small clip from the caudal fin from residents; whole-settler specimens were collected and euthanized in MS-222. Additional settler tissue samples were collected from three sites North, and three sites south of the focal study area. (n ≈ 30 per site). All tissue samples were store in 95% EtOH. Microsatellite genotype data were collected based on standard DNA extractions (DNAdvance kit, Beckman Coulter, Inc., USA), multiplex PCR using the Type-It Microsatellite PCR kit (Qiagen, USA), and screening on an ABI 3730 automated sequencer. Further details on all methods can be found in D’Aloia et al. (2013), Molecular Ecology.