Larval collection:
These larvae were collected near three hydrothermal vent sites (Snail, Archaean, or Urashima) on the Mariana Back-Arc Spreading Center in the southern Mariana Trough in September 2010 on R/V Yokosuka cruise YK10-11. See the Sampling Events Supplemental File for specific locations and dates.

To collect larvae, we deployed large-volume plankton pumps (McLane WTS-LV50, East Falmouth, MA, USA) to sample at 3 or 4 m above the bottom. Each pump sampled for up to 24 hrs at 30 liters per minute over a 63 micron mesh, yielding a filtered volume of up to 41.5 cubic meters. We provide as a supplemental data file a table of sampling events matched to Station Information in the Marine Geoscience Data System (https://www.marine-geo.org/tools/search/Events.php?event_set_uid=1185).

Shipboard sample processing:
Upon recovery, pump filters were rinsed with filtered seawater into trays on ice and examined live to select some individuals for experiments reported by Beaulieu et al. (2015, DOI:10.1111/maec.12207). Within an hour the trays were poured over a 63-um sieve and washed using 95% non-denatured ethanol into a 250 mL jar. Individuals used in the experiments were saved separately, either frozen or preserved in ethanol.

Laboratory sorting and morphological identification:
Samples were poured over nested 300-um and 63-um sieves and washed into dishes for sorting under a dissecting scope at magnifications up to 50X. Individuals were identified to morphotypes at lowest taxonomic level (original categories for morphospecies). All morphospecies were enumerated except for ostracods and copepods which were only accounted as “p” present, “a” absent, or “n” not accounted for. We used the original category plus genetic evidence when available to match our lowest-level identification to the World Register of Marine Species (WoRMS) taxonomic database. Some morphotypes contain morphologically indistinguishable larvae from multiple species (e.g., complex nectochaetes); in this case, our final categories for morphospecies group those individuals with genetic evidence into the higher taxon rank. The table is organized such that the first row listing a final category morphospecies is at the higher taxon rank containing the subsequent rows for that category (in general, the subsequent rows are for specimens sent out for genetic analyses). Note that the polychaetes classified to Infraclass Scolecida correspond to the “Capitellid” larval morphotype in Beaulieu et al. (2015). We thank Dr. Takenori Sasaki for guidance for identification of gastropods.

Additional genetic evidence to assist with larval identification:
A subset of these specimens with 28S rRNA sequences is described in a separate BCO-DMO dataset (Beaulieu et al., 2021, DOI:10.26008/1912/bco-dmo.839476.1, https://www.bco-dmo.org/dataset/839476). We'd like to thank Dr. Hiroka Hidaka, Dr. Shigeaki Kojima, and Dr. Hiromi Watanabe for providing histone 3 sequences for gastropod specimens and Dr. Florence Pradillon for providing cytochrome oxidase subunit 1 (CO1) sequences for polychaete specimens. We thank Bethany Fleming for NCBI BLAST query.