Sampling
Janthina spp. were collected as part of a global sampling effort of neustonic taxa via neuston tow and beach collection from 2005-2010 from: National Oceanographic & Atmospheric Administration (NOAA) Southeast Area Monitoring and Assessment Project (SEAMAP), NOAA Pacific Island Fisheries Science Center, Woods Hole Oceanographic Institute SEA Semester Program, P. Colman, C.K.C.C.

Molecular data
Whole genomic DNA was extracted from dissected foot tissue when possible, or whole body extractions for very small specimens, using the E.Z.N.A. Mollusc DNA Kit (Omega Bio-Tek). A total of 2,217 aligned nucleotides were amplified from four molecular markers using previously published primers. 536 nt of mitochondrial 16S rDNA was amplified using universal primers combination 16Sar/16Sbr (named primer pairs are in the format 5’/3’) [S1] and an annealing temperature of 49C. 1,192 nt of nuclear 28S rDNA (DI-DIII) was amplified using molluscan primers D23F/D6R [S2] and an annealing temperature of 50C. 328 nt of nuclear Histone-H4 was amplified using universal primers HexAF/HexAR [S3] and an annealing temperature of 53C. 160 nt of nuclear Histone-H4 was amplified using universal primers H42FS/H4F2er [S4] and an annealing temperature of 50C. All PCRs followed a general protocol: initial denaturation (95C, 2 min); 35 cycles of (94C, 30 sec; XC, 30 sec; 72C, 1 min); final elongation (72C, 5 min), where X = Annealing temperature. After verifying the size of amplified fragments via gel electrophoresis, PCR products were directly sequenced using an ABI 3730xl (Applied Biosystems, Inc.) automated sequencer by the University of Michigan DNA Sequencing Core. Sequences were aligned using the MUSCLE alignment method [S5] implemented in CodonCode Aligner 3.7.1.1 (CodonCode Corporation) and verified by eye. Two sequences for Littoraria intermedia (mt 16S rDNA, nuclear 28S rDNA) used in this study were previously published in GenBank. Museums holding voucher specimens are identified by prefixes before catalog numbers: FLMNH, Florida Museum of Natural History, University of Florida, Gainesville, USA; UMMZ, University of Michigan Museum of Zoology, Ann Arbor, USA; RMNH, Netherlands Centre for Biodiversity Naturalis, Leiden, The Netherlands; FMNH, Field Museum of Natural History, Chicago, USA. Asterisks indicate no sequence data.

Phylogenetic analyses
For the molecular analysis, best-fit models of nucleotide substitution were selected statistically by Akaike Information Criterion in jMODELTEST 0.1.1 [S6] for each molecular marker: mt 16S rDNA (TPM2uf+Γ), nuclear 28S rDNA (TIM3+I+Γ), Histone-,H3 (GTR+I+Γ), and Histone-H4 (SYM+I+Γ). Bayesian phylogenetic analysis was conducted in MrBayes 3.1.2 [S7] (4 chains, 10 million generations) with a partitioned data set; the model of nucleotide substitution chosen for each partition was the closest approximation to the AIC best-fit model available in MrBayes: mitochondrial, (GTR+Γ); nuclear, (GTR+I+Γ). Convergence was estimated by plotting the average sums of split frequencies every 1000 generations. Bayesian posterior probabilities were calculated after a burn-in of 25%. Maximum likelihood phylogenetic analysis was conducted in PAUP* 4.0 [S8], using the AIC best-fit models of nucleotide substitution for respective markers. Likelihood bootstrap values were also calculated with PAUP* 4.0 [S8] with 300 replicates.

Morphological data
To confirm and add to previous studies [4, 5, 7] supporting Recluzia as a plesiomorphic janthinid, ethanol-preserved oviparous and ovoviviparous Janthina spp., and Recluzia cf. jehennei, were dissected using standard techniques [S9] Morphological data for benthic janthinoideans (=Epitoniidae) came from previously published studies [4, 7] (Table S1).

Table S1: Taxonomic distribution of janthinoidean characters used to support Recluzia cf. jehennei as a transformative janthinid. [Churchill et al, 2011]

Character	Benthic janthinoideans (= Epitoniidae)	Recluzia cf. jehennei	Janthina
Statocysts [7]	yes	yes	no
Stylets in the inner paired salivary glands [7]	yes	yes	no
Cephalic tentacle structure [7]	uniramous	uniramous	branched
Pre-female stages associated with egg mass or float [5, 6]	yes	yes	no
Unpaired labial gland*	large	large	reduced
Metapodial attachment to egg mass or float*	mucus stalk	mucus stalk	mucus sheets

mollusc_vouch_acc_nums.csv