Microbiomes of the polychaete Hydroides elegans (Polychaeta: Serpulidae) across its life-history stages

Abstract

Larvae of the biofouling tubeworm Hydroides elegans (Polychaeta) must contact a bacterial biofilm to induce settlement. To further understand the relationship between particular bacteria and the worm, this study analyzed associated microbiomes at multiple life-history stages with high-throughput 16S rRNA amplicon sequencing. Worms were collected from Pearl Harbor Hawai’i (21°21′27.2″N 157°57′34.8″W), and microbial communities were analyzed from adults, their tubes, spawned eggs, 6-day old nectochaete larvae, and filtered seawater from the larval cultures that included cells of the microalga Isochrysis galbana fed to the larvae. Microbiomes of females sampled at two different times were significantly different: Cellvibrionales dominated the microbiome of adult females in August 2016, whereas Oceanospirillales were dominant in females collected in April 2017. Additionally, in 2017, the microbiomes across all life stages were significantly different from each other; only one OTU in the genus Endozoicomonas was shared between females and their nectochaetes at a relative abundance > 1%. Because of the apparent absence of a consistent microbiome between stages, female worms, their eggs and both trochophore and nectochaete larvae were stained to reveal the presence of bacteria and analyzed with scanning confocal microscopy. The eggs were almost devoid of bacteria, and bacteria seen on larvae were almost all confined to the feeding apparatus. The evidence of inconsistent microbiomes across life stages and lack of bacteria on eggs demonstrates that H. elegans lacks a vertically transmitted microbiome, and bacteria found at each stage came from a varying environment.