Episymbiotic microbes as food and defence for marine isopods: unique symbioses in a hostile environment

Abstract

Symbioses profoundly affect the diversity of life, often through novel biochemical services that symbionts provide to their hosts. These biochemical services are typically nutritional enhancements and less commonly defensive, but rarely both simultaneously. On the coral reefs of Papua New Guinea, we discovered unique associations between marine isopod crustaceans (Santia spp.) and episymbiotic microbes. Transmission electron microscopy and pigment analyses show that episymbiont biomass is dominated by large (20-30 microm) cyanobacterial cells. The isopods consume these photosymbionts and "cultivate" them by inhabiting exposed sunlit substrates, a behaviour made possible by symbionts' production of a chemical defence that is repulsive to fishes. Molecular phylogenetic analyses demonstrated that the symbiotic microbial communities are diverse and probably dominated in terms of population size by bacteria and small unicellular Synechococcus-type cyanobacteria. Although largely unknown in the oceans, defensive symbioses probably promote marine biodiversity by allowing niche expansions into otherwise hostile environments.

Figure 1

Collection sites of Santia spp. isopods in Papua New Guinea. K, Kairiru Island (September 1998); EF, Eastern Fields (December 1999); R, Rabaul (August 2000) and CI, Crown Island (August 2000).

Figure 2

(a) Epifluorescence photograph of the Santia spp. isopod and its ectosymbiotic cyanobacteria. (b) Cyanobacterial cells on an isopod antenna. (c) TEM micrograph of a cyanobacterial cell of an unpalatable red isopod and surrounding micro-organisms. (d) TEM micrograph of a cyanobacterial cell of a palatable brown isopod and surrounding micro-organisms. Scale bars: (a) 1 mm, (b) 25 μm, (c) 10 μm and (d) 5 μm.

Figure 3

Results of in situ feeding assays testing the palatability of: (i) red isopods with their episymbiotic microbial community, (ii) episymbiont-depleted red isopods and (iii) a crude extract of the red isopod/microbial association. A single control bar is shown for comparison with each assay result to simplify the graphical presentation of the data because consumption of control pellets was 100% for all assays, n=10 replicates per assay. p<0.000 1 for (i) and p=0.003 1 and 0.47 (Fisher's exact test) for (ii) and (iii), respectively.

Figure 4

Phylogram representing the most parsimonious tree (522 steps) obtained from 563 bp (246 parsimony informative) of mitochondrial cytochrome oxidase c (subunit 1) data using a heuristic search via simple stepwise addition, characters unweighted and unordered, gaps treated as missing and TBR branch-swapping. Bold numbers represent bootstrap values obtained from 10 000 bootstrap replicates. Sequences were obtained from 28 isopods representing five populations from four localities.

Figure 5

(a) One of two most parsimonious trees with 1175 steps obtained from selected 846 bp (380 parsimony informative) rpoC1 clones. (b) One of 95 most parsimonious trees with 581 steps obtained from 189 AA (116 parsimony informative) characters from the 846 bp rpoC1 sequence fragment. Numbers at nodes are bootstrap values obtained from 1000 replicates. Outgroup and comparison sequences for AA tree were obtained from GenBank.