Urchins Respond to the Epiphytes: Good Evidence? No.

Larval settlement is a key process affecting both the population dynamics and distribution of benthic invertebrates. Decision to settle is affected by biotic influences such as cues released by algae but also by epiphytic bacterial communities. Previous research into settlement and epiphytes using culture-based methods revealed that specific isolates, such as associates of Pseudoalteromonas luteoviolacea strongly induces settlement. However, these effects often do not translate into mixed cultures (more natural model systems). Furthermore, correlative culture independent techniques have shown larvae are responsive to biofilm community composition. However, it is difficult to assess if bacteria are responding to cues from the community as a whole or specific members. Nielsen et al. attempted to address this by examining settlement of the urchin species Heliocidaris erythrogramma and Holopneustes purpurscens in response to the epiphytic communities associated with different Coralline Red Algae.

Adult urchins were collected from Long Bay, Sydney during the reproductive season. Gametes were released and eggs fertilized. Eggs were cultured in autoclaved seawater, under controlled conditions with antibiotics. Five species of coralline algae, three non-coralline macroalgal species and a control group of bio-filmed shell grit were collected from the same habitat as the urchins. Algae were treated with antibiotics to reduce abundance and diversity of epiphytic bacterial prior to settlement assays. To verify antibiotic effectiveness, the two most abundant species of coralline algae were examined and compared between antibiotic and control treatments. Larval settlement assays were then conducted. Bacterial communities were assessed to compare natural variation of bacterial community composition. For the two most common coralline species larval settlement was correlated against epiphytic bacterial community, using TRFLP (Terminal Restriction Fragment Length Polymorphism) to identify OTUs (operational taxonomic units) and then carrying out multivariate statistics on this data. Total epiphytic bacterial DNA was extracted and PCR targeting 16S rRNA genes ran. TRF sizes were determined after electrophoresis by comparison with internal size scanner. To determine in associates of P. luteoviolacea induced settlement in the community TRF lengths was compared to the TRF length of P. luteoviolacea.

In this experiment, the settlement of sea urchin larvae was not particularly specific to certain algal species. However, settlement did depend on the qualitative and/or quantitative composition of the epiphytic bacterial community associated with algae. Larval settlement of H. erythrogramma strongly correlated with bacterial community composition whereas settlement of H. purpurscens was more general. The authors suggest that urchin larvae respond to a complex natural bacterial community and are effected by the relative abundance of inductive and inhibitive OTUs, however there is no description of what they mean with inductive and inhibitive. Nielsen et al. also suggest that patterns of larval settlement correlated with community composition and the relative abundance of certain OUTs cannot be generalized and are species specific. There are further limitations to this study. Firstly, TRFLP provides limited phylogenetic information for microbial OTUs compared to other methods that could have been used instead. However, the study did show that no OTUs correlated with settlement were affiliated with P. luteoviolacea. Furthermore, correlation between larval settlement and bacterial community composition was only analysed for the coralline algae Amphiroa anceps and Corallina officinalis. This study is therefore missing important information and may lead to the wrong conclusions. Additionally the scope of the literature cited is limited, as the authors own relevant work was not considered. Overall, this study provides an interesting but flawed investigation into the effect of biofilms on larval settlement. Rather unfortunately, it is a good example of how it is important to be discerning when reading even the peer-reviewed literature. 

Ref: Nielsen S.J., Harder T., Steinberg P.D. (2015). Sea urchin larvae decipher the epiphytic bacterial community composition when selecting sites for attachment and metamorphosis. FEMS Microbiology Ecology 91:  doi: 10. 1093/femsec/fiu011.