Partner choice in Gobiid fish Myersina macrostoma living in association with the alpheid shrimp Alpheus rapax

Partner choice is an important selective force in mutualistic partnerships. Although several aspects of mutualism have been well-studied, partner choice remains a novel aspect in the study of marine mutualists. Previous research on the goby-shrimp mutualism suggested that visual cues affect the ability of gobies to discriminate between potential shrimp partners but small sample sizes limited the generalization of that result. Here, primary partner choice in Myersina macrostoma was investigated in a series of ex situ experiments. Results indicated that shrimp-associated gobies relied on visual cues in partner choice. The goby M. macrostoma was clearly able to discern between different shelter types, demonstrating a preference for burrows compared to rocks. While they were attracted to larger burrows, they were not visually attracted to larger shrimp partners. This indicates that in wild populations, intraspecific competition for larger burrows might be the driving factor in size assortment in associated partners. Myersina macrostoma could also discern between species of shrimps in laboratory experimental set-ups, indicating a clear preference for A. rapax which they naturally occur with, but not to A. lobidens, which occurs at the same locality but with which they do not naturally associate. These findings offer fundamental building blocks towards the broader understanding of partnership formations in marine mutualists.