Evidence of early chemotaxis contributing to active habitat selection by the sessile giant clam Tridacna maxima

• We investigated chemosensory ability and locomotion of giant clam Tridacna maxima.
• Both larvae and juveniles showed a strong preference for conspecific effluents.
• Juveniles have unexpectedly retained their locomotive ability after settlement.
• Juveniles could discriminate effluents from “favorable” vs. “unfavorable” reefs.
• T. maxima may rely on both conspecific and habitat cues to detect suitable reefs.

Finding a suitable habitat that will maximize the likelihood of settlement is one of the greatest challenges faced by coral reef species. This is the case for giant clams, which exhibit high sensory abilities coupled with a fast decline in locomotion after settlement that may curtail subsequent searches for an optimal environment. The goal of this study was to evaluate the ability of early giant clams to use and discriminate contrasted reef signals. We investigated the chemosensory ability and subsequent locomotion of the widely-distributed giant clam Tridacna maxima, ranging from five-day-presettlement larvae to six-month-old juveniles. Choice experiments were conducted to investigate the response of larvae and juveniles to chemical cues that were potentially associated with conspecifics, and with “favorable” and “unfavorable” reef habitats. Our results suggested that T. maxima may rely on both conspecific and habitat cues to detect suitable reef areas. Both larvae and juveniles showed a strong preference for and actively moved towards conspecific effluents. Two- to six-month-old juveniles were capable of efficiently discriminating effluents from “favorable” vs. “unfavorable” reefs, even without the presence of conspecifics, offering the first direct support for active habitat selection based on chemotaxis in giant clams. Our results expand the range of giant clam species known to exhibit post-settlement locomotion and demonstrate that juveniles (up to six months in the present study) have unexpectedly retained their locomotive ability. Despite a marked reduction in displacement occurring between four and six months, the ability of juveniles to move away from unsuitable areas using effective chemotaxis has positive implications for conservation-oriented initiatives, such as clam restoration projects.