Context-dependent refuge use in the estuarine crab Hemigrapsus crenulatus under varying salinity conditions

Salinity fluctuations impose physiological challenges on estuarine organisms, but immediate behavioral responses may also depend on available habitat options. We characterized adult male Hemigrapsus crenulatus during the first minutes following transfer from 32 PSU to 6, 16, or 32 PSU. In Experiment 1, crabs had simultaneous access to sand and a submerged rock cavity for 720 s. In Experiment 2, the bottom was covered by either sand or shell hash, without a structural refuge, and crabs were observed for 300 s. Because each condition was represented by a single aquarium, observations, descriptive summaries, and principal coordinates analyses characterized individual patterns rather than independent treatment effects. In Experiment 1, 14 of 18 crabs entered the rock cavity: five of six at 6 PSU, five of six at 16 PSU, and four of six at 32 PSU. Refuge use, substrate occupation, refuge-entry latency, and behavioral transitions varied among individuals. In Experiment 2, completed burrowing occurred only in three of six crabs in shell hash at 16 PSU and was absent from the other salinity × substrate conditions. Median observed burrowing latency among these three crabs was 53.5 s (range 22.0-70.5 s). Wall-associated and wall-climbing events were common across the conditions examined. These findings show that H. crenulatus expressed multiple immediate responses following experimental transfer. Structural shelter was frequently used when available, whereas completed burrowing was uncommon without an alternative refuge. These exploratory patterns highlight the importance of habitat configuration and physiological timescale when examining behavioral responses to salinity variability.