A field model of learning: 2. Long-term memory in the crab Chasmagnathus granulatus.

In the previous companion paper, the possibility of learning by Chasmagnathus in field conditions was demonstrated. Here, we study long-term memory inquiring to what extent an internal representation could be maintained in a complex environment. Two 45-min training sessions, each of 15 visual danger stimulus presentations with 3-min intertrials, were given at a 24-h interval. Throughout the first training session and during the first 22.5 min of re-training on day 2, crabs kept the same hiding response level but then, during the second phase of re-training, the re-emerging increased up to the point that 32% of surface crabs ignored the stimulus. Each session was followed by a 22.5-min testing without stimulation. At testing on day 2 after re-training, crabs showed a change in the usual exploring strategy. Results reveal long persistency in responding despite a rest interval of 24 h followed by a gradual decrease in response until it vanishes. The statistical analysis was performed by comparing performances at day 2 (Rescorla in Am Psychol 43:151-160, 1988) and then confirmed through comparisons between day 1 and day 2. However, it is not possible to identify separate and taxonomically well-defined learnings but rather a tangled collection of processes that influence each other blurring some of the diagnostic features of each learning.

A field model of learning: 1. Short-term memory in the crab Chasmagnathus granulatus.

Learning and memory studies have been performed for more than two decades using the crab Chasmagnathus in our laboratory. Here, our research was aimed at disclosing some instances of learning in field conditions. Three experiments were performed non-simultaneously, all with a 22.5-min pre-training preceding the first visual-danger-stimulus, an opaque rectangle passing overhead. In Experiment 1, crabs received a single stimulus followed by 22.5-min testing without stimulation, where the re-emerging latency was considered the basic latency response. In Experiment 2, training consisted of 15 stimulus 3-min apart, followed by 22.5-min testing without stimulation. Throughout training crabs were underground but re-emerged at testing with latencies longer than the basic latency response. Both at pre-training and testing the usual strategy of exploring was the short-near excursions. In Experiment 3, training included three stimulus 22.5-min apart, followed by 22.5-min testing. Crabs left their burrows before the end of each inter-trial, showing a mean latency like the basic latency response, but a sensitization to the stimulus and a preponderance of the fast-far excursions over the usual slow-near. In brief: through 15-3 training, crabs learn that the stimulus is iteratively presented; through 3-22.5 training, crabs acquire sensitization to the stimulus and a different strategy of exploration.