Memory of direction of locomotion in sea urchins: effects of nerves on direction and activity of tube feet

Memory and learning in invertebrates with decentralized nervous systems is poorly understood and sea urchins are no exception. Following up on previous studies of locomotory direction in sea urchins, the regular sea urchin, Hemicentrotus pulcherrimus was used to examine whether they have directionality and ‘memory’ of direction in locomotion. Experiments were conducted in 2011–2013 on urchins collected from the subtidal rocky area off the Aburatsubo coast, Kanagawa, Japan. The sea urchins locomoted mainly with their tube feet. Individuals retained their directional locomotion in two successive trials separated by a resting period. This implies memory of the original direction and is termed “directional memory”. This memory was retained when the resting period was 5 min but disappeared when the period was extended to 10 min. Such directional memory was also observed in cutaway preparations in which the aboral one-third of the test was removed along with the viscera. The radial nerves (RNs) were severed in cutaway preparations to study their roles in memory and activities of the tube feet. The speed of locomotion was proportional to the total length of the RNs. When all RNs were separated from the nerve ring, the preparation continued to attach firmly to the substrate. When all RNs but one were cut at the midpoint, preparations locomoted keeping the ambulacrum with the uncut, longest RN forward. When all RNs were cut at the midpoint, directional memory was observed without the interference of the longest RN. A model in which each ambulacrum assumes one of four modes (leading, trailing, attached, or retracted) with its tube feet being controlled and coordinated by the RN, and where mode switching is caused by the mechanical pull of other ambulacra and or via the RN–ring system accounts for these findings.