Movement patterns and habitat utilization in the spider crab Maja squinado (Herbst) (Decapoda, Majidae) measured by ultrasonic telemetry

Abstract

Using ultrasonic telemetry, the activity and movement patterns (speed, distance and direction), and habitat utilization (depth, substratum) have been analyzed in juveniles approaching maturity and adults of the spider crab Maja squinado in the Ria de Arousa (Galicia, NW Spain). In summer three juveniles were tracked during a 23-day period and in winter five juveniles were tracked for 56 days in shallow rocky kelp forests (< 10 m), located in the outer area of the Ria. The juveniles showed a similar behaviour in both experiments, carrying out non-directional movements in restricted areas. No habitat changes were observed during the study period. Movement speed was slow in summer and winter (9.7 and 4.5 m/day, respectively), and the depths at which the animals were located also showed little variation (4.1 and 4.8 m). Seven adults were released in shallow zones, where they carried out the pubertal moult in summer; they were located in kelp beds, similar to those occupied by the juveniles, for an average of 53 days after release. During this first phase, adults showed a slightly higher speed (22.1 m/day) and depth (7.3 m) than juveniles, although there was no significant orientation observed in their movements. In late summer and in autumn adults began highly directional movements, oriented to the deeper zones (up to 40 m) in the outer and central channel of the Ria, with speeds averaging 76 m/day. This change in behaviour is linked to changes in meteorological conditions (changes in the direction of the prevailing winds and the beginning of a period of heavy rainfall) as well as oceanographic variations (a drop in the temperature and salinity of the shallow waters). The start of this migration may be related to the search for deeper waters which are more stable and suitable for the development of the reproductive processes.