Vertical distribution, seasonal and diurnal migrations and seasonal variability in abundance of Parathemisto gaudichaudi were investigated using the Longhurst Hardy Plankton Recorder (LHPR) for a 4-y period (1971 to 1974) a t Ocean Weather Station (OWS) 'India' (59" OO' N 19" OO'W) in the north-east Atlantic Ocean.The results from this programme were compared with those from the wider geographical coverage of the Continuous Plankton Recorder (CPR) survey and placed in the context of an 18-y series of data (1960 to 1977) from the north-east Atlantic Ocean.The results from the CPR survey showed that P.gaudichaudi was widespread over the northern north Atlantic, being most abundant in the Labrador/Greenland current system and in the oceanic area to the west of the UK from June to October, while at OWS ' I n d ~a ' they were most abundant between July and September.The young juveniles (less than 3 mm in length) accounted for 76 % of the population at OWS 'India'; they were found in the surface waters (0-50 m) during both day and night.There were cunsiderable changes in the vertical distributions of adults by night and day with amplitudes of diurnal migration of 200 m.These migrations would not have been evident if the population had not been divided into size categories to identify the different behaviour patterns exhibited with age.The population data suggest a 6-to 8-week generation time from egg to adult (8 mm), allowing 2 weeks from egg laying to liberation of young from the brood pouch and a further 4 to 6 weeks to grow from 3 mm to an adult with a corresponding weight increase of 120 fold from 0.025 m g (3 mm) to 3 00 mg (8-13 mm).
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Williams et al. (1981) studied this question.