Recruitment and population dynamics of Axiothella serrata (Polychaeta: Maldanidae) on an intertidal sand flat

Abstract

The population dynamics of the deposit-feeder Axiothella serrata Kudenov & Read was investigated in Pauatahanui Inlet using seasonal transect samples processed through 500-μm mesh, and more frequent, fine-sieved samples, during the juvenile recruitment period. New recruits began to appear in October–November, at first as buried aggregations of 4- to 7-setiger juveniles. The aggregations, up to 15 m⁻² and composed of up to at least 900 juveniles, are believed to represent groups of lecithotrophic siblings developing directly from egg masses spawned sub-surface. Once surface-dwelling at about the 6-setiger stage, juveniles dispersed rapidly over the intertidal zone. Frequency distributions of juvenile setiger development indicated that synchronised spawning of adults occurred at intervals during an approximately 6-month period from October to April in 1978–79. Juvenile density peaked at about 29 000 m⁻² in December 1978 (250 μm sieve estimate), and mortality after recruitment ceased was about 96% in 6 weeks. Three age classes, 0, I, and II+, were separable by width size-frequencies in November and February quarterly samples; I and II+ had merged by May. By November, densities of I plus II+ classes ranged from 270 to 550 m⁻² 1977–1979. Density of new recruits varied from year to year, and this was possibly linked to variation in the size of the spawning II+ class. Width modes indicated 110 μm recruits grew into 1600 μm adults in 2 years. Zonation patterns were variable and age specific, differing even between young and old juveniles. Wave erosion may be an important cause of juvenile dispersal.

Keywords:

• Polychaeta
• Maldanidae
• Axiothella serrata
• population structure
• population density
• recruitment
• mtertidal zone
• spawning
• Pauatahanui Inlet