Changes in vertical distribution and aggregative behaviour in response to population density for larval sea urchins ( Strongylocentrotus droebachiensis ) and sea stars ( Asterias rubens )

We examined the effects of the presence of larval conspecifics on larval vertical distribution of four‐arm and six‐arm plutei of Strongylocentrotus droebachiensis and bipinnaria of Asterias rubens , in laboratory experiments, by manipulating population density. Larvae were introduced to experimental columns (10 × 10 × 30 cm) at one of two or three population densities ( S. droebachiensis : 0.1 and 10 larvae·ml −1 ; A. rubens : 0.1, 1 and 10 larvae·ml −1 ). Subsequent changes in vertical distribution were determined from images of the larvae in the columns illuminated by a 532‐nm laser and captured by a high‐resolution camera. Larvae of both species were found higher in the water column in the high than in the low and intermediate population densities. The relationship between vertical swimming velocity and nearest neighbour distance (NND) was measured for four‐arm plutei of S. droebachiensis , and used to determine a range in distances among individuals that may affect potential interactions. The variation in swimming velocity decreased with increasing proximity below a threshold distance among individuals of 10 mm, which was greater than the estimated distance in the high population density. We suggest that the increased larval aggregation near the water surface under high population density is the result of a behavioural response to conspecifics.