Hatchery setting and breeding pairs. (a) Flat oyster sampling locations in the Limfjorden connected to the North Sea in west and to the Kattegat in east. Coloured symbols indicate locations for collections of broodstock at Løgstør (green), Nissum (blue), and for the natural settling event (‘Natural settlement’, purple). (b and c) Experimental set-up in four parental broodstock tanks (‘Small 1-3’ in panel (b) and ‘Large’ with two separates swarming events analysed in panel (c)), and offspring from the five analysed swarming events. For individual tanks, hexagons show broodstock sizes and origins (green = Løgstør, blue = Nissum). For individual swarming events, each coloured segment of the chord diagram represents a paternal full-sib family, and the thickness of the chord represents the number of offspring identified, as shown by peripheral tick-marks. Individual swarming collections all had one single dam, the label of which is in bold. Two instances of sex reversion (one dam to sire and one sire to dam) in parental breeders are indicated by the pink label symbol “R”. Sires contributing to two consecutive swarming events are indicated by the blue label “C”. Tags inside chord diagrams give the total number of observed breeders (dam and sires) as well as the number of effective breeders N_b.