Short Communications
Toroidal vortices are a common result of motion through both liquid and gaseous environments. Their formation and movement are recognized as important in understanding the mechanics of flight (Weiss-Fogh 1977) and swimming (McCutcheon 1977). One species of coral-reef fish, the regal angelfish, Pygoptites diacanthus, makes use of this result of normal swimming to assist spawning success. During spawning, a cohesive, toroidal vortex of gametes is produced which moves upward in the water column, away from the potential predators which crowd the reef. This spinning ring of gametes is not known from other members of the family Pomacanthidae or among other fishes.
Spawning by P. diacanthus was observed between 31 July and 15 September at Enewetak Atoll, Marshall Islands, at several locations including patch reefs, lagoon pinnacles and lagoon-ocean passes. Each male controlled a harem of 2-4 females, spawning with each at dusk, from 15 min pre-sunset to at least 25 min after sunset. Courtship was minimal, the male visiting each female in turn, travelling well-defined paths. Females signaled readiness to spawn by rising up into the water column while spreading their pelvic fins. A male would then approach from behind, place his mouth on the female's side just above her gonadal pore and move his mouth in a nuzzling manner. The pair then spiralled slowly up ı89to 1 m above the coral before releasing the gametes and darting back to shelter.
The courtship is thus very similar to that of other angelfishes (e.g. Thresher 1982). However, as pairs ascended, the male left his position beside the female, dropping down and back until he was below and perpendicular to her body plane, though still in contact with her ventral body surface (Fig. 1). At the peak of the ascent and just as the gametes were shed, the male flexed his tail sending him downwards to the reef, simultaneously catching the gametes in the resultant vortex and sending them rolling toward the surface waters.
After release, the vortices rose with an overall speed of 0.27 m s -1, rising fastest when first formed and slowing as the rotational speed decreased with the friction of passage through the water. Initial ring radius (ro) was estimated at 0-05 m and expanded to approximately 0-5 m at 5 6 m above and 30-40 s after release. Vortices were still cohesive and visible for as long as 60 s after formation, elongating and expanding as the system slowed and eggs began spinning off the edges of the ring. If passage was impeded or interrupted, the cohesion was destroyed, upward motion ceased and a diffuse gamete cloud similar to that of other pelagic spawning fishes resulted.
Such a vortex, because of its motion and cohesiveness, has an effective or 'apparent' viscosity of ...
Figure 1. Pre-spawning positions of Pygoplites diacanthus showing the change in position of the male relative to the female to facilitate vortex ring formation. (A) After the male approaches the female and as they begin the ascent, he is beside and below her. (B) Just prior to gamete release, the male has moved under the female and perpendicular to her. (C) Just after gamete release, the male's tail has moved upwards to catch the gametes in the resulting vortex as he moves downward and back to the reef.