Two pairs of Thalassiosira diatoms 'caught in the act' of asexual reproduction. The pair on the left has just finished binary cell division, their usual, asexual form of reproduction. The thin gap between these two daughter cells indicates that division is complete and these are two independent individuals. The pair on the right is still dividing—still building, WHOI biologist Sam Laney says, the new cell walls and "exterior siding" that will separate them.