Also, as terrestrial vertebrates, we think it normal for individuals to be separate sexes. But there is flexibility in the timing of the change in sex. With Crepidula, for example, if you are a male sitting on a female's shell, you will stay a male for a much longer period of time than if you are with another male. In fact, if you take two males and put them together, the oldest, largest one becomes a female within a few weeks and the other remains a male for months. Other animals are simultaneous hermaphrodites.
Barnacles are male and female at the same time.
And there are oyster species that change sex every few years. The more you know about the animals the more fascinating they become. In fact, much of what we currently understand about the control of gene expression, mitosis, aging, pattern formation during embryonic development, the biology of fertilization, nerve function, and vision, and even the genetic basis for a predisposition to some major diseases have come from studies of invertebrates. A large percentage of the air that we breathe is courtesy of phytoplankton. Not only do they increase the amount of oxygen, but they also remove tremendous amounts of carbon dioxide from the atmosphere, something like 2 gigatons of carbon per year.