The mermaid and siren are oft-confused and argued over as to distinguishing feature or attribute is more appropriate to their respective name. What is interesting of these ideas and descriptions, despite having been scribed in the guise of fiction and fairy tale, are that they denote an accurate account of some evolutionary aspects regarding the species. The Siren was once described as a bird in ancient folklore. It only later became a woman of the water, unfortunately, the key point of its change is missed in the general disbelief in the species as a whole. There was, at some point in the past, a scientific need to have made specifit distinctions between the water-human and the bird-human animals. Wheter it was a mistake in classification, or the siren evolved into an aquatic mammal is not well understood. However, it is evident that any distinction made now would not match what the common folklore would opine as to thei characteristics.(from The Resurrectionist: The Lost Work of Dr. Spencer Black by E.B. Hudspeth via exhibitionsinternational.org)
I will begin with the homogeneous nature of them as a species differing only as dogs may differ in breed- albeit, significant differences indeed.
The mermaid (the female of the species siren oceanus), despite its popularity, was perhaps less common and certainly more elusive than the siren. It would been able to breathe underwater without having need to surface. I speculate the possibility of several variants of the species that exhibit more mammalian traits and therefore require the occasional breath as do the dolphin or whale. The task of discovering any of the animals intact by means of good fortune alone are nearly impossible.
Read also Myth and Error in the Rise of Natural History by S. E. Longwell, Siren-Mermaid by Wilfred P. Mustard, Mythical Monsters by D. S. Lamb