This is the second question in a series about my ongoing worldbuilding project, which seeks to explain the evolution and biology of various fantasy creatures. The first one was: Is petrifying vision plausible?
Now, I ask you to review a concept I developed myself and inform me of its plausibility.
The hydra is a well-known beast featured in Greek and Roman mythology - a serpentine, many-headed beast that lives in swamps. In my project, hydras are portrayed as amphibians that mate like Ceratiid anglerfish - males fuse themselves to the larger female's body and mate with her throughout her lifetime. Eventually, the circulatory systems fuse, and the hydra becomes a chimera.
The reason that Ceratiids to do this (It is thought) is because encounters between individuals are very rare, and mating opportunities must be exploited to the max. Similarly, my hydras are also naturally rare.
To give you an idea of the exact life cycle of a hydra, here's a rough summary:
All the hydras in an area will be born at the same time due to the short period of the hydra's life where it is mature. This is for predator satiation - the predators of young hydras (Namely giants) are overwhelmed by a relative feast of prey, thus letting the majority go.
The male is born and immediately moves to find a female, which may take weeks or months. During this time, he grows from the size of a salamander to that of a man.
When he finds the female, he attaches himself to her body and begins to mate.
Every time fertilization occurs, the female lays an egg. However, hydras are constantly moving, so eggs are often far between.
Eventually, the female will have between 6 and 15 males attached to her. Hydras live only to mate, and she - and all her mates - will soon die.
How plausible is this premise? Is it possible that such a phenomenon would evolve in a creature of said anatomy, and are there any problems that it would cause if it did come about?
If you require any further details on these creatures, think it is unclear or too broad, or have spotted an error, please say so in the comments and I shall amend the question promptly. Also, if you do give it a VTC, please say why, as closing a question without giving criticism is wholly unhelpful.