What is the equivalent scientific term for samsedaja? What kind of beings on earth are born from samsedaja? Are there any beings on earth that are samsedaja (born or arisen from moisture)? Are bacteria samsedaja?
Mahasi Sayadaw says:


  Conception in moisture-laden media such as moss etc. (sansedaja), represents the coming into existence of some larva etc.
  
  http://static.sirimangalo.org/mahasi/Dhammacakkappavattana%20Sutta.htm


In this view, the larva seems to be seen as an embryonic stage.

Nina Van Gorkom says:


  Queen Padumavati was conceived in a lotus blossom and Queen Veluvati in the hollow of bamboo plant. Cincamana who slandered the Buddha took rebirth in a tamarind tree.
  
  http://www.wisdomlib.org/buddhism/book/abhidhamma-in-daily-life_2/d/doc3179.html


These allusions are to cases where a human being was born miraculously outside of the womb; somewhat apocryphal.

She also says most insects are samsedaja, but that doesn't seem technically correct.

As for bacteria, I'm only going to state as an opinion that given that stem cells are not sentient, I'm not convinced that bacteria are either. There seems to have to be a higher level of organization for sentience to be possible. At any rate, there is no reason to assume that just because something is alive and reproducing that it is also sentient; unless the mind takes hold of the organism, there will be no sentience even in the case of a fertilized egg.