No surviving species today is the same as a fossil or hypothetical form claimed to be an intermediate form, but living creatures preserve historical information that is otherwise lost to us in their unique adaptations and the adaptations they share with fossil/hypothetical forms. So whenever the press says a living creature is somehow “ancestral” or “primitive” it’s a furphy. A characteristic or two might retain features from long ago, but the lineages of all creatures alive today have been evolving for exactly the same length of time. Some lineages might evolve very quickly – if only to stay in the one place – but we’ve all had the same timespan to evolve in.
With that caveat consider the Ascidians or Tunicates, filter-feeders who, oddly, make a cellulose coat (‘tunic’) and, at some point in their lives, share certain features with our own Chordate/Vertebrate lineage. So who are the Tunicates? Sea-squirts are the ones we encounter most commonly, but there are a few other fascinating Tunicates who don’t just cling to rocks – Salps (Thaliacea), Doliolids, and Pyrosomes.
Salps can form incredibly large colonial masses and suffer massive die-offs when their population overloads – this happily sends carbon to the bottom of the sea to get buried, and thus form a major part of the carbon cycle.
Doliolids also form colonies, though quite differently to salps. They have a rather complicated alternating breeding system – propagating asexually part of the time, then sexually.
Pyrosomes really piqued my interest because they form huge tubular colonies – many metres long and sometimes wide enough for divers to swim into. They’re also bioluminescent.
Isn’t the natural world amazing?