How is Elysia chlorotica classified as an animal capable of photosynthesis?

Question

Elysia chlorotica is a kind of sea slug. Technically, it's an animal. However, it undergoes a process called kleptoplasty, which is the absorption of chloroplasts from algal prey cells which are then used to produce food. E. chlorotica uses these chloroplasts for 9-12 months at a time, and manages to keep them functioning. Also, E. chlorotica has absorbed some algal genes, however, none are related to photosynthesis, and none are expressed.

So, Elysia chlorotica is a sea slug that is capable of photosynthesis, but only after taking chloroplasts from its prey.

What I'm wondering is how do you classify something like this? I know E. chlorotica is currently listed as a mollusc in kingdom Animalia, but what if it fully absorbed its preys genome and was able to sustain chloroplasts on its own? Would E. chlorotica be considered a plant or an animal? Or something else? Is that even possible?

I know this is a lot, and maybe this forum isn't the right place for my questions, but any insight would be greatly appreciated!

Answer 1

Maybe you are familiar with the endosymbiotic theory which hypotetisays that the mitochondria (and chloroplast) was a little procrayote that got absorbed by larger organisme. At the beggining they were independent in an endosymbiontic relationship but in the process of evolution they merged to one organism from which all eukaryotes (plants). So yes, the process your describe is possible. If it happens than the new organism is no longer E. chlorotica but a another which evolved from it, this creatue will likely still be clasified as an animal since there are non-photosynthetic plants and there are photosynthetic non-plants (algae, cyanobacteria), Therefore photosynthesys is not the definition of a plant. The goal of the tree of life is to draw the "familiy tree" of all the living creatures. So, if we know one creature is an animal that gave rise to another one with a special ability we will still clasify it as an animal.

Answer 2

Phylogenetic classification—which has come to be widely accepted—classifies organisms based on ancestry. Evidence for common ancestry exists at various levels: molecular, cellular, tissue/organ, and whole organism structure. It is by synthesising information from all of these that we draw conclusions of descent, and therefore of classification. Thus there is nothing wrong with classifying E. chlorotica as a mollusc: it has this strange chloroplast phenomenon, but is a bona fide mollusc in other ways (see this article).

But what if, as your question says, E. chlorotica actually integrates genes from its prey into its own genome? (By the way, this has already been reported.) Will it still be a mollusc? Or will we need to place it closer to Vaucheria, its algal food?

This highlights a major limitation of the 'tree of life' concept: the inability to take horizontal gene transfer (HGT) into account. Some biologists have come up with other ways of thinking about evolutionary history—such as a 'web of life' or 'ring of life'—to overcome this.