New Year, New Me: Amazing Metamorphoses in Fish

Like clockwork, the phrase “New Year, New Me” arrives with every flip of the calendar. The phrase itself invokes ideas of dramatic personal change and transformation (and maybe even a new gym membership). But some fish species take “New Year, New Me” to a whole other level.

The word metamorphosis comes from Greek and means “to change form” – you might think of a caterpillar’s dramatic change to a butterfly or a tadpole’s transformation to a frog. Scientifically speaking though, “metamorphosis” can be a bit tricky to define, with some definitions only considering the most dramatic external transitions (i.e., significant and rapid transitions in morphology between life stages), while other definitions include more subtle biochemical and cellular changes (e.g., Youson 2004).

No, not talking about that kind of metamorphosis.

What’s the selective advantage led to the evolution of metamorphosis as a life history strategy? The process likely provides multiple selective advantages, and a major one might be that metamorphosis allows individuals of the same species to occupy different habitats and niches across life stages and minimize intraspecific competition, by either exploiting different environments (e.g., pelagic versus benthic) or consuming different resources (e.g., filter-feeding versus parasitism).

While “true” metamorphosis is common in invertebrates, it is less so in vertebrates. Metamorphosis in vertebrates is often under strong control by hormones, particularly the thyroid hormone (Laudet 2011). While relatively few of the approximately 28,000 described fish species undergo metamorphosis (Youson 2004), those that do include lampreys and two groups of bony fishes (flatfishes of the order Pleuronectiformes and many species of the actinopterygian subdivision Elopomorpha, such as true eels) (Manzon et al. 2014).

Without further ado, here are some North American fish species who definitely take “New Year, New Me” to a new level: the Sea Lamprey (Petromyzon marinus), Summer Flounder (Paralichthys dentatus), and American Eel (Anguilla rostrata)!

Sea Lamprey (Petromyzon marinus)

Sea Lamprey don’t start off life as the fearsome bloodsuckers they’re often portrayed to be, but instead as wormlike, eyeless larvae called ammocoetes. Shortly after hatching in streams, ammocoetes drift downstream and burrow into the substrate where they can live for up to 10 years while filter-feeding on organic matter.

Lamprey ammocoetes begin metamorphosis when (1) water temperatures increase in the spring, and (2) when they’ve accumulate sufficient lipid reserves because they don’t eat while they’re metamorphosing (a process that can take a few months) (Dawson et al. 2015). During metamorphosis, they develop eyes, an oral disk mouth covered in pointy, hooked teeth, and a tongue covered with bony plates. They then migrate out of their natal stream and into the lake or ocean where they attach to and feed on hosts using their oral disk and teeth. This parasitic stage only lasts for 12-18 months, compared to the much-longer filter-feeding larval stage.

The Sea Lamprey’s lifecycle involves spending most of its life as an eyeless, wormlike larvae known as an ammocoete, with much less time spent as a fearsome-looking parasite. Figure from Boroweic et al. 2021, Journal of Great Lakes Research https://doi.org/10.1016/j.jglr.2021.10.015

Summer Flounder (Paralichthys dentatus)

When it comes to amazing fish metamorphoses, flatfish species such as Summer Flounder (Paralichthys dentatus) undergo some of the most well-known transformations. While they start life as larvae that look “normal” (i.e., a fish with an eye on each side of the head) they end up as adults that look like they got run over by a steamroller (and have both eyes on the same side of the head). Summer Flounder larvae are planktonic and develop offshore before using currents to enter estuaries where they undergo metamorphosis to become bottom-dwellers (“benthic”).

Timing of flatfish metamorphosis depends on a variety of factors including moon phase, tide stage, salinity, water temperature, substrate type, and presence of predators. Different species of flatfish can be characterized by which side of the body both eyes are located; Paralichthyids like Summer Flounder are considered “sinistral” flatfish, with the eyes and camouflaged exterior on the left side of the body (i.e., they lie on their right side).

American Eel (Anguilla rostrata)

You can think of American Eels as basically reverse salmon – instead of spawning in freshwater and moving to the ocean as an adult, they spawn in the ocean (the Sargasso Sea) and spend adult life in freshwater (a lifestyle called catadromy). American Eels undergo multiple metamorphoses throughout their lives involving morphological and physiological changes that help them with their long-distance migrations across very different habitats. As marine-living, leaf-shaped larvae called leptocephali, they undergo transformations that allow them to live in freshwater habitats, becoming glass eels and eventually elvers. When it comes time to spawn, however, they undergo different transformations that allow them to return to marine habitats to spawn (i.e., the yellow and silver eel stages).

The life cycle of an American Eel involves multiple metamorphoses and migrations. Figure credit: Illustration by Eric S. Taylor, WHOI Graphic Services, with stage sketches by Salvor Gissurardottir (https://www.whoi.edu/oceanus/feature/a-slithery-ocean-mystery/)

While your New Year’s Resolutions may not involve as dramatic a change as a lamprey, flatfish, or eel, these fish show us that there’s always room to grow!

References

Dawson, H. A., Quintella, B. R., Almeida, P. R., Treble, A. J., & Jolley, J. C. (2015). The ecology of larval and metamorphosing lampreys. In Lampreys: biology, conservation and control (pp. 75-137). Springer, Dordrecht.

Laudet, V. (2011). The origins and evolution of vertebrate metamorphosis. Current Biology, 21(18), R726-R737.

Manzon, R., Youson, J., & Holmes, J. (2015). Lamprey Metamorphosis. In: Docker, M. (eds) Lampreys: Biology, Conservation and Control. Fish & Fisheries Series, vol 37. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-9306-3_4

Youson, J. H. (2004). The impact of environmental and hormonal cues on the evolution of fish metamorphosis. Environment, development, and evolution. In Hall, R.D. Pearson, G.B. Müller (Eds.), Environment, Development, and Evolution: Toward a Synthesis. Vol. 1, (pp. 239-278). Cambridge: The MIT Press., Massachusetts Institute of Technology, Cambridge, MA (2004)

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