A research team of scientists from Freie Universität Berlin and Princeton University has provided insights into the origins of complete metamorphosis in insects.

More than 60% of all animal species are insects. The majority of these species undergo complete metamorphosis, whereby the larva transforms into a pupa and then an adult. A classic example of this process is the caterpillar’s transformation into a butterfly.

During the pupal stage the insect’s body is totally reconstructed, a process that even affects its internal organs. The question as to the evolutionary advantages behind this radical change among such a high percentage of animal species has hitherto evaded a straightforward answer.

One hypothesis is that this process allows for more rapid growth during the larval stage as the adult structures form in the pupal stage after which the animal emerges fully grown. Rapid growth is often advantageous in situations where resources are scarce or seasons are short.

A new study titled “Rapid Growth and the Evolution of Complete Metamorphosis in Insects,” recently published in the Proceedings of the National Academy of Sciences of the United States of America, demonstrates the plausibility of this scenario through a comparison of different species of insects, using a mathematical model.

Researchers from Freie Universität Berlin, Princeton University, and Leibniz Institute of Freshwater Ecology and Inland Fisheries investigated whether insects with a pupal stage grow faster than insects that do not go through this process. The former group of holometabolic insects includes beetles, butterflies, hymenoptera, and flies, while the latter group of hemimetabolous insects includes aphids, crickets, and grasshoppers, which do not have a pupal stage. The larvae of holometabolic insects do indeed grow much faster than other insect types.

In order to demonstrate that rapid growth by means of a pupal stage is evolutionarily advantageous, a mathematical model was created in cooperation with Professor Jessica Metcalf from Princeton.

“The findings produced by combining data from different insect species with and without pupal stages and using a mathematical model strongly indicate that the complete evolution of insects came about as this is the only way of ensuring rapid growth, which is also ecologically beneficial,” says first author of the study Dr. Christin Manthey, an evolutionary biologist who now works at the Max Planck Institute for Chemical Ecology in Jena, Germany.

“There are also other hypotheses out there that try to explain why insects undergo metamorphosis, but these have yet to be investigated,” says principal investigator Professor Jens Rolff, a biologist at Freie Universität Berlin.

“In light of the important role that insects play in our ecosystems as well as in our food production, as both pollinators and herbivores, this fundamental aspect of their biology is vital to better understanding these insects.”

Many marine organisms, like sea anemones, struggle to spread across the ocean, especially if they lack long, mobile larval stages. Unlike their jellyfish relatives, sea anemones do not have a medusa stage, making their dispersal challenging. Their only mobile stage is a tiny larva called a planula.

In many species of sea anemones, the planula persists for only a short period before it settles on the seafloor and transforms into a polyp—a soft, tube-like animal with a central mouth surrounded by tentacles. This brief window reduces the ability of sea anemones to settle in new areas far from where they originated.

Some sea anemones, like the burrowing anemone Edwardsiella carnea, have developed a unique way to overcome the challenges of ocean dispersal. These anemones parasitize jelly-like marine animals called comb jellies (or ctenophores) to spread more easily through the ocean.

The polyps of this species release eggs and sperm into the water, where fertilization happens. The fertilized eggs develop into planulae, which can infect comb jellies by either burrowing into their tissues or being swallowed.

Inside the comb jelly, the planula grows into a worm-like form, which can then be released and settle on the seafloor to develop into a polyp. Comb jellies can host one or many of these parasitic anemones at the same time, and carry the parasitic planulae for long distances, across the ocean.

A study led by Prof. Tamar Guy-Haim from the Israel Oceanographic and Limnological Research (IOLR) and Ben-Gurion University, and the doctoral student Anastasiia Iakovleva, along with Dr. Arseniy R. Morov from the Guy-Haim Lab in IOLR and Prof. Dror Angel from the University of Haifa, has uncovered the first documented cases of parasitic burrowing anemone planulae in scyphozoan medusae (“true jellyfish“).

The study, published in Scientific Reports, identified Edwardsiella carnea planulae in the Mediterranean barrel jellyfish (Rhizostoma pulmo) and the invasive nomad jellyfish (Rhopilema nomadica), using both morphological and molecular-genetic analyses.

These findings indicate a “parasitic spillover”—an ecological phenomenon that occurs when a parasite, typically associated with one host species, begins to infect a new host.

This finding is particularly astonishing because parasite-host relationships are usually evolutionarily conserved, meaning parasites typically infect species they have co-evolved with over long periods, often developing very specific ways to survive and thrive within those hosts. It is rare for a parasite to switch to a different species in an evolutionarily separate group.

To explain their findings, the researchers proposed that the sea anemone’s host choice is driven by the availability of gelatinous zooplankton during seasonal jellyfish blooms rather than due to evolutionary ties.

This research highlights how parasites can adapt to new hosts in rapidly changing marine ecosystems, especially under an exacerbating climate change that is evident in the Mediterranean Sea. The implications of this host switch could be significant, especially as jellyfish blooms have become more frequent and intense in this region in recent decades.

Further research is planned to investigate the broader impact of this parasitism on jellyfish populations, particularly regarding their reproduction, growth, and survival.