Many of us are familiar with the story of The Frog Prince, where a princess kisses a frog, and to her surprise, it transforms into a human prince.

In reality, many frog species produce poison in their skin, the effects of which can range from mild nausea to death, so this display of affection is generally ill-advised. But what if the frog had found a more unusual way to defend itself, one that left our heroine’s lips sealed by the kiss instead?

Biological adhesives: A tale as old as slime

While humans mainly use synthetic materials to make things sticky, our princely frog—like other glue-producing organisms—produces what is known as a “biological adhesive.” These naturally secreted materials are widespread among animals, and are often essential for their survival.

For example, mussels and barnacles produce a type of glue that permanently cements them to the underwater surfaces they call home, while other ocean-dwelling animals, such as starfish, use a different, much more temporary kind of glue to help them move around.

Back on land, the most famous example is spider glue, commonly used to create silk for prey capture. However, our focus here is on terrestrial vertebrates, the stickiest specimens being the feet of geckos and certain tree frogs. These are examples of “dry” and “wet” adhesion, meaning that geckos stick to surfaces without actually producing anything that resembles glue, while the toe pads of tree frogs are covered with a thin layer of slime, or mucus.

Despite their obvious dissimilarities, both of these types of adhesion have been described as “self-cleaning,” and may help us to develop synthetic materials that share this trait. Indeed, the ways that different biological adhesives are formulated and then secreted often involve wondrous feats of natural engineering.

In other cases, however, glue secretion has less to do with complex geometrical operations, and more to do with discharging copious amounts of slime all at once.

Sticky secretions: An unlikely defense mechanism

From traditional medicine and shamanistic rituals to folk tales and myths, frogs and toads are culturally significant across the world. The frog’s poison glands are especially prominent, as they can be used to make weapons, treatments, or even hallucinogens.

To date, studies on amphibian skin-secreted defenses have focused on molecules that function as toxins. However, aside from being purveyors of poison, a small number of species (including the world’s largest amphibian, the Chinese giant salamander) have come up with a more obscure (and much stickier) survival strategy: glue.

Meet the tomato frog.

When stressed, the animal’s skin releases a thick fluid that becomes extremely sticky within seconds. From a frog’s perspective, this stress usually takes the form of an attack by a predator (or princess). The speed with which the viscous secretion—a sticky slime, basically—turns into a glue makes it nearly impossible for a predator to ingest the frog, likely due to the annoyance caused by having its mouth and face coated with glue.

While this tactic may sound crude and inelegant, it is an effective defense mechanism, as it gives the frog time to escape.

Retracing an evolutionary slime trail

Although glue is a rare feature in frogs, it has evolved several times in species that are spread across different continents. My recently published research explores the origins of this remarkable survival strategy, and why it is present in some frogs but not in others.

To answer these questions, we first needed to identify the ingredients responsible for creating the stickiness of frog glue. We did this using technologies ranging from low-tech Lego bricks to high-tech microscopes that can magnify on a nanoscale (a billionth of a meter).

Surprisingly, what we found is that the base ingredients needed to make this glue exist in almost all animals, including humans, but only amphibians have evolved the necessary toolkit for turning them into glue. Even within amphibians, only a select few species—ones that live as far apart as Madagascar, Brazil and Australia—have actually gone on to develop this ability.

In fact, we found that the Mozambique rain frog, which is separated from the tomato frog by about 100 million years of evolution, uses the same base ingredients and toolkit to create its own adhesive secretion. Perhaps the most infamous use of this glue actually involves sticking male frogs to females, as Sir David Attenborough himself has attested.

From biology to biomimicry: A seriously sticky surgical solution

While frog glue is fascinating, it is also extremely fast-acting and flexible, meaning it has huge potential for practical applications. This is where biomimetics comes in: a field that strives to mimic biological processes that nature has evolved over millions of years.

Thanks to our research, we now know, for the first time, exactly how glue is produced by a four-legged animal. Imagine how medical adhesives inspired by frog glue can be used as surgical sealants: not only is it strong and nontoxic, but it’s capable of adapting and sticking to practically any surface.

So the next time you stumble across an unsuspecting frog, be gentle—remember, it might just hold the key to one day healing your heart.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

With 84 species currently described, Characidium may be the most diverse fish genus known to science. This is the conclusion reached by Brazilian scientists after analyzing more than 4,400 specimens of this genus of South American darters endemic to the Neotropical region and known in Brazil as mocinha or charutinho.

In an article published in the journal Systematics and Biodiversity, researchers affiliated with São Paulo State University (UNESP) and the Federal University of Bahia (UFBA) confirm the occurrence of 15 species in rivers located in four ecoregions of the Northeast (Caatinga Nordeste e Drenagens Costeiras, Mata Atlântica Nordeste, Parnaíba, and São Francisco).

Ten of the species had already been described, and four required additional molecular analysis for confirmation as novel species. According to the authors, the remaining species was certainly new to the scientific literature and had yet to be named.

“Some species of Characidium are almost identical in external morphology [body shape and characteristics]. Others not only look very similar but are also very close genetically and can’t be distinguished from each other by the molecular marker most used for this type of comparison,” said Leonardo Oliveira-Silva, first author of the article and a researcher at the Botucatu Institute of Biosciences (IBB-UNESP).

The study began while Oliveira-Silva was a Ph.D. candidate. With his thesis advisor Angela Zanata, a professor at UFBA’s Biology Institute and last author of the article, he explored the main rivers of the Northeast in order to find out how many species of Characidium occurred there.

In addition to the collected material, they analyzed specimens deposited in scientific collections by Zanata and other researchers. The set in question comprised more than 4,400 individuals, the largest ever for this group, and the analysis encompassed morphological data as well as molecular data obtained from tissue samples.

“The collections contain many individuals with only the genus on the label because even specialists in the group find it hard to distinguish between some of the species. In addition, some of the existing descriptions fail to account for the variability within a given species. Hence the importance of defining each one very clearly,” said Zanata.

Recent diversity

Among the findings was a genetic distance smaller than 2% between two species with significant morphological differences: Characidium bimaculatum and Characidium deludens.

“This is a problem because the molecular marker most widely used to detect freshwater species, the gene COI, determines that 2% is the limit to distinguish one species from another, but several recent studies have shown that this isn’t necessarily an absolute truth and that it’s very important to consider the evolutionary history of a species before making decisions about name changes,” Oliveira-Silva said.

The two species in question are therefore closely related, but other genes must be included in the analysis in order to establish whether they are different species and compare their evolutionary histories.

More such discrepancies are expected, partly because Characidium was defined as monophyletic (descended from a common evolutionary ancestor) in a 1993 study based on only 18 species. At that time no molecular tools were available for the analysis of evolutionary history (phylogeny) and the rich diversity of the genus was not yet understood.

Monophyly in this case is the hypothesis that all species of Characidium descend from a common ancestor, belong to the same lineage, and share their evolutionary history. In the study, the researchers noted that a subgroup, which included Characidium chancoense, was closer to the genera Leptocharacidium and Microcharacidium, making Characidium non-monophyletic.

Another finding, obtained by means of a phylogenetic analysis, was that several species of Characidium in the Northeast were very closely related to species in other South American ecoregions, such as Alto Paraná, Fluminense, Paraíba do Sul and Tocantins-Araguaia.

All the rivers and catchment basins concerned are now disconnected, so the most plausible hypothesis is that common ancestors of these species migrated from one basin to another at a time when they and the rivers in them were connected.

The researchers are now expanding their survey well beyond the limits of the Northeast in order to see if the evidence they have assembled so far is confirmed. The aim of the project led by Oliveira-Silva at UNESP is to analyze species belonging to the genus Characidium in all South American rivers in order to establish the most complete phylogeny of the genus to date. In addition, the researchers are looking for species that have not yet been described.

He and Zanata were recently in Peru and Colombia, where they were able to analyze specimens deposited in collections and, through relationships with collaborators there, to obtain tissue samples for molecular analysis.

“Thanks to these collaborations, we’re obtaining tissue from species living in every part of South America and will be able to answer many significant questions about these animals,” Zanata said.