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Hidden crab diversity discovered in coral reefs

A new study has shed light on a surprising exception to the pattern of uniformity across the Indo-West Pacific, the largest and most biodiverse marine ecosystem on Earth. While many species in this region have comparably wide ranges, including fish and shells noted by Charles Darwin, the same pattern has been observed in crabs. 

Chlorodielline crabs, common on coral reefs, have long been difficult to distinguish based solely on appearance, as they look so similar. However, a team of researchers has now discovered that chlorodielline crab species with non-overlapping ranges are often nearly identical, but those that occupy the same region have a unique feature.

The study, which was led by Robert Lasley, former postdoctoral researcher at the Florida Museum of Natural History, revealed that the key to distinguishing these similar-looking crabs lies in their specialized appendages used for reproduction, called gonopods. These complex and species-specific structures have evolved multiple times in different arthropod groups, including crustaceans, moths and butterflies, and millipedes, and are variously used for sperm transfer and clasping.

“They all look the same, until you compare their gonopods, which are structurally complex and very species-specific,” explained Lasley.

In crabs, the gonopods sometimes come equipped with elaborate frills that give them the appearance of a spatula with a mohawk. Lasley wanted to understand if there was any pattern to the variation in these gonopods, which led him to study species across the Indo-West Pacific. Over the course of more than a decade, he collected specimens during numerous marine field forays in the Red Sea, Singapore, Australia, and the Phoenix Islands, searching for crabs hiding among the coral bric-a-brac.

The study’s findings indicate that the key to distinguishing closely related species of chlorodielline crabs is in their gonopods, rather than their external appearance. While the gonopods of species with non-overlapping ranges are often nearly identical, those of species occupying the same region have unique structures, suggesting that their evolution is influenced by interactions with other closely related species.

Chlorodielline crabs are one of the most abundant crustaceans in coral reef ecosystems, where they occupy the base of the food chain. These tiny crabs, most of which are no larger than a kernel of corn, are especially diverse in the coral triangle, a region that stretches from Indonesia to the Solomon Islands and supports 76 percent of the world’s coral species and more than a quarter of all coral reef fishes. Despite their importance, chlorodielline crabs have long been difficult to distinguish based on appearance alone.

The researchers sought to understand why chlorodielline crabs have such wildly different gonopods, specialized appendages used for reproduction. To do this, Lasley first had to determine how chlorodielline species are related to each other, which he accomplished through an analysis of DNA extracted from museum specimens. The experts then added information regarding the range of each species and the shape of their gonopods.

The study’s findings shed light on one of marine biology’s most perplexing mysteries: how do marine invertebrates, such as crabs, remain isolated long enough for evolution to generate diversity? While roaming animals on land become isolated all the time, the larval stage of many marine invertebrates makes this step in the speciation process more difficult to achieve. Naturalists like Charles Darwin saw the Indo West Pacific as one vast body of water, uninterrupted by geographic barriers that would otherwise act as a catalyst in the process of speciation.

However, the results of this study suggest that sheer distance and time can also act as barriers. Many chlorodielline crabs have ranges that extend across the entirety of the Indo-West Pacific, and the genetic analysis revealed that these cryptic species have slowly accumulated differences in their DNA over millions of years. But it wasn’t until close relatives were reunited after an extended separation that those genetic differences visibly manifested in a single, peculiar way. In almost every case, close relatives with overlapping ranges had uniquely shaped gonopods but otherwise looked exactly the same.

“What we can say is these crabs start genetically diverging in different geographic areas, and then the divergence of gonopods is an important piece of the speciation process that happens at the tail end of things,” Lasley said.

The findings of this study have important implications for our understanding of marine biodiversity, as they suggest that far more variation exists in the Indo-West Pacific than previously suspected. The engine driving this diversity has yet to be entirely discovered, but Lasley intends to test his hypothesis that the way these crabs reproduce may play a role in the evolution of their unique gonopods in future studies.

The research is published in the journal Molecular Phylogenetics and Evolution.

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