They can’t decide. For millions of years, crabs escaped from the oceans to land and then returned.

Cancers are sea creatures that are constantly evolving, looking for new things and how they can best adapt. Typically, this type of development involves moving from the ocean to another environment, and in a new study by IFLScience, scientists counted the number of times this has happened over the past 100 million years.

The new study was conducted by an international team of researchers. Here they focused on how many times true crabs, members of the taxonomic group Brachyura, have abandoned marine environments throughout their history and when they did so. In their studies, scientists examined terrestrial movements known as terrestrialization, as well as semi-terrestrial habitats such as intertidal zones and estuaries.

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To answer this question, scientists used three different data sets:

• A detailed description at the DNA level of evolutionary relationships among 344 crab species;
• the entire documented fossil record of crabs;
• Collection of natural history data including morphology and behavior.

The team combined this data in their study and found that real crabs are 45 million years older than previously thought. Additionally, scientists think that they first appeared in the Triassic period and left the oceans 7 to 17 times in the last 100 million years. All this together makes crabs unique among arthropods; most have undergone terrestrial transformation only a few times; This happened about 300 million years ago.

During the study, scientists also discovered that there are two ways of terrestrial adaptation:

• First, crabs moved from sea to land via intertidal zones, beaches, and coastal forests;
• Second, they passed through estuaries, freshwater floodwaters, and riverbanks before reaching forests and jungles.

To do this, scientists believe they probably needed to evolve certain traits that would adapt them to their new environment. The results show that different crabs evolved the same traits independently of each other, in a process also known as convergent evolution.

According to Joanna Wolfe, the lead author of the study, scientists made another surprising discovery during their study: They found that the crabs completely reverted to marine life at least 2-3 times during the same period.

Scientists think the cost of living on land may be so high that the crabs feel stranded and return to the ocean. As a result, scientists concluded that it is actually extremely difficult for crabs to leave the aquatic environment.

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