Crab Population Decline Since the Late Pleistocene

Predation Scars Reveal Declines in Crab Populations Since the Pleistocene

Kristina M. Barclay, Lindsey R. Leighton

Summarized by Nicholas Stanton, a geology major at the University of South Florida. Nicholas is a Navy veteran who went back to school to follow his passion in the study of Earth’s history. Nicholas has worked for USGS for 2 years as a hydrographer, continuing his love for all things science.

Data that were used: The number of customers that visited Joe’s Crab Shack in a month was the determining…. JOKING. The data that was used in determining if there was a decline in crab populations is quite fascinating and it starts with snails. The Tegula funebralis to be specific, but to make it a little easier these can be referred to as a black turban snail. These snails are extremely helpful, telling stories of the past by their healing attributes. They leave remarkable healing scars after attacks from predators, also known as predation scars.

Methods that were used: So, we just count how many snails have predation scars and that accounts for how many crabs there are in the ocean right? Well, there are a few other factors that need to be eliminated before we can equate these two pieces of information. For example, can the failure of crab attacks account for fewer scars due to the snail shell never being penetrated to create a scar in the first place? Or, can the size of the snail and the size of the crab determine the amount of predation scars found? Well, it is a good thing science rules and we have a bunch of cool scientists that have already conducted studies to help in answering these questions. Other studies have inadvertently shed light on this by answering these questions, even though the purpose of their study was not to calculate crab populations. This other study ended up showing that a change in the success of crab predation is not an explanation for a change in repair frequency. Instead, it is likely that the change in repairs is caused by the change in crab population. In the study summarized here, fossil and modern specimens were collected to compare and determine the crab abundance dating back from the Pleistocene to present day, with the maximum geologic age for the fossils extending to 120 ka (thousand years). Most of the fossils were taken in southern California, specifically the Palo Verdes Hills and the San Diego area. The modern specimens also came from these same areas as to reduce biases in the study. A total of 712 fossil specimens were gathered, 261 from the Palos Verdes Hills area and 451 from the San Diego area. The modern specimens came from seven locations in the same two areas previously listed. Collecting these modern samples was tedious to ensure certain biases did not enter the study. Researchers would walk up and down in symmetrical lines spaced two meters apart collecting around ten to twenty snails on each line. This was done until a total of 1,152 snails total were collected.

A dark colored snail with a cone shape that gets narrower as it extends upward. There is scar, which is a jagged vertical scar that covers 2/3rds of the cone shaped shell on the side. This scar was caused by predation from a crab. Left is a side view, right is a bottom view of the snail
Figure 1: These are examples of predation scars on the black turban snail. In box A the red arrow is pointing to a typical repair scar seen in these snails. Box B shows the size of the snail being taken at the time of attack. This helps show the size at which the snail was and the success of the attack. A snail with a high conical shape and a large groove covering nearly 2/3 of its body shows signs of repair from predation scars.

Results: It was discovered that modern black turban snails have fewer scars than those fossils dating back to the Pleistocene. It was concluded from the study that changes in the frequency of repairs since the Pleistocene is indicative of a change in the number of attacks. The maximum size of repairs between the fossils collected and modern samples were similar, showing crab strength had not changed much over the years. This helped eliminate the thought that lower predation scars were due to the lower success of attacks. This tells the researchers that the crab population has declined since the Late Pleistocene, due to the decline in predation scars in snails.

Why is this study important? Paleontology is crucial for understanding the story book of Earth’s past. It is a nice guide in determining questions about Earth’s future, as well. Fisheries have poor data and little money to invest in expensive research on how to maintain their fishing numbers in the ever-declining industry. Overfishing is playing a huge part in the decline of not only crab population, but thousands of species of marine life. For example, Somalia was once a successful fishing port, but due to larger countries overfishing those waters, the economy collapsed. This is a devastating notion that an entire country’s economy can be significantly affected due to overfishing.

The big picture: This study has mapped an entire population of crab, and this can inform fisheries on how to sustain a healthy number, without depleting the entire species. Things such as climate change, long line fishing, and pollution are wiping out our marine diversity swiftly. Paleontologists, as well as all other scientists, understand the effects of climate change and pollution on Earth’s ecosystem. These scientists are on the front lines combating these realities with knowledge, with evidence, and with SNAILS! 

Citation: Barclay, K. M. & Leighton, L. R. (2022). Predation scars reveal declines in crab populations since the Pleistocene. Frontiers in Ecology and Evolution. 

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