Marine Wildlife Club monitors mole crabs at Pescadero Beach

Marine Wildlife Club members extract samples using sand cores and analyze mole crabs at Pescadero Beach with the guidance of Schneider. Used with permission from Vivian Frazita.

Vivian Frazita

Marine Wildlife Club members extract samples using sand cores and analyze mole crabs at Pescadero Beach with the guidance of Schneider. Used with permission from Vivian Frazita.

Apurva Krishnamurthy

Marine Wildlife Club members extracted writhing mole crabs, a vital species of the aquatic ecosystem, from the sandy shore of Pescadero Beach on Oct. 1. They monitored local populations to examine the overall health of the ecosystem in the Sandy Beach Monitoring program from Long-term Monitoring and Experimental Training for Students, or LiMPETS, a state-wide organization dedicated to educating students about coastal ecosystems. 

Working alongside intertidal ecologist and LiMPETS program coordinator Jaclyn Schneider, students collected data to address issues affecting the local mole crab population. Marine Wildlife Club officers decided to organize this project to transform their club into a more gratifying and hands-on experience that would deepen returning members’ understandings of aquatic ecosystems and entice new committed members.

“The main goal was to find an activity that’s fun and educational,” senior and Marine Wildlife Club President Hank Hsu said.  

At Pescadero Beach, members first found an optimal area for monitoring in the swash zone, the ecosystem which mole crabs inhabit. They then measured the distance between the strand line and the water to calculate the vertical transect line, which they gathered data along and took samples of sand across a 50-meter rope parallel to the waves. Each sample was collected using a sand core to extract a section of sand, which was then sifted through to leave only the mole crabs.

With their samples, members determined the mole crabs’ life stages by measuring their carapace with calipers, a measuring device. They also identified the sex of the adult mole crabs by pulling back the telson and examining for pleopods and eggs. The data collected is then published on the LiMPETS database for public use and scientific reports are sent to marine sanctuaries and state parks to notify them of current conditions.

“When they go out there, collect samples and get their hands dirty, they are able to feel like they are closer to the data,” Marine Wildlife Club adviser Vivian Frazita said. 

When they go out there, collect samples and get their hands dirty, they feel closer to the data.”

— Vivian Frazita, Marine Wildlife Club Adviser

Prior to beach monitoring, club members attended virtual training sessions on Sept. 20 and Sept. 27 to learn about mole crabs and how to collect data. During the first meeting, members were introduced to the purpose of the project and learned about the mole crabs’ basic anatomy, how to determine life stage and their role in the food web. The second session covered the pro

cedures to be followed while handling the crabs at the beach.

“I was impressed at how excited everyone was about marine science,” Schneider said. “It was enlightening and fun.”

According to LiMPETS, significant changes in the size of mole crab populations may reflect environmental phenomena. For example, a smaller population may indicate a larger presence of toxic plankton that kill mol

e crabs upon consumption, affect mole crab predators, such as birds and fish, heighten mortality rates throughout the ecosystem and impact human fisheries. In contrast, a higher number of a certain gender or age group of mole crab

s displays that ecological conditions are favorable to them and that their food supply is adequate.  

By establishing a baseline of the ecosystem based on their data, LiMPETS can identify future ecological impacts or human-induced issues like pollution and oil spills. For example, the Cosco Busan Oil Spill of November 2007 resulted in a drop in mole crab populations and recruit numbers in San Francisco’s Ocean Beach. Samples taken by LiMPETS in October 2008 reported that 15% of the crabs surveyed were recruits, a number that nearly doubled in a study done in September 2014. This revealed a link between recruit

numbers and excess oil, allowing LiMPETS to assess the magnitude of the oil spill’s damage.

“I hope they got a deeper understanding of the marine ecosystem in our local community,” junior and Marine Wildlife Club Secretary Harry Chiu said. “Collecting this data will help the environment and having a local impact is a big plus.”

The Marine Wildlife Club hopes to work with LiMPETS in the second semester and learn more about marine wildlife through whale watching trips and aquarium visits.