Mole Crabs and Beaches: Could We Affect Them?

Authors: Halie, Jolene & Matt

Sand Crab Measuring

Everybody loves going to a nice sandy beach. We set out to learn more about the health of beaches by analyzing the abundance of mole crabs at beaches near cities and beaches in more remote areas.

Although they are no bigger than the size of your thumb and seemingly unimportant creatures, mole crabs are very significant animals on sandy beaches. Their numbers are also indicative of the general health of the entire beach because they are in the middle of the food chain. This means that they are affected by both a scarcity in their prey or predators.

So, how do mole crab populations vary from urban beaches to remote beaches?

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Out of the Blue

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A group of students from The Branson School have been monitoring key invertebrates and species of algae at Duxbury Reef in Bolinas, California for roughly 10 years. This January, we found some unexpected things. We saw an octopus, a rare sighting!

The octopus in question was an East Pacific Red Octopus, or Octopus rubescens. Like many octopuses, the Red Octopus has an amazing ability to camouflage, and is able to alter both the color and texture of its skin. We were able to experience its incredible camouflage ability first hand; the octopus first mimicked coralline algae, turning itself a reddish pink, and adopted a bumpy texture, executing a near-flawless impression of the algae. Later, after moving from the side of the pool to the floor, it turned a dark brown, in order to match the rocky substrate at the bottom of the pool.

Another interesting thing about the monitoring was the fact that no one in our group saw a single sea star. Sea stars are a common staple of intertidal life at Duxbury, so this blatant lack of sea stars was not only unusual, but also very concerning.

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Experiencing the Wonders of Marine Biology

We collected data about sand crabs from Ocean Beach. From the data, we came up with a question about the sand crabs, and supported it with the data we collected. We compared our findings to that of other scientific resources, like the “Pacific Mole Crab” article. We were able to see if sand crabs changed from the time the articles were made to now. When we looked at the findings, we could come up with possible causes. We gathered the information and began to put it into a blog. This process helped us become more confident in science and in writing. It’s amazing that our research is contributing to the scientific world.

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Hey Folks! What’s Up With the Sand Crabs?

Have you ever seen a sand crab at the beach? If not, imagine little creatures with grey shells that wiggle backwards and live underneath the sand. You may ask yourself, why should we care about some tiny sand crabs?  Well the answer is simple, they are actually very important to a beach’s ecosystem, as they are the basis of the food web. By monitoring sand crabs, we can indicate the health of the entire beach.

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Feeling Crabby?

The question that we looked at was if the size of males, females, and females with eggs in a sand crab population on Ocean Beach change over a ten year period. Well, based off of recent monitoring at Ocean Beach on September 2013 in comparison to research at Ocean Beach on September 2003, size of males, females, and females with eggs in a sand crab population on Ocean Beach does change over a ten year period.
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Sex in the Beach

Our group research question was, “What is the abundance of Recruits, compared to adult Males and Females, over Time (from 2003-2013) at Ocean Beach from the data collected by a specific school?” Basically, we asked if the number of juvenile sand crabs (aka recruits) has had any significant changes compared to the number of mature adults that are on the beach.

Your initial reaction to this question might be to ask, “Why should we care?”, because until now there’s a large chance you didn’t even know sand crabs existed. To answer that question our group had to take a step back, and understand the crabs role in the marine community. We have since learned that they are important food sources for the shore birds, and local fish so their populations reflect how those predators and even the crabs’ prey are doing. So, by tracking sand crab population,s scientists are able to keep tabs on how healthy places like Ocean Beach are, and to see if that marine community needs more attention.

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Life is a Beach, We’re Just Playing With the Sand Crabs

There are many reasons why it is important for beaches to be clean. Beaches that are covered in litter are not ideal places for life to thrive. Our group found this from the two sites that we visited. The first was the San Leandro Marina. This beach was covered in litter, and clearly was not well-maintained. There was not even a beach, only broken concrete filling the 10-foot gap from the walking path to the water. The only life we found here were seagulls and algae. Seagulls don’t even live on the beach, so they aren’t really affected by this, and algae will grow regardless of the lack of effort in supporting the site. However, compare this to Ocean Beach. This site is well taken care of, and the multitudes of species that live there illustrate this fact. These include the Western Snowy Plover, and the topic of this blog, the Sand Crab.

When our group went out and had an awesome time researching the sand crabs at Ocean Beach, we started to wonder if there were more females in the population of more male.  Our question is concerning the sex ratio between males and females.  Sex ratio is defined as “The proportion of males to females in a given population, usually expressed as the number of males per 100 females.” Are females dominating males in the sand crab population?

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‘Do the Crab Walk’

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On September 14, 2013, students scavenged a 50 meter portion of Ocean Beach for sand crabs. The wind was strong and water was cold but we were persistent to dig deep into the sand under the heavy rain to find sand crabs.  We walked along the beach experiencing the habitat that most organisms can not live in but the  habitat sand crabs call “home sweet home”. From the bigger pregnant females, to the smaller males, we realized the importance of sand crabs.

We learned that sand crab monitoring is important because the data collected contributes to a  better understanding of the health of the beach. Variation in sand crab abundance can also help indicate this. [ We believe that coastal squeeze is going to be a bigger issue in the future and affect the health of the beaches. ] Sand crabs are a common food source for many animals on the beaches in California.  Surfperch, surf scoters, and shore birds along with many other organisms depend on sand crabs to stay alive. Sand crabs are crucial to the ecosystem if not the most important and abundant species on the beaches in California.
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