Digging deep to study the Horseshoe Crab

Over the past months, you may have followed discussions about our efforts to count and tag horseshoe crabs on several area beaches and Avalonia’s Sandy Point Island.

On Wednesday, July 17, Avalonia volunteer taggers Binti Ackley and Beth Sullivan were joined by Professors Jennifer Mattei and Mark Beekey, and four students from Sacred Heart University to do further studies of the population on the Island. Thanks to Avalonia member and Pine Point science teacher Jon Mitchell we were transported, gear and all, back and forth, and he joined the group.

The Sacred Heart team has been studying all aspects of Horseshoe Crab biology: life spans, migration routes, mating studies, habitat quality, nesting and hatching studies as well as all stages in between. A great deal of information has been amassed over the last decade and they continue to compile and add to the data base.

On this morning, we arrived at the beach at low tide, giving us the greatest expanse of beach where we expected to find nest depressions, as well as mud flats and small pools where we might find newly hatched juveniles. Horseshoe crab nests are mostly dug during the high tide and at night. This morning there were dozens of depressions along the high tide line indicating nesting activity over the last week. We gently scraped away layers of sand until we uncovered the clusters of eggs. Newly deposited eggs are glistening grey/blue, not much bigger than grains of sand, and are somewhat clumped together.

Horseshoe crab eggs  look like small pearls mixed in the sand.

As the eggs mature, they appear to become more beige or peach colored, and the ones that were closest to hatching were nearly translucent and the minute crab could be seen, at times, moving inside the egg. Samples of eggs were taken from dozens of different nests along the shore. They will be evaluated for numerous things. Some will be checked for heavy metals, including lead. This is often found in habitats near where hunting has occurred and the lead shot becomes absorbed into the sand area and into the eggs. They will also evaluate for the caloric content of the eggs to see the quality of the “food supply” inside of the egg, usually measured in the fat content, and may indicate health or viability.

Another interesting aspect of examining the nests was the presence of an entire community of organisms that live within the nest. The organisms are dependent on the eggs, the spawning fluid or milt, the sugar based material that holds the eggs together, and the bacteria that develop within the nest. There were worms, isopods, amphipods and even beetles that spend their life cycle within horse shoe crab nests. Many nests had deep holes poked into them and footprints all around from numerous bird species, notably sandpipers and oystercatchers that probe for the nutritious eggs. 

Examining a nest for eggs. Note the bird foot prints surrounding the area. 

In the mudflat pools we were shown how to really pick out the newly hatched, juvenile crabs, that were so small they were easily mistaken for tiny bits of gravel. The very smallest were probably hatched within the last weeks.

Newly hatched horseshoe crabs.

We also found juvenile crabs that ranged up to two inches across that were probably the young of last year. These burrow to forage in the mudflats and may be found by their trails in the sand. There were also weighed, measured and categorized. 

A one year old juvenile.

Underside of a juvenile.

We also scouted for adult crabs, though they were hard to find because of the time in the moon cycle as well as the tide levels. We did find at least five and they were weighed, measured and photographed, and samples taken from each to assess health and heavy metals in their blood.

This Horseshoe crab was tagged at least three years ago.

Taking samples from an adult horseshoe crab.

Documenting a tagged visitor to Sandy Point.

The scientist team checked soil samples, plant material, other invertebrates, small fishes and snails that share the habitat with the crabs.

It was pretty amazing to explore the web of life and interdependency to be discovered along a special stretch of sandy beach.

We are excited to have a connection with the Sacred Heart team and look forward to sharing their data and learning more about a special inhabitant of our Avalonia Island.

Written and photographed by Beth Sullivan.

Find out more about Project Limulus.