Horseshoe Crabs on Nantucket – Ancient and Fascinating

Spring on Nantucket means the arrival of peepers, daffodils blooming and American oystercatchers on the beach.  It also means the arrival of one of our more interesting marine species – the horseshoe crab, which begins showing up on protected and sheltered beaches to mate and spawn in May and June.

These prehistoric looking animals aren’t actually crabs – they are most closely related to trilobites which were around 400 million years ago.  Fossil records show that the horseshoe crabs we see on our beaches today do not look very different from the horseshoe crabs that swam around when dinosaurs walked the earth!

These unique and fascinating creatures may be in trouble – anecdotally, populations are much lower than they were 30 years although it’s only recently that serious efforts have been made to document how large horseshoe crab populations actually are.  People that grew up on Nantucket can remember a time when the beaches were covered with horseshoe crabs in the spring! Horseshoe crabs are in high demand as bait for conch and eel fisheries and for use in the medical industry.  Some states currently have a moratorium on horseshoe crab harvesting but the populations may still be in trouble. Check this great website for more information on threats to horseshoe crab populations.

So, to try and get a handle on horseshoe crab populations on Nantucket, in 2009, a group of island researchers from the Nantucket Conservation Foundation, Maria Mitchell Association and the UMASS Boston Nantucket Field Station began monitoring horseshoe crabs.  The monitoring work we do now will help establish a baseline of information so in the future we can see if populations are dramatically increasing of decreasing.  In May and June we start visiting beaches on and around the days of the full and new moons at high tide and count the numbers of horseshoe crabs we see near the shore.

We use PVC poles strung with rope to help us define a survey area and count all males, females, and mating pairs within the area.  Because of the timing of high tides, we are sometimes out surveying beaches in the middle of the night – if you see flashlights out on the beach at Warren’s Landing at 2 am over the next two months – it’s most likely us searching for crabs! Luckily we aren’t alone in these late night endeavors – this effort is being performed all along the East Coast by various conservation, university and government groups.

Male and female horseshoe crabs are fairly easy to distinguish.  Females tend to be quite a bit larger than males, going through 2-3 more molting cycles before reaching maturity.  Additionally, the front set of walking claws on male horseshoe crabs are modified into pinchers that look a lot like boxing gloves which allow the males to hold on to the back of the female crabs shell during mating.

 

We monitor horseshoe crab populations in May and June at high tides around the new and full moon due to the unique reproductive biology of these animals. Horseshoe crabs become mature adults in deep ocean waters with males reaching sexual maturity around 9 years old and females around 11 years old. Once mating season starts, typically in May and June, the horseshoe crabs move to shallow areas, particularly around sandy protected beaches near intertidal sand flats.  Mating takes place in shallow water where many males seek out and court a few females.  Females spawn and lay eggs in nests in the sand from the low water up to the spring high-tide and each nest can contain up to 4,000 eggs which each female producing ~80,000 eggs in one season!  The highest amount of spawning occurs with the evening high tides during the full and new moon cycles – this is when we target our beach surveys to help document the largest amount of crabs on the beach.

In addition to counting the number of crabs – male and female that we encounter, we are also tagging each crab with an individual tag to help us identify them.  White circular tags are placed on the back of the crab’s main shell and each tag has an individual identification number.  This can help us figure out how crabs move around the island.  Most of our crabs are being tagged in Madaket Harbor and we tend to see them there again but a few of those crabs have made their way into Nantucket Harbor as well.  Now this is something you can help us out with!  When you see a crab – alive or dead, with a tag on it – write down the tag number and remember where you found it.  Reporting tag numbers is very very easy – just visit this website and fill out the online form – you will get information back on where the crab was tagged!