This is the time of year that I love the most. It is the time of year when the estuary starts to come to life and dinosaur-looking animals come onto shore to do their “thing”.
It is the horseshoe crab mating season!
For the past 3+ years I’ve had the opportunity to witness this event as part of my research on understanding horseshoe crab mating behavior and distribution.
During the late spring to early summer of every year, American horseshoe crabs (Limulus polyphemus) leave their deep-water winter grounds, and crawl on to the shallow waters of estuaries and embayments of the U.S. Atlantic and Gulf Coasts to mate. At times of high tide, a female horseshoe crab, often dragging a male clasped to her back, will dig a nest in the sediment at the high tide water line where she will lay clusters of 2,000 to 4,000 eggs. The eggs are externally fertilized by the attached male and/or several lone males called satellites.
Horseshoe crabs have existed for more than 350 million years. Nicknamed “living fossils,” they have been doing this every spring since… who knows? Probably since the age of the dinosaurs.
At some locations and at particular times, you can see thousands and thousands of horseshoe crabs on beaches, often resembling cobble stone streets.
Many species of birds and marine invertebrates rely on the horseshoe crab and their mating season. Migration times and routes of birds like the red knot (Calidris canutus) are closely linked to the mating season and mating locations of horseshoe crabs. As these birds migrate from the southern tip of South America to Northern Canada and the Arctic (almost non-stop!), they have specific sites along their route where they briefly stop to feed and refuel on the protein-rich eggs of horseshoe crabs. The foraging behavior and bioturbation (the process of disturbance of the sediment by a living organism) of horseshoe crabs have an impact on bottom sediments and communities in estuarine systems, providing new habitats for epibiotic and benthic organisms, like shellfish and worms (check out Sara’s work on parasitic worms!)
So if you’ve read my profile and my related links, you’ve already gathered a glimpse as to why I’m excited about horseshoe crabs. But why have I dedicated 3+ years to studying and observing what is essentially horseshoe crab sex?
This time of year is when horseshoe crabs show up on our beaches more abundantly, and also, when they are very vulnerable to being harvested and over-fished.
Though horseshoe crabs are not your typical fish that you eat – actually they’re really not that palatable – they are still important to us for other economic reasons.
In the 1800s, horseshoe crabs were used for fertilizer and animal feed. However competition from better non-horseshoe crab fertilizers and unpalatable livestock due to being fed horseshoe crabs, made the use of horseshoe crabs in these early industries obsolete.
By the 1970s, a new industry grew and the practice of using horseshoe crabs as bait became popular for catching American eel (Anguilla rostrata) and whelk (Busycon spp.). Because horseshoe crabs don’t run away, they are easily captured and thus widely available. Due to increased demand of eel and whelk for food cuisine, horseshoe crabs are now at risk of being over-harvested.
If you’ve read my previous blog post about the “nobility” of the horseshoe crab and their blue blood, you know about how important their blood is. (If not, learn more about it here!) Horseshoe crabs are extremely valuable in the biomedical field and to modern medicine. Their blood has a property called Limulus amebocyte lysate (LAL), which is able to detect bacterial toxins in human blood. Currently, horseshoe crabs are harvested in large numbers to extract LAL from their blood, which is used to test for bacterial contamination in vaccines and intravenous drugs.
In response to over-exploitation, in 1998 the Atlantic States Marine Fisheries Commission (ASMFC) initiated a fishery management plan to limit the number of horseshoe crabs taken by the fishery and to conserve and protect horseshoe crabs. The plan called for new fishery regulations, reporting of landings, spawning and egg surveys, mandatory catch reporting, studies on the use of horseshoe crabs by the biomedical industry, and habitat delineation by each U.S. Atlantic state.
This leads to the research I have been doing.
I have been trying to discover where and when horseshoe crabs mate in New Hampshire, specifically in the Great Bay Estuary. This location is at the northern most geographic range of the horseshoe crab and experiences extreme changes in weather conditions and water quality. Studying horseshoe crabs from this population provides valuable insight into the potential threats of climate change.
With growing threats of climate change and additional pressures from coastal development, the health of estuarine ecosystems and its species are at risk of deteriorating. Estuaries are important environments for many marine and terrestrial species, and provide a suite of ecosystem resources, benefits, and services to society. Horseshoe crabs have existed in these estuaries for millions of years and have established important ecological relationships with other members of the estuarine ecosystem. Learning about the ecology and behaviors of horseshoe crabs in response to these environmental and anthropogenic changes can serve as an indicator of the estuary’s health and status.
I always tell people that if you were to travel back in time, you would see the same animals on the shoreline as you do now. It amazes me (and the people I tell this to) that the horseshoe crab has existed for so long yet we still know very little about them.
So when you walk on those beaches this summer, be mindful of where you step; you don’t want to be disrupting what horseshoe crabs have been doing for millions of years. Appreciate the horseshoe crabs for existing for so long and leave them be; let the horseshoe crabs do their “thing”!
(For frequent visitors of Great Bay Estuary, if you see any large groups of horseshoe crabs in the estuary, email me (firstname.lastname@example.org)! I am determining where large aggregations exist in the estuary in efforts to understand this population and the public's help is greatly appreciated!)