Florida Flatworms & Horseshoe Crabs

Latest update July 7, 2018 Started on June 3, 2018

Funded by the Harvard Museum of Comparative Zoology and the Harvard Department of Organismic and Evolutionary Biology, I am on a quest to find the flatworms living on Atlantic horseshoe crabs in order to research phylogeography and symbiosis around the Florida peninsula.

June 3, 2018
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In The Field

Field Notes 7/6/18 pt. 2

And it all leads up to Key West, the final site of my expedition in this charismatic chain of mangrove-speckled isles. This site is also expected to be our most difficult, in part due to being in the southernmost part of Florida in the dead of summer in antagonism of the preference of horseshoe crabs for cool water temperatures, and in part due to the high degree of human development and foot/boat traffic in Key West which not only deters horseshoe crabs but also can harm their spawning sites.

Our destination in Key West was, of all places, the backyard mangrove beach of a Courtyard Marriott Hotel. I distinctly remember my low expectations of the yield of any of these field sites as we pulled into the parking lot and eyed the 20 meter beach. Yet as I leisurely exited the car and grabbed our gear, I heard the exuberant shriek of one of my research assistants followed by her frantic disposal of shoes and phone in order to jump into the shallows. Second later she emerged victorious clutching multiple horseshoe crabs that wriggled in her grasp - triumphant, with the least effort, at our most unlikely field site.

The next two hours offered so many horseshoe crabs that we nearly grew exasperated, a new friend spotted crawling across the shallows just as we tossed one of its companions back. The three of us worked in a frenzy of spotting, catching, extracting, and repeating.

In the midst of the excitement we even got to interact with some interested civilians who were staying at the hotel. One woman yelled at us to leave the critters alone - a champion of marine life if I ever saw one - only to calm down once she realized we had permits and were conducting research. Another couple was fascinated at the archaic appearance of these critters and listened to our entire saga about their biology and ecology, with the penultimate pleasure being letting their young daughter touch one.

The excitement finally ended when we began recapturing the same horseshoe crabs and ran out of collection vials for their Bdelloura symbionts. We toasted Arnold Palmers to the exciting end to our day, enjoyed a slice of tourist-garnished key lime pie, and bid Key West ado.

As we drove back on the cozy two-lane highway stretching the entire length of the Keys, I was filled with nostalgia for the resolution of this adventure. Not only was this my last field site in the Keys, but also the last field site of my entire summer expedition in Florida. Horseshoe crabs have grown on me as both charismatic and fascinating; their Bdelloura candida symbionts I am sure will instill me with their genetic treasures in the months to come in lab.

Thank you for joining me on this adventure!


Field Notes 7/6/18

The relentless simmer of a Floridian July radiated through my shoes this afternoon during our survey of Cudjoe Key, yet the heat did not deter my research assistants who (voluntarily, mind you) waded knee-deep into the murky depths to locate our horseshoe crab friends by the rustic yet reliable method of ‘feel-by-foot’.

We found a considerable amount of horseshoe crabs, yet unlike in Islamorada or other sites in Florida, these harbored notably fewer Bdelloura candida. Furthermore, in addition to the expected presence of horseshoe crab molts that signify strong, growing populations, Cudjoe Key hosted a considerable amount of dead horseshoe crabs.

I suspect that the lack of Bdelloura and the frequency of dead Limulus are due to the recent increase in pollutive runoff in Cudjoe Key following heightened urban development on the island starting in 2014.

Our next and final site brings us to balmy Key West, the southernmost isle of this chain of mangrove landmasses and our greatest challenge, as horseshoe crabs are sparsest here among all of the Floridian field sites. Adventure awaits!


Field Notes 7/5/18

As we ventured further south into the Keys, landmasses melded into sporadic isles of mangrove. Our next field site was Bahia Honda State Park, a notorious beacon for manatee and horseshoe crab populations.

However, upon our arrival over 80% of the park was shut down due to the devastating Hurricane Harvey and Hurricane Irma. Dead seagrass and debris littered the coastline rendering surveying both impossible and unsafe.

Having grown up in southwest Florida, I’ve grown accustomed to the ravaging effects of hurricanes. Sometimes the aquatic turmoil transcends that of terrestrial damage, and thus I speculate that the strength of Irma may have disrupted and shifted some horseshoe crab habitats along the coastlines, especially here in the Keys where it first made landfall. In any case I am again reminded that nature is humbling.


Field Notes 7/3/18

Successful sampling on both the eastern Atlantic coast and the western Gulf coast now give way to the most extensive and anticipated expedition of the summer: The Florida Keys. The Keys are romantically coveted as a Caribbean escape, the hideaway of Hemingway and the haven of the rustic ocean lover. For scientists, they offer not only the warmest waters in the continental United States but also some of the most biogeographically diverse coastlines - a bold and humble chain of barrier islands extending from the southernmost tip of the Everglades towards Cuba.

The unique position of the Keys between the Gulf and the Atlantic suggests that they could serve as biogeographic barrier that is causing the genetic differentiation between Gulf and Atlantic B. candida populations, since their horseshoe crab hosts may not cross their threshold. This is coupled by the fact that the Gulf Stream and its nutrient-rich flow only passed the eastern coast of Florida and does not encroach upon the Keys or the westward Gulf.

However, the conditions of the Keys make it the most difficult area to find horseshoe crabs, which descend to deeper waters if the surface temperature is too high. Given that we were scouting in the middle of the summer in the already warmest waters of the country in a place with limited coastlines, this posed quite a challenge.

Hence my two trusty research assistants, Elena and Kayla, and I set off ready for a challenge. Our first stop was to a humble marina in Islamorada to meet Jesse, a freelance fisherman and citizen scientist who constantly impresses me with his awareness and passion for the critters around him. Jesse took us out on his boat as we scoured the mangrove isles, fighting glare and heat, to no avail.

Three hours in and it seemed like the notorious difficulty of this field site had beaten us. Just as we were returning to the marina Jesse spotted a large female about 6 feet underwater, and before he had turned off the engine I was in the water swimming towards it. The triumph was palpable as I lifted it into the air and handed it onto the boat for my research assistants to extract the Bdelloura.

On to the next isle!

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Field Notes 7/2/18

Back to the west coast to delve back in the mangrove ecosystems of Sanibel, a quaint island ecosystem on the Gulf of Mexico. Sanibel is home to the J.N. Ding Darling National Wildlife Refuge, unique in its harboring of large populations of migratory birds and - but of course- horseshoe crabs!

On this expedition I was able to collaborate with Dr. José Leal, director and curator of the Bailey-Matthews National Shell Museum, and Carmen Hoyt, a recent Duke marine science graduate. Together we conducted transects along the south side of the refuge with eyes peeled for the characteristic horseshoe-shaped carapace that mulls around sandy shallows. Harder than it sounds when everything on the floor of the tannin-browned waters is in varying shades of brown.

Today was neither a spring tide nor a high tide, so I expected a low turnout of horseshoe crabs, but today they decided to be social and grace us with their presence. In my excitement I jumped barefoot into the shallows to catch them, saving the fun B. candida extractions for my trusty research assistants.

Following this expedition both the eastern and western coasts of Florida have been successfully surveyed for B. candida with ample geographic dispersal. High hopes for the expedition to come in the Florida Keys, which offers not only the greatest challenge to find horseshoe crabs - warmer water, smaller populations -but also the greatest reward, as the Keys could serve as the natural geographic barrier causing the genetic differentiation demonstrated between the Atlantic and Gulf populations. Adventure is out there!

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Field Notes 6/27/18

Back to Florida's east coast for a daring night survey. Horseshoe crabs in Florida don't have a specific mating season like their more northern, cold-water counterparts, so finding them can be especially tricky. Based on local fisherman legends, horseshoe crabs come close to shore to mate and lay their eggs in the sand during high tide, and especially spring tide, which is the strongest tide occurring on full and new moons.

Just so turns out that June 27th's spring tide was at 9:07pm in this little corner of the world. Thus, my intrepid research assistant Clay https://www.bcbarlow.com/ and I trudged through the shallows of Phil Foster Park under the cover of moonlight.

Accompanied by the occasional ghost crab and some daring scuba divers, we circumnavigated the humble island to no avail until Clay spotted a mating pair off the corner of a dock. Before I knew it I was waist-deep in the water, feeling around blindly for the smooth carapaces lurking in the sand.

As seen in the photo below, here we see an obvious case of sexual dimorphism in which the female (right) is much larger than the male (left), not due to age but rather practicalities of horseshoe crab mating rituals. The female burrows into the sand to lay eggs with the male attached to her back, depositing sperm on top of those eggs as the female crawls forward. A mating pair will lay and fertilize eggs multiple times during a spring tide, with the male attached to the female the entire time, so it makes sense for the male to be substantially smaller such as not to tire the female out as she carries his weight all day.

Thus another successful capture on the east coast. By now my field sites are filling in quite nicely in sampling different localities around the peninsular coasts. On to the next adventure!


Field Notes 6/19/18

Today my research assistants and I ventured Mosquito Lagoon, nestled within the Indian River estuarine ecosystem of eastern Florida, on the hunt for flatworms and their horseshoe crab hosts with collaborators from the University of Florida's Whitney Labs.

From a humble boat ramp harboring curious manatees, we drifted east on a small boat through the mangrove flats. The water was turbid as we traced the coastline back west, delighted by a pod of dolphins that calls this cove home. Brave Mr. Taylor waded through the shallows as we searched by boat. Mosquito Lagoon hosted many carcasses and shed molts of horseshoe crabs, the live ones eluding us until the latter half of the expedition.

Under a hot sun we greeted two large old horseshoe crabs and a subsequent mating pair, all of which were loaded with B. candida. Curious as to why horseshoe crabs here harbor such a multitude of flatworms while their southeastern counterparts boast much smaller quantities. In either case, we collected enough flatworms to keep me busy in lab for quite some time!

*Extraordinary thanks to Mark Martindale, Adam Pancetti, and Jacob Taylor of Whitney Labs for providing the manpower for today's expedition! *Photography by Clay Barlow https://www.bcbarlow.com/

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Field Notes 6/13/18

Today it's peak spring tide and a full moon day, which bodes well for horseshoe crabs being close to shore to mate. Ventured up to St. Pete to collaborate with Eron Higgins, a marine science instructor at Eckerd College who works with the Florida Fish and Wildlife 'Horseshoe Crab Watch'. We conducted a beach transect and found at least 10 horseshoe crabs all attached for mating.

Here, where time as well as abundance was on our side, Eron and I waited until a mating pair was finished laying and fertilizing eggs to check them for B. candida, enjoying the time in between to sit back and watch nature happen. We watched the male horseshoe crabs attach themselves to the backs of females, going along for the ride as the female dug herself into the sand to lay eggs. For critters that are nearly 400 million years old, preceding dinos, they are quite charismatic.

Field work is now picking up steam with three days in a row of successful sample collections. Time soon to start targeting the eastern coast of the peninsula - adventure is out there!

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Field Notes 6/12/18

Stoked to have collaborated with Ryan Young and Rising Tide Explorers, an ecotourism kayaking company in southwest Florida dedicated to exploring and learning about the unique wildlife and mangrove-estuarine ecosystem of the Ten Thousand Island National Wildlife Refuge.

I partnered with Young as the 'expert biologist' of our four-hour kayaking expedition through the mangrove tunnels and mud flats of Rookery Bay. Accompanying us was a family to whom horseshoe crabs were wholly unfamiliar. As Young pointed out oyster beds and soaring ospreys, I told the kiddos how to spot horseshoe crabs from the vantage point of their kayaks, promising that they could help me with my research if we found any.

Lo-and-behold, back in the sandy shallows of the mangrove forest, we spotted some. Ecstatic and happy to put a show on for the kids, I stood up in my kayak and dove through 4 feet of murky water (from tannins released from the mangroves), surfacing with a handful of squirming L. polyphemus. After wading to a shallower area I showed the family how to safely hold the horseshoe crabs, and they passed them around before holding them still so I could extract the B. candida commensals off of their undersides.

After about 7 L. polyphemus captures, Young guided us back to shore to avoid the oncoming afternoon thunderstorms. The family and I were chatting all the while about the importance of horseshoe crabs and mangrove ecosystems to the rich diversity of wildlife here in the Gulf of Mexico.

Beyond the successful fieldwork of that day, what thrilled me the most was seeing the pure interest and adventurism of the kids to an animal that is not nearly as charismatic as panda bears or elephants. Nature has the grand capacity to fascinate and inspire kids, and it was incredible to help facilitate that innate passion for wonder.

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Field Notes 6/11/18

Gold! Located the expedition's first live horseshoe crabs off the coast of Sarasota's Indian Beach in collaboration with Armando Ubeda of the Florida Sea Grant program. Ubeda had heard from local fishermen of a popular breeding ground of horseshoe crabs nestled in a protected cove of Indian Beach. His intel was spot-on and we waded along the shoreline, locating several mating pairs of horseshoe crabs.

The majority had B. candida nestled among their legs and book gills. The B. candida vary are small, white worms of the phylum Platyhelminthes and are visible to the naked eye, which made collecting them a breeze before we placed the horseshoe crabs back in the water.

A couple cool notes about handling live horseshoe crabs: their unique book gills allow them to breathe both in and out of water such that holding them out of the water to collect the B. candida does not seem to bother them. They're also quite squirmy and will often arch their bodies and extend their long tails - this motion is similar to what they do when they are overturned by a wave and need to get off of their back to move again. Thus their tail does not have a stinger like stingrays, but rather is used for motility and balance.

Note in the photos below: 1) a male and female horseshoe crab attached for mating, in which the female deposits eggs while moving forward and digging herself into the sand, and the male (attached near the female's tail) externally fertilizes them as he moves forward with her; 2) proportionate size of a standard horseshoe crab found in Florida, in which adults tend to be smaller than those found in more northern waters; 3) The underside of a male horseshoe crab who has lost many of its appendages yet still appeared unhindered in mobility; and 4) The underside of another male L. polyphemus with all of its appendages intact.

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Field Notes 6/6/18

Kayaked 3 knots through mangrove forest to the north end of Delnor-Wiggins Pass State Park. Low tide rendered the waters turbid with mangrove tannins, yet was still able to snorkel around the flats in search of horseshoe crabs. These flats have been a notorious mating ground for horseshoe crabs in the past, yet note in the picture of me snorkeling below its proximity to developed high rises and the shadow of a dredge in the background. Dredging in particular not only upsets the estuary bottom, which horseshoe crabs use for laying eggs and feeding, but also contribute deterring noise pollution.

Conducted a 3 mile beach transect of the shoreline during high tide, surprised to locate not one but rather six freshly dead horseshoe crabs washing up along the shore. Sadly offshore was another dredge from whose current came directly the six corpses. Bdelloura candida, as an ectocommensal of the horseshoe crab, cannot survive without a live host, and thus no samples could be gathered on this expedition.

The good news: horseshoe crabs are in the area! Must locate breeding sites that are not near dredges.

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Expedition Background

As an undergraduate student studying Integrative Biology in the Harvard Department of Organismic and Evolutionary Biology, and under the tutelage of Professor Gonzalo Giribet, I traveled to Florida in June 2018 to collect Bdelloura candida, the ectocommensal planarian (flatworm) found on Limulus polyphemus, the Atlantic horseshoe crab.

A previous study (Riesgo et al. 2017) established that the B. candida population in the northern Gulf of Mexico is genetically divergent from the Atlantic population that was sampled from Georgia to Maine. Thus the Florida peninsula serves not only as an area that must be sampled between these divergent populations but also as a natural geographic barrier between the Atlantic Ocean and the Gulf of Mexico.

This is where I come in. Having grown up in sunny southwest Florida nestled between the Gulf of Mexico, Ten Thousand Islands, and Everglades National Park, I desperately wanted to delve more into Florida's marine science and this was my chance to do so.

My mission is to sample areas all around the Florida peninsula in order to triangulate the biogeographical barrier demonstrated by the divergent haplotypes of B. candida populations. I'm thrilled to see what this research reveals not only for flatworm-horseshoe crab relationship but also for a variety of symbiotic relationships in which the phylogeography of the symbiont may mirror that of its host.


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