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Florida Flatworms & Horseshoe Crabs

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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Mission Underway

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.