The Symbiotic Relationship Between Horseshoe Crabs and Killifish In St. Joe Bay, Florida

This summer, we vacationed at our good friend's rental guest house that sits in a beautiful marsh a few yards away from the clear waters of St. Joe Bay. 
How wonderful it is to stay in this house
called "Bay Window", is the subject of another post.
So stay tuned if you are curious. Click on the link if you can't wait ...

This post is about something I observed while wandering around in the clear waters of St. Joe Bay. 
There's a video below, but you know that, because the first thing you did after opening this post, was CLICK THAT VIDEO.
You know you did...




As usual, I had my GoPro Hero 5 and my selfie stick with me as I waded in the bay. I love this camera and wowsers, have I put some miles on it between obstacle course races and wet and wild nature photography. This was the perfect combo for wading in the shallow bay. With my selfie stick and the right GoPro fittings, I could poke the GoPro under the water and right into the face of the marine life creeping over the sandy bottom.
 (I even took it out there at night with a light ... yet another post to come)

So what was the thing I noticed while wading? First, you need to understand that St. Joe Bay is crawling with Horseshoe Crabs... big ones, mid-size models, and compact youngsters. They were everywhere, day and night, always moving across or plowing the bottom in search of buried clams, worms, and other invertebrates.

With the GoPro setup I had, I didn't just walk by them, I stopped and spent time with them as I filmed. The Horseshoe Crab's response to me standing still and filming was to basically treat me like part of the scenery and carry on plowing and feeding. The fish also ignored me if I stood still. And that was how I got to see the relationship that a group of shallow-water fishes called, "Killifish", had with the Horseshoe Crabs. 

History: A kill is a body of water, most commonly a creek, but also a tidal inlet, river, strait, or arm of the sea. The term is derived from the Middle Dutch kille (kil in modern Dutch), meaning "riverbed" or "water channel". (from Wikipedia)

Most fisherfolk just call this group of fish, "mudminnows". They are excellent bait and an important food fish for bigger fish and seabirds. 

The relationship I witnessed was repeated over and over again as I roamed the shallows. Almost every actively feeding Horseshoe Crab was accompanied by a school of killifish who were also actively feeding directly behind (and even under) the Horseshoe Crabs.  

As Horseshoe Crabs plow and dig up the bottom while feeding, they eject a constant flow of water and sand from underneath their body.

 This outflow from under the backside of the Horseshoe Crab probably carries with it bits of the Horseshoe Crabs meal as well as very small invertebrates dug up by the Horseshoe Crab. It is this bounty that the Killifish were feeding on.

Two species of Killifish were most abundant in this moveable feast, Longnose Killifish (Fundulus similis), and Gulf Killifish (Fundulus grandis). They dominated the scene by sheer numbers in the extreme shallows. Other fish that appear on the video in smaller numbers were Mojarra (Eucinostomas harengulus), 
Sheepshead Minnow (Cyprinodon variegatus) and
Pinfish (Lagodon rhomboides)


This type of relationship where one species benefits, while the other isn't harmed or helped, is called "Commensalism".
Other types of Symbiosis include "Parasitism" and "Mutualism".

I did an internet search for "Symbiotic Relationship between Horseshoe Crabs and Killifish" and the only thing that popped up was my video above that had only been on the internet for 4 minutes at that time.

I'll probably be getting a call from the NOBEL PRIZE COMMITTEE sometime this week.