In 2008 I published a short post on TreeHugger: A Picture is Worth....Dock Spiders Eat Fish?, taken at the end of my dock on Shoe Lake, Muskoka, Ontario. I had never seen such a thing before, and didn't know that spiders ate fish. It's evidently not very common. The best comment to the original post was from my daughter, who wrote:
HOLY GEEZ THAT'S OUR DOCK????? EWWWWWWWWWWWWW SERIOUSLY I AM NEVER GOING TO BE ABLE TO SWIM IN THIS LAKE AGAIN
Although my favorite comment was "You're gonna need a bigger dock...."
In 2012, I received a note from Dr. Martin Nyffeler, who said he specializes in the feeding ecology of spiders. He had noticed the photo in TreeHugger. I sent him all the information I had and there they are, two photos of the Shoe Lake Spider in this fascinating study, Fish Predation by Semi-Aquatic Spiders: A Global Pattern
According to the study, there are fish-eating spiders on every continent except Antarctica. While it was thought that the main meals for spiders are insects, "evidence suggests that fish prey might be an occasional prey item of substantial nutritional importance."
Here is a closeup of the Shoe Lake Spider, which according to Dr. Nyffeler, is a "Dolomedes scriptus feeding on fish (probably green sunfish Lepomis cyanellus)" Evidently the spiders release a neurotoxin that kills the fish.
These spiders possess large strong chelicerae capable of piercing the skin of vertebrates and are equipped with powerful venoms containing hundreds of different neurotoxins, some of which are specific to vertebrate nervous systems.
Why was the fish dragged onto the dock? They need a dry place to eat.
The behavior of always first moving a fish prey to a dry site prior to feeding can be explained by the spiders' extraintestinal digestion – first pumping digestive enzimes into the prey and thereafter sucking in the dissolved tissue through the mouth opening; otherwise the digestive enzimes would be diluted in the water and, thus, become ineffective.
Read the whole study, which is available because Dr. Nyffeler published it on a Creative Commons scientific publisher instead of the usual expensive fenced version. Thank you for that.