A Low Tide Bonanza of Life

We have recently had a series of very low tides and this has revealed some interesting animals either on the beach or on shallow flats.

The state shell of Florida is the magnificent horse conch which has a bright red body and foot. It is not a true conch but is more closely related to the tulip shells. I have often wondered about the striking red body color and its purpose but have found no answer. The horse conch is predatory on other molluscs and is the largest gastropod in American waters. A slightly smaller marine gastropod and food for the horse conch is the lightning whelk, which is unusual in that it is sinistral or left handed. If you hold the shell by its front or pointed end facing down, the flared part of the shell where the foot emerges is to the left side. I came across a large adult feeding on a hard clam on a tidal flat. They lay their eggs in a long string of capsules which are sometimes washed onto the beach.

I found a small lightning whelk shell washed up on the beach which was empty, although it likely had a hermit crab in it previously. But the most interesting feature was the attachment of three symbiotic anemones (Calliactis) to the shell. These of course are retracted when out of the water but they benefit from attaching to the mollusc shells inhabited by hermit crabs and probably also provide some protection and perhaps food to the crab with their stinging tentacles.

A smaller marine gastropod, the moon snail, lays its eggs in a collar mixed with sand. It has been shown that the design of this collar is useful in maintaining the eggs on top of the sediment and upright. The moon snail bores through the shells of its prey with an acidic secretion.

My grandkids’ favorite marine invertebrates are the sea squirts which as adults are sessile tunicates fixed to the bottom or some hard object. They are filter feeders and have an internal compartment filled with water. When squeezed they squirt the water out in a fine stream- they thus become nature’s water pistol ! They are interesting since they have a free swimming larvae which is similar in structure to the early chordates/vertebrates and are indeed in the same phylum despite their strange morphology.

My favorite jellyfish is the upside down jelly or Cassiopea. It has symbiotic algae in its tissues and lies on its back to gather sun for the algae. It appears to be a photosynthetic animal/plant partnership which also obtains food from catching zooplankton with its venomous nematocysts. It is a warm water species and is only occasionally found in numbers in our area of SW FL. Indeed during my 20 years of experience in this area it has rarely been able to overwinter and grow to a large size. Thus global warming has apparently not yet had a significant effect on its ability to colonize more northern habitats.

So keep an eye on the tide tables and weather events such as strong winds which can contribute to the occurrence of exceptionally low tides. This is an opportunity to observe some fascinating subtidal animals which otherwise are not easily seen.

Bill Dunson

About Bill Dunson

Bill Dunson, born in rural Georgia, skipped 12th grade and went directly to Yale. Bill subsequent-ly received a PhD in Zoology from the University of Michigan, studying softshell turtles. Bill is Professor Emeritus of Pennsylvania State University thanks to a career spent entirely at that institution, teaching and doing research on the physio-logical ecology and ecotoxiciology of reptiles, amphibians and fish. Always curious about nature, Bill has dedicated his life to learning and sharing his knowledge with others. He has served on many advisory boards here in Southwest Florida to preserve the water that gives life to our region.

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