This week we officially entered fall. It is always a somewhat bittersweet time when the freedom and warmth of summer gradually slips away just as more of the daylight slips into darkness. But, in this transition, there is a brief window when you can explore the darkness while the night air is still relatively warm and comfortable. This is an especially neat opportunity to observe coastal waters.
There is so much activity in the water at night that often gets missed. Fish are moving in and out with the tide and many creatures are coming out of their hiding places or up to the surface at a time when it is harder for their predators to see them. This week’s full moon helped to illuminate some of this activity, as has the recent intensity of phosphorescent plankton in the water (although this is much harder to see with a bright moon).
The glowing sparkles in the water are from tiny plankton called dinoflagellates – these are single-celled organisms with flagella (tails) that whip them through the water. These dinoflagellates have a chemical in their bodies that is activated and glows when they are disturbed. This is their attempt to scare off predators. If you throw a rock into the water at night or swish a paddle around, you might see these plankton glow.
Or, if there happen to be tiny fish jumping, they also can send off arrays of sparkles on the water. This is what I saw on a recent night – buckets of tiny little fish breaking the surface and putting on a light show. It is common for people to call any small bait fish a “minnow,” but technically minnows are freshwater fish. These little fish are Atlantic Silversides (Menidia menidia). They are named for the metallic silver stripe that runs along the sides of their bodies. They have greenish backs and are nearly clear underneath with round white bellies – a perfect camouflage both from the bottom and the top. Silversides are also sometimes called spearing or shiners.
These slim, silvery fish are usually only about 2 inches long, although they can get bigger as they move further offshore – up to about six inches at the largest. Because they are small, they often swim in thick schools for protection from predators. They also often like the forestry hiding places of eelgrass. This keeps them mostly in shallow waters where they are safer from predators like striped bass and bluefish as well as seabirds including cormorants and terns.
Silversides are common along the entire east coast from the Gulf of St. Lawrence all the way down to Florida. They are a critical part of coastal ecosystems. They feed on plankton and other little invertebrates and then serve as food for the next level of predators. They can be in fully salty water as well as in some fresher estuarine conditions.
Seeing silversides at night chasing the phosphorescent plankton is a truly remarkable experience. It’s their moment to shine, literally, when they can tuck in close to shore and avoid the hungry sportfish further out. The water is still relatively warm and provides good conditions for them – and a chance for us to see them on warm, dark early fall evenings. But, soon the season will cool and they will head out deeper where the water stays at a stable temperature.
There is always more to be seen in and on the water and every set of conditions from temperature to tide to light offers a new combination of living things and activities to be observed.
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