Spring is here - the writing is in the water

Plankton water sample. Photo credit: Tina Saey (flickr user tinahesmansaey)

Plankton water sample. Photo credit: Tina Saey (flickr user tinahesmansaey)

For those of us that live in colder climates, the first sign of spring is not when the calendar announces “First day of spring.”  I assure, the calendar is wrong.  Instead it’s when the air temperatures finally warm well above freezing, birds start chirping outside the window, and ultimately flowers start to bloom.

However, if you’re a marine biologist, the first signs of spring have nothing to do with birds or flowers.  In fact, you often haven’t noticed a temperature change yet.  But the animals living in the water detect the 1-2 degree Celsius change.  For many marine organisms, especially invertebrates, that slight increase in temperature is the signal for reproduction, for mating, for spawning, for releasing of larvae.  A close inspection of a sample of ocean water under a microscope will tell you very quickly if spring has arrived yet.

A sample of zooplankton containing copepods and veligers (larval molluscs). Photo credit: Cynthia Trowbridge

It’s in the plankton

Water is full of microscopic organisms.  Even when water that may look clear, may actually contain hundreds of small animals, known as zooplankton.  Some of these zooplankton will live their entire lives in the water, subject to the currents.  However, for many others life as a zooplankton is merely a stopover on the way to another habitat and life style.  It is one of these organisms passing through the plankton that is the first sign of spring in the oceans.

A cyprid larvae; the final larval stage of a barnacle before settling permanently and metamorphosing into an adult.  Photo credit: By M. C. Mingorance Rodríguez via Wikimedia Commons

A cyprid larvae; the final larval stage of a barnacle before settling permanently and metamorphosing into an adult.  Photo credit: By M. C. Mingorance Rodríguez via Wikimedia Commons

Remember, the acorn barnacle, Semibalanus balanoides?  This barnacle, which inhabits the harsh rocky intertidal environment, and who’s larvae can survive freezing in winter ice, is also the first to release its larvae in response to those first signals that spring is near (not uncoincidentally, this is probably why it has to be able to survive freezing in ice).  When the barren water of winter, devoid of much zooplankton, begins to be populated with barnacle larvae, or cyprid larvae, you know that spring is near.

But the story of barnacles starts much earlier.  Before that first 1 degree rise in water temperature.  Before the ice sets in.  The story actually starts almost a year earlier (or perhaps it’s just the beginning).

The story of the barnacle

Barnacles start their lives as a zooplankton in the early spring.  After only a short stint amongst the plankton, the larval barnacle finds a hard surface to settle on.  However the decision of where to settle is not a simple one.  This first decision a larval barnacle makes will determine much of the rest of its life because after the larval barnacle settles, it cements itself to the hard surface, and metamorphoses into an adult.  Adult barnacles don't have the ability to move from that spot.  The young barnacle must “choose wisely” and select a spot with shelter, room to grow, abundant food, and other barnacles.

Shelter is important for a young barnacle to protect itself from predators as it grows (and thus also the room to grow).  As I mentioned, barnacles can’t move once they settle, so food must come to the barnacle for it to snatch out of the water.

Why does a barnacle also need to be next to other barnacles? 

Organisms that live a sessile life (or non-mobile) have challenges when it comes time to mate.  A sessile organism cannot travel to find a mate.  Most marine organisms solve this problem by broadcast spawning.  They release millions of sperm and eggs into the water at the same time and the odds take over.  If an individual releases enough eggs or sperm into the water, the odds are at least some of them will successfully meet and result in a fertilized egg, which can then develop into the next generation.  As long as enough individuals are in the same general area, this strategy works.

ARKive video - Acorn barnacle reproduction

Barnacles don’t do this.  Barnacles mate like most mammals do.  So here’s the conundrum: how does a barnacle that can’t move, get near enough to another barnacle to directly mate?  The first step comes back to the decision made by the larval barnacle to settle near other barnacles.  But even with close proximity, there is still a challenge; the barnacle still has to somehow reach out of its protective shell and into the protective shell of its neighbor in order to mate.  The barnacle solved this problem by evolving a really long penis.  In fact, the common intertidal acorn barnacle, Semibalanus balanoides, has relative to body size, the largest penis in the entire animal kingdom; penis length is up to 8 times their body length (check out here to see how the barnacle compares to other animals).

Among a group of barnacles, the assumption is that the likelihood of mating is closely correlated to the distance to the nearest neighbor.  And in fact, researchers have found that the nearer a barnacle is to other individuals, the more likely it is to harbor fertilized eggs.  As it turns out, though, the story is not so simple.  Close proximity and a long penis are the primary factors that determine the odds of successfully mating, but environmental conditions can shift the balance. 

Consider again, the acorn barnacle Semibalanus balanoides, which lives in the in the rocky intertidal, a habitat characterized by waves.  In areas with intense wave action, extending a long penis and hitting the mark becomes a problem.  How does the barnacle cope?  The shape and length of the barnacle penis is different between areas with and without waves.  Acorn barnacles that live in area with consistently high wave action have shorter and broader penises than individuals from wave sheltered sites.  As a result, in wave dominated areas, the nearest neighbor must be closer together to successfully mate.

What does this mean for the larval barnacles in the water?

For a young barnacle, finding the right habitat is not so simple.  Survival and success depends on finding a hard surface (which is easier said than done in the ocean), finding an area with abundant food, finding an area with some protection against predation, and finding other barnacles (in close enough proximity to overcome potential waves).

So the next time you’re scrambling around the intertidal and are about to step on a barnacle, remember the incredible feats that barnacle overcame to find that spot.

Resources:

For more barnacle videos: Arkive

References:

Hoch, J. M. (2008). Variation in penis morphology and mating ability in the acorn barnacle, Semibalanus balanoides. Journal of Experimental Marine Biology and Ecology, 359(2), 126-130.

Yuen, B., & Hoch, J. M. (2010). Factors influencing mating success in the acorn barnacle, Semibalanus balanoides. Journal of Crustacean Biology, 30(3), 373-376.

Check out more about the intertidal:

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