"A good scientist is a person in whom the childhood quality of perennial curiosity lingers on. Once he gets an answer, he has other questions." ~Frederick Seitz, President, Rockefeller University
Science is looking at the world and continually asking the questions, “How does that work? Why does this happen this way?” The scientific process always begins with questions. Why is the sky blue? How do salmon know when it’s time to spawn? What’s the quickest route to work? We conduct experiments to try and find the answers to these questions. One of our first assignments for this course was to write down twenty questions about the ecological community we found at Nahant. In a funny way, when you start asking questions, the more questions you discover you have. Also, the more you talk to other people, the more ideas come to you.
When our class first went to the Nahant Biological Station, I didn’t know what I wanted to study. I walked up and down the shoreline, gazing into the tide pools, and simply didn’t find myself very interested in anything I was looking at. As we got farther along into the semester, I started to panic. I still didn’t have an idea of what I wanted to do. I started talking to my classmates, and it was one of them that just offhandedly said, “You could look at anything. Like mussels and their movement. Byssal threads are pretty neat to look at.” Byssal threads are pretty neat to look at. They’re how mussels attach themselves to a rocky substrate, and, when mussels are small, they’re how they move around. Suddenly I had a lot of questions.
What is the life cycle of mussels?
Why do mussels live in groups?
Do mussels have to defend themselves against predators?
Is there one part of the intertidal that mussels live in more often?
What's with the crazy name Mytilus edulis anyway?
What is the mussel distribution at Nahant?
What percentage of those are juveniles?
Is there a different distribution for juveniles and adults?
I had never really given much thought to mussels before, but the more I thought about them, the more interesting they became. I started to do some research to find out what questions I could answer, and which ones I could not.
What is the life cycle of the blue mussel?
As adults, mussels are typically found in dense patches
called mussel beds. They are broadcast spawners, meaning both males and females
release their gametes (eggs and sperm) into the water at the same time, and
fertilization takes place outside the body (Bertness, 1999). Spawning typically
takes place in the late spring and early summer (Engle and L
oosanoff, 1944)
though there is some variation in spawning time (White, 1937). Once eggs are
fertilized, a three-month existence as planktonic larvae occurs, going through
three morphologies (or body-plans) (Bertness, 1999). The first is the trochophore
larval stage, which is named for the specific feeding structures and methods the
larvae employ (Rouse, 1999). Next is the veliger larval stage, which has two
very, very, very thin shells (Aquascope 2000). Finally they become a
plantigrade larvae, where the shell is not completely formed yet and their
“foot” is visible and used for locomotion. It is at this stage that they begin
to settle on filamentous algae like Ascophyllum nodosum (Bertness, 1999)
typically in the summer months of June through August (Engle and Loosanoff,
1944). Juvenile mussels are about 0.5 – 2.0 millimeters in size when they first
settle, and after they have approximately doubled this size many move into a
mussel bed with adults (Bayne 1964). By waiting until they are bigger to move
into a mussel bed, juveniles are probably better able to compete with adult
mussels for food resources while they take advantage of the protection from
predators and wave action (Bertness, 1999). This recruitment strategy (ways new
members are added to a population) of juvenile mussels helps maintain mussel
beds and the spatial dominance of Mytilus edulis in the rocky intertidal
(Petraitis, 1995).
Why do mussels live in groups?
The rocky intertidal is an incredibly harsh environment to set up house in. Organisms have to deal with the flux of air and water exposure during tide cycles, a slew of weather conditions throughout the year in conjunction with tides, compete for food, space and other resources, and avoid getting eaten by predators. In New England, annual temperatures can range from -20ºC in the winter to 40ºC in the summer (Bertness, 1999). Combine this with a typical daily temperature flux of 10-20ºC (Bertness, 1999), and you begin to see the challenge. The other most common threat to the rocky intertidal organisms is that of desiccation, or drying out/running out of water while exposed to the air during low tide. One way mussels combat these obstacles of their environment is to live in groups. By clumping close together they can keep more moisture in the mussel bed and protect individuals from thermal stress (Bertness and Leonard, 1997). Dense aggregations (groupings) of mussels also provide protection from predators and buffer individuals from wave stress (Bertness, 1999). Living in large groups also helps when it comes to reproduction, since mussels are broadcast spawners and proximity and timing are critical to successfully reproducing (Denny et al. 1985).
There are, however, some down sides to living in such a large group. Competition for food and space is the major one. Mussels are active filter feeders, which means they use cilia (tiny hair-like appendages) to pump water over their gills (Bertness, 1999). In dense beds, competition for food is intense (Bertness, 1999). Space for growth can also be an issue in large mussel communities. M. edulis can grow to a length of 25 millimeters or more in the first year of growth (Richards, 1946) and if there’s not enough space on the substrate, mussels have been known to grow on top of one another (Bertness, 1999).
In the larval stage, mussels hang out in the ocean as
free-swimming plankton. Currents can carry them hundreds of kilometers away
from their birthplace (Bertness, 1999). This is often a cause of high mussel
mortality (death), as juveniles find themselves in environments they can’t
survive in, or sometimes are just swept out to sea, never to see land again
(Bertness,
1999). If they do come ashore in a favorable environment, juvenile mussels in
the plantigrade larval stage can move around using their byssal threads (Bertness,
1999), a thread-like protein-based collagen that is five times stronger and 16
times as “stretchy” than a human tendon (access excellence, 1997). Using their
byssal threads like climbing ropes, mussels less than 2 centimeters are
extremely mobile (Bertness, 1999). After they get much bigger though, they’re
too heavy to use their byssal threads to move, but they can still use them to
attach themselves (and re-attach themselves should they get knocked off) very
firmly to many forms of substrate (Bertness, 1999).
Do mussels have to defend themselves against predators?
Yes. Lots of predators in the rocky intertidal find
mussels to be a very tasty snack. The common green crab, Carcinus meanas,
and rock crabs, Cancer irroratus and Cancer borealis present the
highest predation threat to mussels (Bertness, 1999). However dog whelks (Nucella
lapillus),
sea
stars (Asterias forbesii and Asterias vulgaris), and herring gulls
(Larus argentatus) all prey on mussels as well (Smith and Jennings, 2000;
Bertness, 1999). In response to predators, mussels have developed several
strategies to avoid being eaten. One is growth and size; larger mussels tend to
be immune to most predators, because they take too long to apprehend and break
into (Bertness, 1999). Second is living in a community of other mussels, known
as a mussel bed. Juveniles live and hide in and amongst larger adults,
benefiting from the size protection from predators (Bertness, 1999). Mussels
also exhibit phenotypic plasticity, or the ability to change certain
characteristics and traits when certain cues are given. In this case, when in
the presence of predators such as crabs and dog whelks, Mytilus edulis
expend energy to make their shells thicker and more difficult to crack or drill
into (Smith and Jennings, 2000; Leonard et. al. 1999).
(above image © Sinauer Associates, 1999)
Is there one part of the intertidal mussels live more than others?
The rocky intertidal community at Nahant, like all rocky
shores, exhibits what is known as zonation, or the observation of
distinct
“bands” of different organisms running parallel to the waterline. Many factors
influence the zonation patterns observed on shorelines around the world,
including how much wave-exposure there is, whether it is open coastline or in a
bay or inlet, temperature ranges, resource availability, and competition and
predation between organisms (Bertness, 1999). Generally speaking, an individual
organism’s vertical range within an intertidal community is determined by
environmental factors at the upper portion (i.e. desiccation and temperature)
and by ecological interactions at the lower portion (i.e. competition and
predation) (Bertness, 1999). At Nahant, Mytilus edulis can typically be
found from the mid to upper intertidal, usually in conjunction with the
Ascophyllum nodosum canopy. Our study site at Nahant is in a wave-protected
inlet where predation by dog whelks, starfish, and crabs limit the abundance of
mussels (Bertness, 1999).
The Cycle of Science
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