This page contains information about the rocky shore near DEI that may be helpful for teachers who want to incorporate information about this topic into their classroom. From time-to-time, we offer teacher workshops at DEI that are related to the rocky shore environment.
The rocky shoreline has a diversity of marine plants and animals. The seaweed covering the rocks can be quite slippery, so proceed with caution. The accompanying photos are taken as one would move from the upper to lower shore.
One of the first things you will encounter near the upper shore, especially during the Spring, are granite ledges covered with a thin film of green algae. These are short, filamentous strands that are usually consumed by one of three species of gastropods (periwinkles), but in this photo, no periwinkles can be found. The white quadrat measures 30-cm x 30-cm (inside dimensions) and a 1-cent penny is used for scale.
Periwinkles (Littorina littorea) are the primary consumers of young macroalgae (the green algae in the previous photo) as well as small "benthic" (bottom) diatoms that colonize the rocks. There are three species of periwinkles on the rocky shore at DEI. Littorina littorea is the only periwinkle that is harvested commercially. This species can attain sizes of about an inch-and-a-half (38 mm) from the tip of its spire to its ventral margin near the aperture (total length), but most are less than an inch in length.
This close-up of Littorina littorea shows distinct disturbance lines in the shells of each individual that can be used to age the animals. This snail is found on both sides of the Atlantic. In Europe, it extends from the White Sea to Gibraltar. In North America, it ranges from Labrador and Newfoundland to New Jersey. It was accidentally introduced to the West Coast of North America, and can be found from Washington to California. Sexes are separate, and reproduction typically occurs in the spring when as many as 100,000 fertilized eggs are released per individual into the water column. Development of planktonic (swimming) larvae occurs over a 5-7 week period followed by an abrupt settlement of juveniles to the bottom. It takes 2-3 years for sexual maturity, which may occur at sizes of around 10-15 mm. Click here to see some photos of this species from the Baltic Sea.
This is one of two conspicuous species of macroalgae that occur on the rocky shore. This is Fucus vesiculosus, or bladderwrack. The fronds of this seaweed show a central midrib and spherical, paired air bladders. This species is not harvested commercially along the downeast shore, but is elsewhere in Maine. Fucus is a dioecious macroalgae, which means that individual "plants" are either male or female. During the spring and early summer, the tips of the algae (receptacles) swell as gametes (eggs or sperm) develop in conceptacles within the receptacles. Once fully developed, the gametes are released on the incoming tide where they combine in the water column. A wonderful resource on the life-cycle and development of Fucus, and other seaweeds can be found on Dr. Michael Guiry's seaweed site.
Here are the receptacles of Fucus vesiculosus (photo taken on 3/13/2012).
This is a close-up of the "holdfast" portion of Fucus, where the plant is attached to ledge.
As one walks toward the mid-tide zone, there is an abrupt transition in species of macroalgae from Fucus vesiculosus to the other major fucoid species, the knotted wrackweed, or Ascophyllum nodosum. Most people refer to Ascophyllum as rockweed.
Like Fucus, Ascophyllum, uses a holdfast to attach to ledge and other rocks. (The small, spiral-shaped organisms on the ledge (foreground) and on one of the stipes of the plant is a polychaete worm that lives in a calcareous, coiled tube and is called Spirorbis borealis.)
These individuals of Spirorbus borealis were found on the underside of a small rock. To see other examples of this species in Europe click here.
A close inspection of the holdfast area of Ascophyllum will reveal a suite of small organisms, including polychaete and nemertean worms, snails, amphipods, and bivalves such as the blue mussel. The periwinkle, Littorina littorea, appears in this photo.
A dense stand of Ascophyllum nodosum occurs in the mid and lower intertidal zone along the rocky shore near DEI. This species is harvested commercially in many areas of the Northern Hemisphere where it grows (e.g., France, Scotland, Ireland, Norway, Iceland, Nova Scotia, New Brunswick, Maine). Commercial uses include stabilizers and thickening agents for paints and some foodstuffs, soil conditioners/amendments (fertilizers), animal feeds, cosmetics, and pharmaceuticals. For more information about its commercial uses, there are many interesting sites you can visit, including these two: FAO, the seaweed site, and several commercial sites, including one from Norway and one from Nova Scotia. A recent report on the short-term effects of seaweed harvesting on algal biomass and associated fauna was released by Drs. Thomas Trott (Department of Biology, Suffolk University) and Peter Larsen (Bigelow Laboratory). A published account of the dynamics of Ascophyllum during its first year was conducted on nearby Swans Island by Drs. Steve Dudgeon and Peter Petraitis.
Like Fucus, Ascophyllum nodosoum is dioecious (male and female plants are separate). Gametes are stored near the tips of the branches of each plant in receptacles, and release of gametes occurs sometime in late March to early May. The receptacles dry out during low tide, squeezing out the eggs (from the oogonia) and sperm (from the antheridia) that are washed into the water by the incoming tide where fertilization occurs. Fertilization success and subsequent recruitment of sporelings have been investigated by several researchers in Maine (D r. Robert Vadas and colleagues; Dr. Steven Dudgeon and colleagues) and show high mortality of these small plants due to a variety of factors; therefore, Ascophyllum relies on vegetative growth mainly to replace biomass lost annually to consumers such as periwinkles, storms, and commercial harvesting.
This close-up of the receptacle of Ascophyllum was taken on May 1, 2004 at a rocky intertidal site near DEI. It shows the swollen receptacles at the tips of an individual. The circular "dots" that can be seen inside each receptacle are the conceptacles. It is not possible to discern the gender of this plant because it may have already released its gametes. The conceptacles of female plants are olive-drab whereas the conceptacles of the mature males are orange due to the caratenoid pigments in the sperm.
These are "spent" receptacles. (May 1, 2004)
It is possible to follow the growth rate of Ascophyllum through the year because individuals can be aged. This photo shows four nodes, or air bladders. The two on the right are closest to the holdfast. The two on the left appeared approximately one year later. Therefore, the distance between the base of one node to the base of the next node represents one year of growth. Counting the nodes on a single stipe from the holdfast to the tip of the plant will reveal its approximate age. It is an approximate estimate of age because when a plant has been damaged due to ice, storms, wave action, disease, or herbivory from snails, it can begin to grow again vegetatively (asexually). This means that if you counted 10 nodes for a single individual, the plant was at least 10 years of age. It could be that the individual is really 25 years of age, or older.
In the spring or winter, you can measure the previous year's growth of individuals of Ascophyllum by recording the distance from the base of the apical node (the node furthest from the holdfast at the very end of the plant) to the tip. A close inspection of this photo reveals that new nodes (air bladders) are beginning to form at the end of each tip (to the far left). A penny is approximately 19 mm in diameter. Using the penny for scale, the estimate of last year's growth is 90 mm (about 3 1/2-inches).
This photo shows damage to an individual of Ascophyllum that is typical of herbivory. The culprit is a small snail, like the one in the upper portion of the photo near the shiny penny. The resilience of this species should be noted, as a new reproductive structure has begun to grow from the damaged tissue. The surface cells of Ascophyllum have the ability to regenerate lost tissue (totipotency).
Another example of damage due to hervibores (snails).
This is an example of an epiphytic red alga, Vertebrata lanosa, growing on Ascophyllum. This is an obligate relationship (restricted to a specific host) for this red alga. Other members of the red alga group provide valuable biogels for DNA and other molecular studies and research.
This diminutive snail (Littorina obtusata) is the primary herbivore of Ascophyllum. Typically smaller than a penny, it lives within the dense stands of Ascophyllum and can easily be counted during low tide surveys. Littorina obtusata is also known as the smooth periwinkle. It ranges from Greenland to New Jersey in the northwest Atlantic, and in Europe, where it is known as the yellow or flat periwinkle, it ranges from northern Norway to the south of Spain. The smooth periwinkle reproduces in the summertime around DEI. Sexes are separate and fertilization of the eggs occurs internally in the female. A white, kidney-shaped egg mass is laid on the stipes of Ascophyllum, but you can also see these from time-to-time on the blades of eelgrass, Zostera marina. Each egg mass can contain up to 300 eggs. Juveniles crawl out of the eggs about a month after the egg mass is laid. Sexual maturity occurs in about 2 years. Littorina obtusata comes in many different color variations.
This is another herbivore commonly found in the rocky intertidal near DEI -- the amphipod Gammarus oceanicus. It grazes mainly on detritus, or non-living particulate organic matter, but it also can graze on the living Ascophyllum stipes or shoots.
Another grazer of microalgae, including benthic diatoms, filamentous forms, and juvenile stages of several macroalgae, is the tortoiseshell limpet, Testudinalia testudinalis. This organism can grow to about the size of a penny as an adult.
Other rocky intertidal organisms that can been seen, counted, or measured include the ubiquitous northern acorn barnacle, Semibalanus balanoides. Barnacles, like amphipods, are arthropods called crustaceans. Other crustaceans include crabs, lobster, shrimp, and krill. Barnacles are filter feeders and ingest particles (microscopic plants and small animals) that are suspended in the water column.
Another common organism of the rocky shore is the blue mussel, Mytilus edulis. It, too, is a filter feeder, and has been shown to ingest everything from microscopic plants (phytoplankton) to zooplankton such as copepods and the swimming larvae of a variety of organisms such as barnacles, clams, and snails. This photo shows two byssal threads produced by the foot of the mussel used to attach itself to hard objects. Typically, the byssal threads attach to ledge or rocks and this provides stability so that they do not get tossed around in swift currents or during storms, but in this case, the threads are attached to a periwinkle. Sometimes periwinkles become entrapped in dense beds of mussels that attach multiple threads to a periwinkle shell. This biological tethering can result in periwinkle mortality because it reduces the area that the snails can forage. A closer look at this periwinkle shows several old attachment sites of mussel byssal threads.
A variety of intertidal predators can be found along DEI's rocky shore. This is one of the most common, the dog whelk, Nucella lapillus. Dog whelks are predatory snails that feed primarily on barnacles and mussels. They evert their proboscis that exposes a radula that they use to drill a very small hole (straight-sided, as when you use a drill bit to drill a hole in a piece of wood). When the hole is complete, the tissue of the mussel or barnacle are digested and removed by the predator.
A small rock turned over on March 13, 2012 revealed approximately 20 green crabs, Carcinus maenas. This is a small handful from under the rock. Green crabs are an invasive species, which means that they were not always located along our shores. The green crab invaded Maine in the late 1800's, and was discovered in Portland Harbor around 1900. Green crabs were unknown to people in downeast Maine in the vicinity of Beals Island before the early 1950's. When they arrived, they decimated soft-shell clam populations. No natural predator can keep green crab populations in check. The weather, especially severely cold winters, is about the only thing known that can check the growth of green crab populations. Green crabs are native to the British Isles. They arrived in North America near Long Island, New York around the time of the Civil War (ca. 1865), and migrated both north and south. Today, on the east coast of North America they range from Virginia to Newfoundland. In the rocky intertidal, they prey on everything from small mussels to periwinkles, limpets, worms, and they are also cannabalistic.
Examining the underside (ventral side) of the green crab reveals its sex by exposing its telson, or terminal segment of the body. The shape of the telson indicates the gender of the crab. Here, the telson is wide and is shaped like an upside-down "U." This is a female crab, and the telson is so wide because it functions to protect eggs that, like lobsters, will remain with the female for many months prior to hatching. In males, the telson is more V-shaped.
In males, the telson is more V-shaped. For two crabs of similar size, the female telson is 2-3 times larger than that of the male. (Photo courtesy of Eric B. Tan)
Green crabs are exotic or invasive species. Read about how green crabs have influenced the ecology of rocky shores here on the East Coast of North America and on the West Coast. For a perspective on green crab invasions in Canada, click here.
DEI appreciates the help and assistance given by Dr. Robert Vadas to improve this page. Thanks, Bob!