Barnacle Zone   1 comment

Barnacle Zone



The barnacle zone, situated just below the blue green zone, is composed of a horizontal band of numerous white barnacles.  Adult acorn barnacles Semibalanus balanoides are enclosed in a calcareous shell. The volcano shaped shell consists of a basal part cemented to the rock; a wall composed of six non-movable plates and one or two pairs of movable plates at the top that allow the barnacle to seal itself from the outside environment during periods of stress or alarm.The movable plates can be separated to allow protrusion of the thoracic limbs, and the cirri, for feeding.

When protruded from the shell and spread out, the cirri form a net with which the barnacle scoops  water immediately above the shell opening . When retracted the cirri come together, enclosing any food particles and upon further retraction , food is brought into the mantle cavity where it is scraped off the cirri and transferred to the mouth by appendages called maxillae.Cirri have small lateral extensions which help in holding food particles.

When barnacles are crowded together, they often grow upward from a small base as shown below. Because the base is not as wide as those in less crowded barnacles, they are easier to dislodge.When this happens, more space becomes available for other organisms to occupy.

The barnacle Semibalanus balanoides is a hermaphrodite that internally fertilizes the eggs of a neighbor. Fertilized eggs are brooded for a while and the resultant larvae (Nauplii) are released into the plankton where they develop into a Cyprid larval stage (see below) that eventually leaves the plankton and attaches to the rock surface where it develops into a young barnacle. Two photographs below show sperm removed from the testis of a mature barnacle and a developing embryo taken from an ovary belonging to the same specimen.  During  winter, rocks and pools may be covered with ice or pummeled by wave action (View the sixth and seventh photographs below), killing a number of animals and thereby opening free space for other organisms to attach. Because of the high reproductive potential, barnacle populations often recover from the effects of environmental extremes in a short time.

Note the small white juvenile barnacles attached to the blue mussel Mytilus edulis above and the rock surface in the photograph below.


Barnacles are well adapted for living high in the rocky intertidal for some of the following reasons:

1.They are cemented to the  rock surface protecting them from heavy wave action.

2. They have a protective shell.

3. The shell is white and therefore tends to reflect light keeping the inner body somewhat cooler.

4. They have internal fertilization and brood their young for a short period.

5. They can close their shell for long periods of time. This protects them during prolonged periods of bad weather.

6. They can “breathe” out of water.


During the spring and early summer months the filamentous green alga Cladophora spp. (Chlorophyta) often coats rock surfaces. They tend to disappear later in the summer.The alga consists of a colony of branched filaments each attached to the substratum by a basal hold-fast. The entire plant is flexible and conforms to the incoming wave reducing the amount of drag and minimizing the chance of being torn loose.



The brown algae Fucus spp and Ascophyllum nodosum (Phaeophyta), described in the Brown Algal Zone, can attach here, however they are often damaged by pounding waves as seen below..


Porphyra spp., a red alga (Rhodophora) , consists of a flat thallus (Blade),  and is attached to the substratum by means of a holdfast. A smaller species is described from the Blue-green zone. Porphyra resembles the green alga Ulva (Sea lettuce). They have a broad, thin, flaccid thallus with ruffled margins arising from a  hold-fast. Porphyra is extensively cultivated in Japan and sold under the name nori. It is used in a wide variety of foods and is high in protein, minerals and vitamins.


Blue mussels (Phylum Mollusca, Class Bivalvia) can be found in all of the zones.  Recently ,however, it has essentially disappeared in mid-coast Maine and other  intertidal locations. It grows well however in the water column, attached for example to rope mooring lines, and lines suspended in the water used to commercially grow mussels.

Like the great majority of bivalve molluscs they feed by capturing small particles of plankton, organic detritus and bacteria in mucus on the gills as the respiratory current passes through them. The mucus and the food are swept to the lower margin of the gills by special tracts of strong cilia and are then brought anteriorly along the food groove at the gill margin to the  palps which then  transfer food to the mouth.

Blue mussels are held in place by strong byssal threads secreted by a byssal gland near the base of the foot. The byssal substance consists of heavily cross-linked, highly viscid, adhesive collagen like proteins that undergoe a quinnone-mediated tanning after release from the byssal gland. The gland is applied to the rock, then the protein is secreted and finally the foot is drawn away forming the thread at which point the thread hardens.


Blue mussels have separate sexes. Sperm and eggs are released simultaneously into the water column. Fertilized eggs develop into swimming larvae (Trochophore Larvae) that reside in the plankton for a period of time and then settle on the bottom in areas generally occupied by other mussels. The young juveniles move around seeking a suitable site and eventually attach themselves to a spot with their byssal threads. The video below shows how the foot is extended and retracted allowing the mussel to move itself around.


Blue mussels are preyed upon by the dogwhelk Nucella lapillus described below and starfish. Both of these predators tend to keep barnacles and blue mussels from establishing populations down lower in the intertidal.


Ice formation can cause extensive damage to mussels. When mussels die their shells open and are torn loose providing more space for other organisms to occupy.


Some of the adaptations that allow blue mussels to thrive here are:

 1. Byssal threads that secure mussels firmly to the substratum.

2. A relatively thick shell for protection.

 3. The ability to keep the shells closed for long periods  during stressful times.



The predatory snail Nucella lapillus (Dog Whelk) feeds almost exclusively on barnacles and mussels. It is responsible for helping to prevent barnacles and mussels from colonizing areas in the lower brown and red algal zones. Nucella is unable to live in the upper part of the barnacle zone because of the dry environment at this level. The animal has a thick shell and an operculum that guards the opening into the shell. The animal positions itself on top of its prey and a shell dissolving chemical is secreted on a particular spot from a gland in the mid-ventral portion of the foot. Once the shell is softened the snail extends its proboscis and applies the radula to the site, scraping the loose material away. The process is repeated many times before the shell is penetrated. Then the proboscis is extended through the newly formed hole and the radula is used to scrape mussel  flesh into the mouth.



Nucella lapillus radula X400. Note the sharp central teeth used to scrape away softened calcium carbonate on mussel and barnacle surfaces. A completed hole through a blue mussel shell is shown below.

Mytilus edulis shell with a bore hole created by a feeding Nucella lapillus


The dogwhelk  lays its eggs in capsules that are attached to the rock surface. Development occurs within the capsules.

Note the color variation in a local population of dogwhelks. Would different color patterns be an advantage or disadvantage to dogwhelk populations?



Some of the adaptations that allow the animal to survive in the rocky intertidal are:

 1. The ability to “drill” and consume prey through a thick shell (Blue Mussels for example).

 2. A relatively thick shell that protects the snail against predators and the effects of heavy wave action.

3. A broad, flat, foot that allows adhesion to the rock surface.

 4. Egg capsules that protect the developing young until they are released.



Limpets are gastropods (Phylum Mollusca, Class Gastropoda) that have a conical rather than spiral shell. Limpets move about and feed on algal films when covered with water or while the rocks are still wet, with their toothed radula (shown below). Between tides they tend to cling tightly to the rock, minimizing desiccation and predation. Constant scraping on rock surfaces subjects the radular teeth to considerable wear. As they are worn down or lost they are replaced by new radular teeth pushed up from behind.

Radula Teeth X100




Limpets are well adapted for life in the rocky intertidal through:

1. Possession of a conical shell that offers little resistance to oncoming waves.

 2. A relatively thick shell that protects against predators and the pounding created by heavy wave action.

 3. A broad, flat, foot that allows strong adhesion to rock surfaces.




Other Animals 

Many of the animal species  found in the Bluegreen Zone are also found here. These as well as new types are shown below. Most of them were found between abutting barnacle shells or inside empty barnacle shells where small algae grow or where sediment is found.


A. Phylum Protozoa


1. Ciliated Protozoans


i. Peritrich Ciliate X400 Vorticella spp.


Peritrich Ciliates are characterized by bands of cilia situated at the anterior part of the cell. They are attached to the substratum by a stalk containing a contractile thread. When disturbed they contract as seen in the video below. They feed on small microorganisms that they filter from the water.




ii. Hypotrich Ciliate X400 (Euplotes spp.)


The hypotrich ciliate shown above literally walks using ventrally located large cilia. Each of the large cilia are actually a group of fused smaller cilia. They are also capable of swimming. They graze on a variety of microorganisms as they move across solid surfaces. In the video below, the animal moves along what appears to be the outside covering of a dead blue-green filament filled with small diatoms.


The same species, shown below, moves over the surface of a Cladophora filament. Note the feeding pattern.



B. Phylum Nemertea

The following video shows a nemertean worm located inside an empty barnacle shell.




C. Phylum Nematoda

Nematodes (Round Worms) are  multi-cellular organisms that live in large numbers in almost every freshwater and marine habitat. Longitudinal muscles that run the length of the animal contract on one side and then the other, generally moving the animal forward or backward. Nematodes have a complete digestive system with mouth and anus and most species living in marsh pools feed on microscopic organisms such as bacteria that they suck into their mouths using a muscular pharyngeal bulb (similar to drawing liquid into an eyedropper by squeezing and releasing the bulb) located between the pharynx and intestine. General nematode movement is shown in the video below.




D. Phylum Rotifera

A few rotifers have been observed, however they are too small and too active to photograph clearly.




E. Phylum Mollusca, Class Gastropoda


Juvenile Littorina obtusata are common in the Barnacle and Brown Algal Zones. They can be seen on the surface of barnacles in the photograph below.



 A small juvenile (X40) is shown in the video below moving in and out of its shell. The tentacle, black eye-spot, foot and operculum are visible.



F. Annelida: Class Polychaeta

Fabricia spp.

Fabricia spp., a filter feeder,  is a small polychaete species, about 6 mm long, with an obvious tentacular crown  of ciliated filaments. It can form a temporary tube and is capable of leaving the tube and moving around. It has a pair of anterior and posterior eyespots that are sensitive to light. The first video features an adult while the second highlights a juvenile worm.




F. Phylum Arthropoda (Class Arachnida, Mite)


 The mites, about 3 mm long, have four pairs of legs attached to a stubby, oval body.




G. Phylum Arthropoda  (Class Crustacea, Order


Amphipods are crustaceans that are laterally compressed. Gammarus is a genus of amphipods common to both marine and freshwater environments. For the most part, they are scavengers.  As food passes through their digestive tract, with the help of enzymes and bacterial enzymatic breakdown, they digest food and assimilate it to provide for their needs. The remaining material passes out of their digestive tracts as fecal matter and becomes food for other organisms. They can walk on the sediment surface or swim rapidly on their sides. The specimen in the first photograph was found under a mat of dead eel-grass.


The 10 mm long gammarid  amphipod in the above photograph was moving in and out of crevaces in the Blue-green Zone at low tide, presumably feeding on algae. They are also found in the Barnacle Zone.


The following video features a much smaller salt marsh pool/rocky intertidal  amphipod. The head bears a pair of compound eyes, a pair of antennules slightly above the eyes, two antennae below the eyes, and two maxillipeds underneath the mouth. The thorax, consisting of 8 segments, is attached to the head followed by the abdomen. The maxillipeds are the first thoracic appendages, followed by 2 pairs of gnathopods (2nd and 3rd thoracic appendages). The gnathopods are followed by 5 pairs of pereiopods (4th to the 8th thoracic appendages). The abdomen has 6 pairs of abdominal appendages. The first three are periopods; they are constantly waving back and forth. The remaining three pairs are uropods.Amphipods can crawl as shown in the photographs above, swim sideways or tuck their abdomen under their body and flick it backward creating forward movement.




H. Phylum Arthropoda, Class Crustacea, Order Isopoda

Amphipods are flattened from side to side while isopods are flattened from top to bottom. The video below shows a small isopod removed from an empty barnacle shell.




I. Phylum Arthropoda (Class Crustacea, Order



Several species of copepods live in the Barnacle Zone . They are a common component of marine ecosystems, especially planktonic communities. A long (sometimes short) pair of antennules are visible extending laterally from the head. These are used to propel the animal forward as shown in the second video. Between the two antennae lies a single median eye. Thoracic appendages are visible along the wide portion of the body. The thorax narrows and joins a thinner abdomen. In the photographs below the cephalothorax is the region of the body from the point of the thorax arrow to the right; the thorax is the section of the body from the point of the thorax arrow to the tip of the abdomen arrow and the abdomen comprises the rest.


Note the movement of the harpactcoid copepod  using its antennae in the first video while the second video shows movement by dorso-ventral contractions of the body.




J. Phylum Arthropoda (Class Insecta, Family

Chironomidae, Sub-Family Orthocladinae)

Halocladius variabilis

This species is a non-biting midge that in the larval stage scrapes food with its Labial Plate from the bluegreen mat or the surface of small algae attached to barnacles or large brown algae such as Fucus or Ascophyllum. The adults mate  and deposit their eggs in intertidal areas. The eggs develop into larvae also shown below, that after several molts reach a length of about 8 mm. Refer to additional photographs of Halocladius in the Brown Algal Zone section.



brown-algal-zone-halocladius-1st-proleg-x400-b-in-polysiphonia-lanosa-bi-nov-2011 - Copy.jpg


The following video shows a whole specimen moving back and forth.



Posted November 25, 2011 by zottoli

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  1. Pingback: Boring molluscs and bevilled edges | Solway Shore-walker

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