The winter beach may appear barren and bare of any marine life. Broken shells and a bit of seaweed may be the only signs of life. However, visit a rocky substrate such as a jetty and you may observe a huge colony of northern rock barnacles (Semibalanus balanoides). The barnacle is the only crustacean that stays in place for its entire adult life. The problem is getting established so that large numbers of barnacles live next to each other. Barnacles tend to settle and grow in areas that are already barnacle colonies. During reproduction the fertilized eggs develop into larvae that swim, grow and molt. These young larvae are called cyprids and will settle by cementing themselves on a suitable substrate. A barnacle will spend its adult life in an upside down position, its head cemented to a rock wile moving its feathery legs to capture small food particles.
Barnacle feeding schedules are determined by the tides. At high tide the legs are beating in the water, trapping phytoplankton while at low tide they close, protecting the animal within from desiccation. The largest barnacles may have several white plates that grow over each other. They grow their exoskeleton by molting. In addition to rock jetties barnacles can also be found attached to other animals such as spider crabs and horseshoe crabs. Barnacles are also very competitive with other colonizers including blue mussels and rockweed for the limited space on a jetty. They are subject to predation by marauding sea stars. Because of these biological pressures and the physical impact of wave action their populations exist in patches on the rock surface.
One of the most intriguing and often overlooked animals is the tubeworm Spirorbis borealis which lives attached to the surface of rockweed (Fucus vesiculosus) and eel grass (Zostera marina). Animals that are attached to the surface of a host such as a plant are called ectosymbionts. Examine the blades of rockweed to see this tiny animal. You will observe a series of coiled tubes in rows. These polychaete worms remove calcium carbonate (lime) from the water to form their tubes which range from 2-4 mm long.
The animals are hermaphroditic and release their eggs into the water after a period of brooding in the tube. The young swimming larvae quickly organize their tubes and attach to a macrophyte. Like the barnacle they can exist in enormous numbers and it is easy to find discarded rockweed in the wrack line covered with them.
Spirorbus is a consumer and uses its feathery tentacles to filter feed small planktonic cells in the water. There are many species worldwide and it is eaten by those organisms that graze on the algae. Use a hand lens to observe the details of this interesting animal.
One of the most commonly misidentified organisms at the beach is Bugula neritina. At first glance it looks like a small clump of brown seaweed, but in reality it’s an animal and belongs to the bryozoan group. This colonial animal is composed of individuals called zooids that have specialized functions such as feeding and reproduction. These zooids collectively obtain nutrition by using their tentacles to filter plankton from the water.
Bugula is a common fouling organism and often grows on the bottom of boats, docks and pilings. It frequently washes up on beaches in large clumps where it gets stranded in the wrack line. It’s this appearance that gives it the name of moss animal. This bushy animal grows up to 15 cm tall and can also attach to shells and algae.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.