We will keep adding to this lecture throughout the term. Here is a very abbreviated introduction to Crustacea or crabs, shrimp, lobsters and barnacles
They are characterized by a five segment head, two antennae and a long trunk divided into two or more tagmata.
Crustaceans are the dominant arthropods in the marine environment but it's not because of large forms. It's the smaller microscopic crustacea that are important as the primary herbivores. The oceans contain huge numbers of microscopic planktonic. Crustaceans that are the ocean’s primary herbivores. These small microscopic animals feed on the primary autotrophs which are single celled. It’s hard to imagine the impact of these singled celled plants and the microscopic organisms that feed on them until you realize that they can be so abundant that they are the only food that some whales eat.
Copepods and Krill are some of the most common crustaceans found in marine environments.
Copepods use their antennae in swimming.
Krill are an important food source in the marine food chain (with 99 million tons harvested per year). They can reach densities of 30,000 individuals per sq. meter. You observed brine shrimp in laboratory that have a similar shape. In some species, six legs attached to the thorax can form a very effective "feeding basket" used to collect phytoplankton from the open water. Krill swim by using their abdominal legs usually at a pace of 5–10 cm/sec.
As can be seen from the pictures below, there is a lot of diversity in shape. The world of the crustaceans is actually a world of bizarre shapes and adaptations.
Most sport biramous appendages that are highly modified for various functions. Review evo 101.
Parasitic crustaceans.
Lernaeocera is a parasitic copepod found in the gills of marine fish. The life cycle is similar to that of the anchor worm, Lernaea spp., mentioned in the first lecture. The anchor part is considerably stouter, and the egg masses are larger and more convoluted.
Gas exchange is through diffusion across the body surface in small forms; gills are used in larger forms. The heart is ostiate, located on the dorsal surface, and pumps blood into the hemocoel surrounding the internal organs. In large crustacea blood vessels extend from the heart to different parts of the body before releasing the hemolymph into the hemocoel (compartment made by sinuses).
Coxal, antennal or other more proximal glands: (ectodermal and coelomic organs) along with the gills are the organs commonly used for excretion.
Basic characteristics also include larvae that can be highly modified in many species.
In Daphnia eggs hatch in the brood chamber and the juveniles, which are already similar in form to the adults, are released in approximately two days when the female molts. Note also small thoracic appendages. This is because Daphnia move through the water in a series of“hops” produced by rapid strokes of feathery paired antennae.
Many small crustaceans have a similar life cycle, and in many species females carry their eggs in brood pouches.
Crustaceans usually have separate sexes, and fertilization is also usually internal.
The large crustaceans copulate by pressing their ventral sides together. The male transfers sperm into the female's sperm receptacle by using a pair of hollow swimmerets that serve as copulatory organs. The female deposits her fertile eggs in a sticky mass which she attaches to the underside of her abdomen. The young hatch as swimming larvae, and molt several times to achieve the adult body form. Crabs have a more complex mating ritual, since the male must wait for the female to molt before copulating,
Barnacles are unusual sessile crustaceans. The body is surrounded by pairs of fixed calcareous plates and is protected by another pair of plates which close the opening. Barnacles are divided into two groups: the acorn barnacles (Balanomorpha) in which the plates attach directly to the rock, and goose barnacles (Lepadomorpha) which are attached on the end of a stalk.
Inside the “shell” however is a segmented body with six pairs of segmented legs used for filter-feeding from the water.
Some odd parasitic crustaceans share a similar larval stage, called a “cyprid”, with more familiar barnacles. All use the first antennae as a form of attachment, the barnacles attach themselves upside down on a rock with their feet in the water.
Surprisingly, barnacles outcross even if hermaphroditic, acting males having proportionately probably the largest penis in the animal kingdom. It reaches out of the shell and transfers its sperm to a female neighbor. After fertilization the female barnacle (or female acting barnacle) broods its eggs until they hatch as a minute cypris larva which swims until it is ready to settle head-down on a suitable surface.
