THE ARTHROPODA - SHRIMP, CRABS, SPIDERS INSECTS, ET AL.

Arthropoda means jointed leg, and refers to the jointed chitinous exoskeleton that is the hallmark of the most diverse phylum, with over 500,000 species. The Phylum Arthropoda consists three great groups: 1) the Superclass Crustacea, which includes shrimp, lobsters, isopods, and many other classes; 2) the Superclass Unirami, which includes the millipedes (and relatives, and the insects; and 3) the Chelicerata, which includes the horseshoe crabs, the scorpions, and the spiders, among others. The Crustacea are mostly aquatic, the Unirami mostly terrestrial, and the Chelicerata, is split between marine and terrestrial forms.

Above, is Ligia occidentalis, the rock isopod, or "cucarachas del mar". It is an active scavenger on rocky beaches and be disquietingly abundant. It is also quite large reaching in excess of 10 cm. It is especially densely populous along the sea wall at Puerto Penasco. Apparently, although acting fully terrestrial, they cool by evaporation and must replenish there fluids from the sea. Curiously they often seem to migrate endlessly in one direction along the sea wall, but they must turn around some time. They also exhibit a marked diurnal color cycle, light colored during the night, as above, and dark during the day. Isopods do have some fully terrestrial members, the most common of which is the so called "pill bug" or "sow bug", so common in rotten but fairly dry vegetation

The fiddler crab, Uca princeps

Uca, the fiddler crab is a very common crab above the high tide mark. It digs deep burrows into the mud and silt that it races into when disturbed. Like many other decapod crustaceans, Uca straddles the terrestrial and aquatic ecosystems. It is very common at Estero Morua among the halophytic plants.

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Some decapod crustaceans, such as hermit crabs (right) have a soft abdomen which they protect with the shells of gastropods. Much of the life of these crustaceans is spent squabbling with other crabs for a good shell. Hermit crabs are nearly omnivorous scavengers and filter feeders.

The hermit crab, Clibinarius digueti

A very peculiar group of crustaceans are the barnacles. This group, thought for many years to be mollusks, has a free living larval form which attaches its head to a solid substrate, sticks its feet up in the water for filter feeding, and develops a hard calcareous shell.
The acorn barnacle, Chthalamus fissus. This is probably the most common barnacle in the Gulf of Mexico, strongly dominating the high intertidal zone. It is the common prey of the thorn snail Acanthina (see Phylum Mollusca).	The barnacle Tetraclita spp. Tetraclita is generally more abundant in the upper middle tidal range, below, but overlapping with Chthalamus. Tetraclita is larger and has indistinctly separate plates with deep thin grooves.

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