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Sea of Cortez 810
Common Invertebrate Phyla in
The Sea of Cortez

porifera - sponge, Sea of Cortez, Baja MexicoPhylum Porifera

Probably the most primitive animals known, sponges exhibit an asymmetrical body plan and only a cellular level of organization.  With no true circulatory system, both gas exchange and feeding is accomplished by filtering water through the sponge’s tissues.  The beating of small flagellated cells called choanocytes is responsible for this water movement.  Sponges are broadly differentiated by the composition of their supporting structures (spicules).  These spicules may be calcareous (Class Calcarea), Silica-based Hexactinellida), or made of a protein called spongin (Demospongiae), the latter are often referred to as “bath sponges”. 

bundosoma mexicana anemone in Puerto Penasco, Sonora MexicoPhylum Cnidaria
Corals, Anemones, Jellyfish,
Hydroids, Gorgonians, Sea Pens

The word Cnidaria has a Greek origin, meaning “stinging nettle”, and anyone who encounters some of the more toxic species of this phylum will quickly appreciate this etymology.  The stinging power of cnidarians comes from the characteristic cell, the cnidocyte.  Cnidarians also show a tissue level of organization, and radial symmetry.  Cnidarians are classified into six classes, although only four are common.  These are:  Anthozoa (Anemones and Corals), Scyphozoa (Jellyfish), Cubozoa (Box Jellies), and Hydrozoa (Hydroids and Siphonophores including the Pacific Man o’ War, Physalia pacifica).    

ctenophorePhylum Ctenophora
Comb Jellies

Members of the phylum Ctenophora were originally classified as a single phylum with the Cnidarians within Coelenerata, but lack the stinging cnidocytes of the cnidarians and are now their own phylum. Ctenophores are called comb jellies due to the rows of beating cilia that provide limited propulsion as well as prey capture.  A relatively small phylum, containing only about 100 described species, the ctenophores can nonetheless become very abundant locally to the point of becoming a nuisance for fishermen.  Comb jellies are identified by the rows of beating cilia that lie along the body, and the presence of a pair of tentacles in the middle region of the body.  Most are colorless, but some deep-water species are dramatically pigmented.  Although most are translucent, the beating cilia can create a shimmering rainbow of colors along the body. 

platyhelminthes flatworm Sea of Cortez, Baja, MexicoPhylum Platyhelminthes

Flatworms are among the most primitive of the bilaterally symmetrical and cephalized (have a head region) animals.  They lack a true respiratory system and body cavity (acoelomate).  Most classes are parasitic, only the turbellarians are considered free-living. Of these, there are some spectacular examples in the Sea of Cortez, including the Mexican skirt dancer.  Flatworms are extremely thin to maximize surface area for gas exchange, and many are brightly colored, a warning about their often toxic or noxious compositions and secretions.    

ribbon worm, nemertea, in Sea of Cortez, Guaymas, Conora, MexicoPhylum Nemertea
Ribbon Worms

With only about 1000-1300 known species, ribbon worms are a comparatively small group of worms, but they are nevertheless one of the most highly visible worm species in many seas.  Some species of ribbon worms can approach the length of a blue whale (30m), and may contract and expand their body sizes dramatically.  A two-foot zebra worm in the Sea of Cortez may expand to over 25 feet when its body is extended to hunt at night.  Hunting is facilitated by the presence of an eversible proboscis.  This proboscis is the key characteristic of the phylum, and in some species may be equipped with a neurotoxic stylet.      

eurethoe complanata fireworm in Baja, Sea of Cortez, MexicoPhylum Annelida
Segmented Worms

Annelids, the segmented worms, are a large (15,000 spp.) and diverse phylum of worms.  Successful in a wide variety of environments, the most familiar non-marine varieties are earthworms and leeches.  Within marine environments, tubeworms are among the most conspicuous annelids that scuba divers might encounter, while there are myriad interstitial and subterranean species such as fireworms.  Segmentation has allowed for tremendous specialization and annelids possess other advanced characteristics, such as a true coelom, a one-way gut, and a closed circulatory system.   

octopus in Puerto Penasco, Sonora, MexicoPhylum Mollusca
Bivalves, Snails, Nudibranchs,
Octopuses, Chitons, Limpets etc.

The mollusks are a large and morphologically diverse group.  The >100,000 members in phylum Mollusca vary in shape and size from tiny slugs and snails, to giant clams and squid.  There are four common classes of mollusks that one might expect to find while tidepooling or scuba diving, as well as several smaller or uncommon classes.  The major classes include: Gastropoda (snails, limpets, slugs and nudibranchs), Bivalvia (oysters, clams, scallops, etc), Cephalopoda (squids, octopus, cuttlefish, nautilus), and Polyplacophora (chitons).  Cephalopoda includes some of the most neurologically advanced invertebrates known.  The Sea of Cortez is gaining notoriety for the abundance of jumbo Humbolt squid in the waters near Santa Rosalia.

lysmata californica in Puerto Penasco, Sea of Cortez, Sonora, MexicoPhylum Arthropoda
Crabs, Lobsters, Barnacles, Shrimp,
Amphipods, Copepods, Isopods, etc.

Arthropods are certainly one of the most successful and widely dispersed of the modern animal phyla, representing 80% of described species.  The majority of arthropods are terrestrial, with only about 50,000 species contained within the predominantly aquatic subphylum Crustacea.  Some of the key features for identification of arthropods include a rigid chitinous exoskeleton, a segmented body, and jointed appendages.  They exist in nearly every conceivable habitat, and can range in size from nearly microscopic copepods, to crabs with 12-foot leg spans.      

tamaria sea star Sea of Cortez Baja MexicoPhylum Echinodermata
Sea Stars, Sea Cucumbers, Brittle Stars,
Sea Urchins, Crinoids, Feather Stars, etc. 

The largest entirely marine phylum, the echinoderms are widely distributed throughout the world’s oceans.  Well-known members of this phylum include the sea stars (starfish), sea urchins, sea cucumbers, and brittlestars.  Key identification features include pentaradial symmetry (internalized in some species), a water vascular system, and tube feet.  The name echinoderm literally means “spiny skin” and is quite reflective of many members of this phylum.  Mouths are central among the arms.  Hundreds or thousands of tube feet that facilitate movement give these seemingly sedentary animals remarkable motility and strength.   

tunicate in Sea of Cortez, Sonora, MexicoPhylum Chordata
Tunicates, Salps, Lancelets

As the closest relatives to humans within the invertebrates, the chordates, specifically the urochordates, bear much more superficial resemblance to sponges than to vertebrates.  The urochordates most often encountered by scuba divers and snorkelers are the tunicates and salps.  Beginning life as a mobile, tadpole-like larvae, most urochordates settle to the sea bottom and attach, becoming a filter-feeding or photosynthetic organisms (tunicates).  Some form massive chains of pelagic adults (salps) that are often mistaken for jellyfish or ctenophores.  Despite the primitive appearances of the adults, juveniles possess a notochord, pharyngeal gill slits, a hollow dorsal nerve tube, and a post-anal tail, placing them squarely within the chordates. 




Updated August 28, 2009

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