The ecdysozoans

Ecdysozoans

Include 8 phyla that are linked through molecular data and have the ability to shed their external cuticle through ecdysis or molting.

This clade is very diverse but only focusing on Nematoda, Onychophora, and Arthropoda.

Ecdysis

Process in which the outer cuticle or exoskeleton is periodically shed, n=and a new, larger covering is formed. ex. snake

Cuticle

Outer covering found in some ecdyzoans, can be alone or on the outer surface of the exoskeleton; does not have attachment points for muscles.

Exoskeleton

Chitinous outer covering found in some ecdysozoans; does have attachment points for muscles, can be impregnated by CaCO3.

Metamorphosis

Life cycle that includes immature (larva/pupa/or nymph) and mature (adult) forms. Benefit= resource partitioning.

Chelicerae

Specialized mouth parts of the chelicerates

Pedipalps

Modified leg like appendages of chelicerates; used to capture/manipulate food.

Mandibles/ Maxillae

Mouthparts found in some arthropod subphyla.

Cephalothorax

Fused head and thorax region

Pedicel

In spiders, a small waist that connects the cephalothorax and abdomen

Ecdysozoan characteristics

Ecdysozoans are triploblastic, bilaterally symmetrical protostomes with a complete digestive tract. They vary in coelom type and may or may not have body segmentation. Unlike lophotrochozoans, ecdysozoans grow by shedding their outer cuticle or exoskeleton in a process called ecdysis (molting). During ecdysis, the animal secretes enzymes to break down the old cuticle, discards it, and forms a new, larger one. This growth period is brief, and animals often remain hidden while the new covering hardens. An example is the soft-shell crab, which has recently molted and is still forming its new exoskeleton.

Phylum Nematoda

Nematodes, or roundworms, are abundant ecdysozoans found in many environments, with some species being common parasites. They are unsegmented and have a consistent shape due to their internal structure. Their muscles run lengthwise (longitudinal only), causing a whip-like movement. Nematodes are pseudocoelomates and lack circulatory and respiratory systems; their pseudocoelom helps with these functions and serves as a hydrostatic skeleton. Males are usually smaller than females and can be identified by structures like copulatory bursae or spicules, used in internal fertilization.

Phylum Onychophora

Onychophorans, or velvet worms, are segmented, terrestrial ecdysozoans with both a pseudocoelom and a true coelom. They have leg-like appendages called lobopods and use mandibles as mouthparts. Velvet worms are known for their unique mating behaviors and have special slime glands that shoot sticky slime, like a sprinkler, to capture prey.

Phylum Arthropoda

Arthropods are the largest and most diverse animal phylum, making up about 75% of all known animal species, with millions more yet to be discovered. They are segmented coelomates and the only invertebrates capable of flight. Unlike other ecdysozoans, their exoskeleton contains chitin.

Key Characteristics Contributing to Their Success:

• Segmentation: Body segments are fused into specialized regions for efficient movement.

• Chitinous Exoskeleton: Provides waterproofing and protection.

• Jointed Appendages: Allow for fast and flexible movement.

• Highly Developed Senses: Includes vision, chemoreception, and sensory hairs.

• Rapid Reproduction: Short life cycles and large numbers of offspring.

• Varied Respiratory Systems:

  • Gills (aquatic species)

  • Book gills (horseshoe crabs, large spiders)

  • Trachea and spiracles (terrestrial species)

• Metamorphosis: Reduces competition between adults and immature stages by separating habitat and food sources (e.g., mosquito larvae are aquatic, adults are terrestrial).

Arthropod Subphyla:

Subphylum Chelicerata (e.g., spiders, scorpions, ticks):

• No antennae

• Body: cephalothorax & abdomen

• Mouthparts: chelicerae and pedipalps

• Respiration: book gills and/or trachea

Subphylum Crustacea (e.g., crabs, lobsters, shrimp):

• Two pairs of antennae

• Body: cephalothorax & multipart abdomen

• Mouthparts: mandibles and maxillae

• Respiration: gills

Subphylum Myriapoda (centipedes & millipedes):

• One pair of antennae

• Body: head & trunk

• Mouthparts: mandibles and maxillae

• Respiration: trachea

Subphylum Hexapoda (insects):

• One pair of antennae

• Body: head, thorax & abdomen

• Mouthparts: mandibles and maxillae

• Respiration: trachea (some larvae have tracheal gills)

• Only arthropod group capable of flight