Spiralia

Long nosed weevil edit

Chaetoblack

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EB1911 Mesozoa - Rhopalura giardii

Lepidodermella squamatum (cropped)

Sorocelis reticulosa

Symbion pandora (unlabelled)

Spiralia is a large group of animals that are primarily characterized by the spiral cleavage pattern observed during their embryonic development. This group encompasses a diverse array of organisms, including both protostomes and Lophotrochozoa, with members ranging from mollusks, annelids (segmented worms), and platyhelminthes (flatworms) to lesser-known phyla such as Nemertea (ribbon worms) and Bryozoa (moss animals). The concept of Spiralia is based on both morphological and molecular data, which have provided insights into the evolutionary relationships among these animals.

Characteristics

The defining feature of Spiralia is the spiral mode of cleavage, a distinctive pattern of cell division in the early embryo. In spiral cleavage, the cells divide at oblique angles to one another, resulting in a spiral arrangement. This type of cleavage is determinate, meaning the fate of each cell is determined early in development. Spiralian development is also marked by the presence of a blastopore, which can develop into the mouth or anus, depending on the specific organism.

Classification

Spiralia is considered a clade within the broader group of Protostomia. The classification has evolved over time with advances in molecular phylogenetics, which have helped clarify the relationships between the various members of Spiralia. The group is divided into several subclades, including but not limited to:

• Lophotrochozoa: This subgroup includes animals that have a lophophore (a crown of ciliated tentacles used for feeding) or undergo trochophore larval stage, such as mollusks, annelids, and bryozoans.
• Platyzoa: Comprising mainly flatworms and other small, often microscopic, worm-like animals.

Evolution

The evolutionary history of Spiralia is complex and dates back to the Cambrian explosion, a period of rapid evolutionary diversification approximately 541 million years ago. Spiralian animals are thought to have originated from a common bilaterian ancestor. Molecular studies have provided evidence supporting the monophyly of Spiralia, suggesting that the spiral cleavage pattern is a derived trait that emerged early in the evolution of this group.

Ecology and Habitat

Spiralian animals are found in a wide range of habitats, from marine and freshwater environments to terrestrial ecosystems. Their ecological roles are diverse, including predators, scavengers, and filter feeders. The adaptability and ecological diversity of Spiralia have contributed to their widespread distribution and evolutionary success.

Research and Significance

Research on Spiralia has significant implications for understanding animal evolution, developmental biology, and the evolutionary relationships among protostomes. The study of spiral cleavage patterns, in particular, has provided insights into the mechanisms of early development and the evolutionary conservation of developmental processes.