Bonellia (annelid)

Bonellia is a genus of marine worms belonging to the phylum Annelida, specifically the class Echiura. Echiurans, sometimes called spoon worms, are a small group of marine animals characterized by a non-segmented body and a prominent proboscis used for feeding.

Bonellia species are notable for their extreme sexual dimorphism. Females are typically relatively large, ranging in size from a few centimeters to several decimeters in length. They possess a sausage-shaped body, often green in color, and dwell in rock crevices or burrows on the seafloor. The most striking feature is the female's proboscis, which can be significantly longer than the body and is used to collect detritus and other food particles.

In contrast, males are drastically smaller, typically only a few millimeters in length, and lack a gut. They live parasitically within the body of the female, either in the uterus or attached to the body wall. Fertilization occurs internally within the female.

Sex determination in Bonellia is environmentally determined. Larvae that settle on the seafloor develop into females. However, larvae that come into contact with an adult female develop into males. This process is influenced by a chemical signal produced by the female.

Bonellia species are found in various marine environments around the world. They play a role in benthic ecosystems, contributing to bioturbation and nutrient cycling. The unique reproductive biology of Bonellia has made it a subject of considerable scientific interest in the fields of developmental biology, endocrinology, and evolutionary biology.