Porifera: Sponges Anatomy, Types, Reproduction, Ecology & Symbiotic Relationships

Study Notes

Porifera: The Anatomy, Types, Reproduction, Ecology, and Symbiotic Relationships of Sponges

The phylum Porifera, commonly known as sponges, is a diverse group of aquatic invertebrates characterized by their porous body structure. Sponges are among the oldest extant Metazoan lineages, having arisen over 600 million years ago. With over 9,300 species, they are grouped into four major classes: Calcarea, Demospongiae, Hexactinellida, and Homoscleromorpha. These sessile organisms filter-feed by pumping thousands of liters of seawater per kilogram of sponge per day, making them crucial for benthic-pelagic coupling and the biogeochemical cycling of nutrients in various ecosystems.

Sponges Anatomy

Sponges lack tissues and organs, but they have several types of specialized cells, including collar cells, which trap and digest food, and spicules, which provide support and protection. They have an internal skeleton, or endoskeleton, composed of short, sharp rods called spicules, which are made of silica, calcium carbonate, or spongin, a tough protein. Spicules grow from specialized cells in the body of the sponge.

Types of Sponges

The four major classes of sponges are:

Calcarea (calcareous sponges): These sponges have a calcareous endoskeleton and primarily inhabit shallow waters.
Demospongiae (demosponges): These are the most common sponges, characterized by a siliceous or spongin endoskeleton and found in a wide range of habitats.
Hexactinellida (glass sponges): These sponges have a hexactinellid endoskeleton, which is made of silica and has a unique structure, giving them their name.
Homoscleromorpha: These sponges have a homoscleromorphean endoskeleton, which is composed of spicules made of calcium carbonate and are found in deeper waters.

Reproduction in Sponges

Sponges reproduce both asexually and sexually. Asexual reproduction occurs through budding, where new sponges grow from a parent sponge, maintaining its body plan and characteristics. Sexual reproduction involves the production of eggs and sperm, which are released into the surrounding water through the osculum. If sperm enter a female sponge through a pore, they may be trapped by collar cells and delivered to eggs inside the female body, where fertilization takes place. The resulting zygote develops into a larva, which is motile and covered with cilia that propel it through the water. As the larva grows, it becomes more similar to an adult sponge and loses its ability to swim.

Ecology of Sponges

Sponges have symbiotic relationships with various species, providing shelter for algae, shrimp, and crabs in return for nutrients. They are also a source of food for many species of fish. Sponges that live on coral reefs have symbiotic relationships with other reef species, providing shelter and support. These sponges are vital to benthic-pelagic coupling and the biogeochemical cycling of nutrients in tropical, temperate, and polar ecosystems.

Symbiotic Relationships in Sponges

Marine sponges form symbioses with diverse microbial communities that can be transmitted between generations through reproduction. Vertical transmission is widespread but not universal, and microbes are vertically transmitted during a select developmental window. The properties of the developmental microbiome depend on whether a species is a high or low microbial abundance sponge. While adult sponges seed the next generation, there is a degree of horizontal acquisition of symbionts, with larvae sharing ~45% of their bacterial taxa with the parents and ~55% of the bacterial taxa associated with sponge larvae being acquired horizontally. The symbiont communities of sponge developmental stages show topological congruency and a phylogenetic signal, suggesting that the developmental stage of sponges inherit a diverse bacterial community.

Description

Explore the fascinating world of sponges in the phylum Porifera. Learn about their anatomy, classification into different types, reproductive strategies, ecological importance, and symbiotic relationships with various marine species.