Sponges: phylum Porifera

Introduction

Sponges are so simple that most people wouldn't recognize them as animals. Unlike other animals, sponges lack organs and "true tissues," and they have only a few types of cells. In animals, a tissue is defined as a group of similar cells that acting together as a functional unit and are separated from other tissues by a membrane of extracellular matrix. Sponges have some specialized cells, but they don't meet this strict definition of tisues.

Almost all sponges live in the ocean. They are suspension feeders, meaning that they capture small particles suspended in the water. Many sponges live in environments like coral reefs, where they continually pump water though their bodies to capture food particles. Unlike most animals, sponges don't have any sort of gut; their specialized choanocyte cells capture bacteria, engulf them by phagocytosis, and perform intracellular digestion. Not only do sponges consume bacteria, they also have large numbers of bacteria living within their bodies. In fact, the diversity of the sponge microbiome is immense; there may be many more bacterial species living within a sponge than there are in the surrounding water or sediments. This fact, coupled with the efficient water pumping of sponges, may have allowed sponges to substantially change the chemistry of the world's oceans through the actions of their internal bacteria.

Sponge phylogeny

Sponges are often considered to be basal animals, meaning that this group was one of the first to branch off in the animal cladogram (although it's not entirely clear whether sponges represent the very first branch). There has been some debate about whether the phylum Porifera are truly a monophyletic group. Some researchers (and some editions of Campbell) have suggested that the various groups of sponges should actually be classified as separate phyla, but the most recent evidence seems to say that the Phylum Porifera is a true monophyletic group that includes all the sponges.

Leucosolenia

Leucosolenia is a member of the class Calcarea, and it has spicules made of calcium carbonate. Our slides show whole mounts or chunks of Leucosolenia. You probably won't be able to make out the individual cells or see much of the body structure; the main thing you can see here is the spicules.

Remember to adjust the condenser on your microscope to make these transparent spicules easier to see.

Scypha

Scypha, like many sponges, is roughly tube-shaped. In this cross section you can see canals that open to the outside (incurent canals). Water is drawn in through these canals and then pulled through small passages to inward-facing canals, called the radial canals. The radial canals are lined with choanocytes, described below.

This picture shows a longitudinal section of Scypha at slightly higher magnification. The choanocytes, or collar cells, are the most distinctive cells of sponges. Each one bears a flagellum that moves water through the canal and a collar that helps to capture food particles from the water. These cells line the radial canals that carry water into the central space of the sponge, called the spongocoel. The choanocytes form a tight layer of tall, densely packed cells. There are a couple of other cell types visible as well, but you won't be tested on the names of these cells.

References & further reading

General Information about sponges

Campbell Biology, Chapter 33. In particular, look at fig. 33.4 (10th ed.), which diagrams the structure and function of a sponge.

Porifera on Animal Diversity Web.

Porifera on Invertebrate Anatomy Online.

Sponge on Wikipedia.

Sponges and their microbiomes

Sponges are animals, and they consume bacteria and other microorganisms. However, they also have other microbes that live inside their bodies, mixed in with the sponge's own cells. In fact, a sponge could be described as a blend of animal cells and bacterial cells. It's not entirely clear how these various cells interact. There are a great many articles on this topic, many of them highly technical. In the past, the bacteria living inside sponges were very difficult to isolate and study. In recent years, researchers have circumvented this problem by grinding up sponges, sequencing the DNA, and identifying the microorganisms by their gene sequences. They have found a remarkable diversity, which may have had substantial effects on ocean ecosystems and chemistry.

Diversity, structure and convergent evolution of the global sponge microbiome. Nature Communications, 2016

Different Sponge Species Have Highly Specific and Stable Microbiomes, MBL Scientists Report. Press release from Marnine Biological Laboratory.

Were early seas transformed by sponge microbiome? New Scientist

Sponge symbionts and the marine P cycle. PNAS. Bacteria living in sponges have accumulated large amounts of phosphate, altering the phosphorus cycle of the world’s oceans and thereby affecting the abundance of O₂. See also: Phosphorus sequestration in the form of polyphosphate by microbial symbionts in marine sponges.