Internal Morphology & Physiology

Left image shows general internal morphology of a sea anemone (Ruppert et al. 2004), compared with a sectioned and DAPI stained microscopic image of a specimen at 4x magnification.

Most sea anemones have hydrostatic coelenterons as their skeleton, although some secrete a protective layer of chitin, which gives some rigidity, but is not considered particularly supportive, as it is usually restricted to the pedal disc or below the introvert on the column.  The coelenteron is a multifunctional space within the anemone, providing functions such as digestion, circulation, absorption and in some cases brooding.

There are usually well developed endodermal longitudinal muscle bands in the septa that are anchored to the oral and pedal discs, used for retraction.  Ectodermal muscles are restricted to the oral disc and tentacles, which is particularly formed for effective capture of prey and feeding, whereas the circular muscles in the column help to elongate the animal.

Image shows principle muscle fields showing major axis and direction of contraction. 1. Longitudinal muscles, 2. Radial muscles, 3. Circular muscles, 4. Oblique muscles (Shick 1991).

As part of phylum Cnidaria, sea anemones possess the simplest nervous system of the metazoans, in that they have nerve nets.  These nerve nets join together at the pharynx, septal insertions, pedal and oral discs and across the mesoglea. Although considered a simplistic system, the communication across the nerve net is complex and studied extensively, and it is thought that there is some communication for the anemone to be able to determine when it should or should not fire it’s energetically expensive nematocysts.

Acontia were found in this specimen that were white and threadlike, attached somewhere within the anemone, probably to the borders of the mesenteries.  These acontia were observed to be coiled and moving independently within the tentacles, a video of which can be seen below.  It is believed that acontia are primarily a defence mechanism, as the nematocysts within the acontia are detachable and hence the animal is not bound to the predator when these are fired.  The type of nematocysts found in acontia are usually basitrichs, also known as b-mastigophores, which does appear to be the type of nematocyst found in this specimen, however, microbasic mastigophores and microbasic p-mastigophores are also found. 

Nematocyst cells within the acontia, taken at 40x magnification, right hand image shows them in cross section.  Scale of 10um relates to both images.