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Introduction

Clytia hemisphaerica (phonetic) is a small hydrozoan-group cnidarian, about 1 cm in diameter, that is found in the Mediterranean Sea and the North-East Atlantic Ocean.

Anatomy and Life Cycle

Clytia hemisphaerica reproduces sexually. Ovulated eggs are fertilized externally and take approximately 24 hours to develop into planula. The ciliated planula will swim freely until the proper external cues, for instance, experimental treatment with CsCl, trigger the metamorphic process; the planula can undergo its metamorphosis into a polyp as soon as three days after fertilization. Once the proper external cue is received, the planula stops swimming and attaches itself to a substrate via its aboral or aboral-lateral pole (what was previously the front end of the swimming planula). After attaching itself to a substrate, the planula contracts along its oral–aboral axis and so forms a flattened holdfast to anchors itself to the substrate. Once the planula is securely on its substrate, a stalk grows out of the holdfast and, eventually, a hydranth, also known as a gastrozooid (an individual polyp specializing in feeding) forms on the anterior end of the stalk. At this stage, the planula is now considered a primary polyp, and this polyp can propagate vegetatively by extending its stolon to form a connected colony of multiple polyps. Polyp colonies are essentially considered immortal; as long as they receive the proper nutrients, they can continuously replace their old parts and expand across their substrate. In addition to the feeding gastrozooids, a mature polyp colony has reproductive gonozooids that produce baby medusa by budding. These medusa reach maturity after 2-3 weeks.

Planula stage of C. hemisphaerica

Like most cnidarians, Clytia has relatively simple morphology. However, despite containing relatively few cell types and lacking elaborate organ structures, the medusae have much greater anatomical complexity than their polyp form. Adult medusa are on average 1 cm in diameter. They are almost entirely transparent, their gonads, radial canals, short stomach, and four-lipped mouth being their most clearly visible anatomical structures. Each medusa has four gonads positioned midway along each endodermal radial canal. The gonads themselves are transparent, allowing for visualization of the oocytes within.

Head of a C. hemisphaerica polyp

Baby C. hemisphaerica medusa

Adult C. hemisphaerica medusa, photo captured from the side

Each medusa typically has around 32 tentacles, each of which are covered in stinging nematocyte cells. These nematocytes are considered a specialized nerve cells despite the fact that they are comprised of a pressurised capsule (nematocyst), a rapid-firing, harpoon-like dart and lethal toxins made for killing prey. Clytia’s nervous system is well-organized and highly specialized. Two parallel condensed nerve rings run around the periphery of the medusa’s bell; the outer rings is responsible for integrating sensory inputs, while the inner ring coordinates motor responses. Specialized balance organs known as statocysts are also located between tentacles. Medusa also have both smooth and striated muscle that allows for the contractions necessary to swim smoothly through the water.