No. 46–Jellyfish, Part I

With thousands headed daily to local beaches and having recently received a call regarding jellyfish spotted in Battery creek, these fascinating creatures seemed an apt topic for this month’s column.

The cnidarian phylum is one of the most fascinating, diverse and confusing I’ve yet delved into. Fossil traces in a few locations world wide establish their presence during the Vendian period 650 to 540mya. While the corals are well represented, the sparse fossil record for their soft bodied relatives contributes to considerable disagreement regarding the phylogeny of the phylum.

As best as I can tell, the animals most of us recognize as jellyfish are of the order Semaeostomae in the class Syphozoa found in the subphylum Medusozoa of the phylum Cnidaria. The phylum also includes Anthozoa (corals and anemones), Cubozoa (box jellies) and Hydrozoa (many look like seaweed or reef building corals, includes the Portuguese man-of war) but conflicting sources make further classification difficult. For instance, some sources list box jellies as a separate class while Ruppert, Fox and Barnes classify them as scyphozoan.

Cladistic infighting aside, jellyfish are remarkable creatures. The diverse complexities of methods of reproduction, species morphology and life cycles are prohibitively vast for full discussion here. A very general outline may be drawn, however, if we set aside the details that make each species so unique.

Most jellyfish are structurally quite fragile being encased by a single layer of epidermal cells and a gastrodermal layer lining their digestive chamber. Different species have varying amounts of an elastic substance called mesoglea sandwiched between the two layers. Sensory receptors on the edges of the bell called rhopilia anchor a rudimentary nervous system controlling movement and are sensitive to light, gravity, vibration and scent.

There are two separate and distinct stages in the life cycle of jellyfish. There is the adult medusa with its bell shape and tentacles familiar to all and there is the phase polyp seen only by those who know what they’re looking for.

Jellyfish reproduce sexually with the male releasing sperm into the water column from gonads located in the gastrodermal lining. Fertilization occurs within the female and embryos develop, depending on the species, either within the gastro cavity or in brood pouches on the oral arms surrounding the oral opening.

The larval jellyfish (planula) swims free of the mother and cements itself to a hard surface on the bottom and develops into a polyp. The polyp develops over time and by asexual reproduction begins to bud a stack of individual disc shaped clones which swim free to become medusa.

You will recall the earlier reference to “two separate and distinct stages in the life cycle”, they are in fact separate generations with the polyp being the offspring of the medusa through sexual reproduction and the medusa being the offspring of the polyp through asexual reproduction.

We’ll explore this subject further in the next issue and discuss some of the species of jellyfish found in our waters.