Friday, September 26, 2008


In the life cycle of Scyphozoans, the medusa is the form that predominates. Nevertheless, scyphozoan life cycles also typically include a polyp form called a scyphistoma. The scyphistoma feeds with a ring of tentacles around the mouth. It undergoes asexual reproduction via transverse fission, and short stacks of genetically identical organisms, which will eventually bud off to become physiologically, anatomically separate individuals, are formed at the oral end of the polyp. The tranversly fissioning polyp is called a strobila. The individuals that bud off to swim freely in the water column are ephyrae, which are baby jellyfish. It is the adult jellyfish that makes gametes and reproduces sexually. A nice series of photos illustrating the life cycle of Aurelia labiata can be seen here. The scyphistoma typically exhibits strobilation only during a certain season. At other times it feeds as any solitary polyp would.

Thursday, September 25, 2008


Even though I said earlier that the jellies were my favorite cnidarians, these magnificent, flamboyant sea pens are also in the running. These are colonial Anthozoans, with a single large polyp specialized to support the colony from a central axis, and tiny feeding polyps dispersed along the margins of each "leaf." As in all members of this subclass (Alcyonaria), the gastrozooids have 8 pinnate tentacles.

These animals get to be over a foot tall and they live with the lower, fleshy part of the central poylp planted in soft sediment. And if their appearance in the bright aquarium lights is not thrilling enough, there's more. These animals can bioluminesce, sending eerie, blue-green waves of light along the colony.

Wednesday, September 24, 2008


Thanks to Miriam Goldstein at the Oyster's Garter for writing this delightful post on tunicates; it's entertaining, information-rich, and downright sassy. What more could you want when reading about our closest relatives among the invertebrates?
You mean our closest relatives don't have a face or even a head? If you want to read more about the details that support the assertion that vertebrates and urochordates are sister taxa (and you should), PZ Myers at Pharyngula gives a great explanation about the evidence supplied by developmental genetics. And you can find out what a pharyngula really is; it's more than a blog. There's even a link to his previous post about the Hemichordata, so you can read more about the other non-Echinoderm deuterostome phylum.

Monday, September 22, 2008


The cnidarians exhibit phenomenal diversity within a simple body plan. This phylum includes the sea anemones, corals, sea fans, and hydroids. My definite favorite are the jellies. Claudia Mills, a researcher at Friday Harbor Labs, where I did my graduate work is one of the world's experts on gelatinous zooplankton, which includes familiar, large jellyfish (the scyphozoans), small hydromedusae, large pelagic hydrozoans like the Portuguese Man of War, and comb jellies (ctenophores). The Ctenophora are a separate phylum from the Cnidaria, which include the jellyfish, anemones and corals, as well as the Cubozoa, the scariest class of cnidarians. The sting of all cubozoan species is intensely painful and some are deadly.
Jellyfish blooms are currently in the news, although the causes of population fluctuations are not well understood.
There is a lot of information, some of it conflicting, about what to do if you are stung by a jellyfish. Here is a great place to get the lowdown on the most recent scientific research. They publish an online newsletter every six months that is an annotated bibliography of recent publications on jellyfish stings.

Friday, September 12, 2008

animal phyla

There are about thirty-something animal phyla that are recognized by invertebrate zoologists. There are two reasons the total number of phyla is imprecise. There is some subjectivity involved in determining whether a clade should be considered a single phylum or whether the group exhibits sufficient diversity to be divided into several. It's the taxonomic argument between the "splitters" and the "lumpers." In addition, new data can increase our understanding of evolutionary relationships. For example, based on new molecular evidence, the vestimentiferans are no longer considered a separate phylum, but are now recognized as a group of highly derived annelids.

There are quite a few phyla that are easily named by non-experts, even if they don't know the scientific names. The Arthropoda is the largest and includes the insects, crustaceans, centipedes, spiders, and scorpions. Echinodermata, Mollusca, and Annelida also include critters familiar to most, as do the Cnidaria and Porifera. There are also various other phyla of worms (flat, round, and others) that are speciose, although less well-known by those who haven't taken an invertebrate zoology class. However, in addition to these groups, which include many large-bodied representatives, there are also quite a few so-called "minor phyla" that contain only a few members. Even though they are not numerically dominant, these groups are important to our understanding of the evolution of organismal diversity. Dave Barry's take on the recently described phylum, Cycliophora, shows that minor phyla can appeal even to humorists.

Figuring out how many phyla there are and who belongs in which phylum is straightforward compared with the task of understanding the evolutionary relationships among phyla. Kenneth Halanych gives a very clear description of our current understanding of the phylogeny of invertebrate phyla. This paper was published in 2004, and there are already minor changes to the tree that I will discuss in future posts.

Friday, September 5, 2008

Nothing in biology…

During the term, we will be studying animals without backbones. We will be focusing on marine invertebrates and will delve into comparative anatomy, functional morphology, development, ecology, physiology, and behavior. However, it's important to emphasize that nothing in biology makes sense except in the light of evolution. The famous essay by Theodosius Dhobzhansky says it as well as anyone. It beautifully captures the themes of unity and diversity that will underlie our explorations throughout the term.
So this is first and foremost an evolution class.

Tuesday, September 2, 2008


This is the blog for the Invertebrate Zoology class at College of the Atlantic for fall of 2008. I'll be using this as an additional forum to communicate and share information with students currently enrolled in the class, although I welcome readers and comments from all who share an interest in invertebrates. I am Helen Hess, professor for the course and new blogger.