The Longest Animal in the World

Various siphonophore - figure from Dunn et al. 2005

Various siphonophore – figure from Dunn et al. 2005

One of the oddest creatures we’ve encountered on our past few dives is the siphonophore. In fact, it’s been touted as the longest animal in the world with some species growing up to 130 feet long (Praya sp). Siphonophores belong to the group Cnidaria, which includes corals and true jellyfish. You’ve probably heard of the most notorious siphonophore of them all: Portuguese Man O’ War. This species can be found on the surface of the ocean, causing surfers, snorkelers, and divers considerable pain if contact occurs with the siphonophore’s venomous nematocysts (specialized stinging cells that cover the tentacles). While the venom is usually not deadly, there have been a few deaths, primarily caused by allergic reactions to the stings.

man o'war

Portuguese Man O’ War

Portuguese Man O’ War tentacles can extend more than 165 feet and they are known to travel in “packs” of 1,000 individuals or more. Well, I’m not sure that last sentence is fully correct; siphonophores are not really individuals at all, but rather hundreds, sometimes thousands of individuals all working together to cause you and other innocent creatures tremendous amounts of pain. The siphonophore is actually a colony of organisms or as some people like to call it – a “superorganism”.

In William Beebe’s account of his dive with Otis Barton in the Bathysphere back in 1934, he mentioned an encounter with siphonophores at a depth of 320 feet:

At this level they appeared like spun glass…These are colonial creatures like submerged Portuguese men-o’-war, and similar to those beautiful beings are composed of a colony of individuals, which perform separate functions, such as flotation, swimming, stinging, feeding, and breeding, all joined by the common body of a food canal. Here in their own haunts they swept slowly along like an inverted spray of lilies-of-the-valley, alive and in constant motion.

Siphonophore. Photo by Lia Barrett.

Siphonophore. Photo by Lia Barrett

This creature produces asexually, essentially budding off clones of itself in order to grow. However, each bud may be phenotypically different; that is – a siphonophore is comprised of zooids which each possess a specialized function. Unlike other known colonial organisms, the individual components of a siphonophore are not independent; rather, they rely on one another to survive. As Beebe mentioned above, each zooid develops separate functions related to reproduction, digestion, and motion.

Let’s take a look at a siphonophore diagram so you can get a better idea of what’s actually going on. Specifically, we’re looking at one suborder of siphonophores known as Physonectae (Portuguese Man O’War are part of the Cystonectae suborder which differs slightly in structure).

Diagram of siphonophore body plan

Diagram of siphonophore body plan (from Dunn et al. 2005)

(b) A Physonect is a common type of siphonophore we see in the Cayman Trench. The top part called the Nectosome (composed of Nectophores) is specialized for propulsion and controls movement through the water. The Siphonosome has many repeating units comprised of different types of zooids (individuals with specialized functions). The Bract is a protective component, and Tentacles are repeated structures throughout the siphonosome.

(d) Each repeated unit of the siphonosome contains the following specialized zooids: Tentacles (of the GAstrozooids), Palpon – may have excretory or defense functions, GAstrozooids – responsible for digestion, GOnophores – reproductive function.

Physonect siphonophore. Photo by Lia Barrett

Physonect siphonophore. Photo by Lia Barrett

Despite how different each of these components look, they are all descendants of a single fertilized egg. Through a highly regulated process of budding, the siphonophore elongates into a chain of repeating units, each containing their specialized zooids for reproduction, digestion, and protection.

You’ll see in the video below, the real-time unraveling of a siphonophore we encountered on our dive from May 14th.  We happened upon this floating, diaphanous chandelier, which soon transformed into an undulating string of gold. Completely stretched out, this siphonophore was approximately five feet in length and no wider than a broomstick.

Their unique colonial structure begs the question: what is an individual? Do we consider each part of the siphonophore an individual? Or is the entire colony itself form an individual? A great illustrative analogy is provided in this video:

 

——

Sources:

Dunn CW, Pugh PR, Haddock SHD (2005) Molecular Phylogenetics of the Siphonophora (Cnidaria), with Implications for the Evolution of Functional Specialization. Systematic Biology 54(6): 916-935

http://animals.nationalgeographic.com/animals/invertebrates/portuguese-man-of-war/

http://www.siphonophores.org/

http://www.ville-ge.ch/mhng/hydrozoa/sipho/siphonophora.htm

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~ by twonakedapes on June 10, 2013.

2 Responses to “The Longest Animal in the World”

  1. Fabulous photos, narrative, history, videos! Bravo Karl and Annick from AuntAnn

  2. […] of hundreds, even thousands, of individuals (or zooids) – similar to a siphonophore (see The Longest Animal in the World post). Unlike a siphonophore, however, these pyrosome zooids are essentially the same and […]

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