The Puzzle of Tunicates

The Puzzle of Tunicates

One of the smallest inhabitants of the reef is the tunicate. Tunicates do not come up much in the creation-evolutionism debate, which is a real pity because these tiny organisms are truly unique and worthy of our study.  This article will introduce you to these beautiful denizens of the deep and explain how their incredible design could not have evolved.

Tunicates are invertebrates but are classified in phylum Chordata with the vertebrates due to a specialized structure called a notochord.  This notochord functions as a pseudo-backbone, but only in the larva. After metamorphosis, the tunicates lose their notochhord.  Tunicates take their name from their tough, leathery tunic, made of cellulose.  This tunic only exists in adults. The larval form is almost tadpole-like in appearance.  As a larva, the tunicate is mobile, and swims until it finds a suitable spot to settle down.  Once it settles down, it undergoes metamorphosis and turns into the adult form.

Tunicates have two siphons, one for intake, the other for outflow.  In order to feed, tunicates bring water in through the upper siphon, called the oral siphon.  The water is drawn into the siphon by the beating of tiny hairs called cilia. The water passes through the siphon and is passed through a strainer called the branchial basket. Since tunicates feed on microscopic plankton, the branchial basket serves as a filter, to remove the plankton from the water so that the tunicate can feed on it. The water is also filtered for oxygen, allowing the tunicate to breathe. Once the food and oxygen have been removed, the tunicate expels the water, along with waste products, out the second siphon, known as the atrial siphon.

For such a small creature, tunicates are surprisingly intricate. They have a heart and circulatory system, as well as a nervous system in the larval stage. Oxygen is contained in plasma and is circulated in much the same way as vertebrates.  The heart can actually reverse the circulatory system. Why it does this is unclear.  Perhaps it is meant to cleanse the circulatory system of any detritus build up, as the tunicates circulatory system is in series, rather than parallel like vertebrates, and potentially open to debris build-up.  Some tunicates accumulate heavy metals in their bloodstream as well, which is believed to deter predation, as the heavy metals are toxic.  Some forms of chemical wastes, rather than being excreted out the atrial siphon, instead pass directly across the surface of the tunic into the water.

Tunicates are hermaphrodites. This means they contain both male and female reproductive organs in their body.  However, they do not self-fertilize.  When they reproduce sexually, the male gametes are either released and drawn into the incurrent siphon where they fertilize another individuals eggs, or both male and female gametes are released simultaneously in something called broadcast spawning.  This involves large groups of tunicates releasing their gametes simultaneously and allowing them to mingle freely. Alternatively, tunicates can reproduce by asexual budding, where a mini tunicate simply grows from the side of the tunicate and pinches off.

Tunicates are a problem for evolution in a number of ways.  They do not have any fossil evidence connecting them to any supposed ancestors. This is not too surprising since they are soft-bodied and thus not easily fossilized. Evolutionists have proposed tunicates arose from a hybrid of a protosome and another, harder bodied, chordate. This supposedly occurred by means of horizontal gene transfer. However, there is no observable evidence that such a horizontal gene transfer occurred, or that such a hybrid occurred.  Further, this same study excluded tunicates from the chordates based on molecular phylogeny, so the study does the standard evolutionary model no favors.

Even if everything in the previous paragraph was accepted as fact, tunicates still present huge problems for evolution. How did their characteristic incurrent and excurrent siphons evolve? Why does their circulation switch directions periodically and how did that evolve? How did their bloodstream adapt to accumulate heavy metals to deter predation without poisoning them? Where did the information for the brachial strainer come from? All these questions must be answered, as must the age-old question of metamorphosis.  When the complex design of something as seemingly simple as a tunicate provides a strong challenge to evolution, what does that say about its ability to explain more “complex forms”? Evolution has no answers.

From a creation worldview, tunicates are easy to explain. The original tunicate kind was made with all the tools it needed to survive in the ocean.  God designed the tunicate to function in a very similar manner to how it functions today.

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