Fins to Feet

1.2 Behold, the Tunicates!

Read the the introduction to this Natural History here (Section 1.1).

Disclaimer: If it wasn’t wholly obvious to everybody reading this blog, none of the artwork, photography and video footage included below was created by me.

Every story has a beginning – and this intrepid writer has chosen to plant his first battle standard on the Cambrian ocean floor sediments where the strange and wonderful Tunicates first evolved and thrived (probably). Perhaps someday in the distant future, I will finish my closing chapter on Human evolution, recline in my worn out computer-chair, take a cold, long and well-deserved sip of Dr. Pepper and smile sheepishly. For now, such things must remain pipe dreams. I have only just begun my journey on the long, winding and wondrous road of Vertebrate evolution. Let the games begin!

NOTE: Ciona intestinalis appears to be the model tunicate organism used by many of my sources – many of the specifics (especially numerical figures) I use in the succeeding paragraphs apply solely to this species. Such figures will be denoted by an asterisk.

1.2.0 Tunicates

They might not look the part, but animals resembling these colorful, if unremarkable, colonial sea squirts may have been the progenitors of all vertebrate life on the planet.

Tunicates or Urochordates are members of an animal group that includes Sea Salps, Sea squirts, Pyrosomes and many other sac-like filter-feeding creatures.

Ever since they were first recognized as a distinct taxonomic grouping by Jean-Baptiste Lamarck in 1816 (a prolific naturalist from the early 19th century who is often maligned in biology textbooks these days for his incorrect pre-Darwinian model of evolution), biologists variously assumed that sea squirts and sea salps (and their cousins) represented some strange and little understood lineage of mollusks (perhaps close relatives of bivalves like mussels and oysters) or annelid worms. One of their most peculiar features, however, was the motile, tadpole-like larval form that, like vertebrates, had a distinct head and a muscular tail. Still, few suspected that we shared a closer common ancestry with these lowly sponge-like creatures than with the vibrant and astonishingly successful arthropods or the uncannily intelligent cephalopod mollusks.

In fact, as we will see, the likeness of the tunicate larva to the tadpole is not entirely accidental.

Thanks to the efforts of a gifted Russian embryologist named Alexander Kowalevsky, tunicate larvae were observed to possess the following important characteristics:

– Gill slits (used in adult life for the purpose of filter feeding)

–  A flexible, supportive rod-like structure called the notochord that runs along the length of the tail. This rod is composed of cartilage – the same stiff yet flexible tissue found in the human nose and ear. It may be thought of as a sort of proto-backbone, although the comparison is not entirely sound.

– A tail

– A dorsal (located along the back) nerve cord

Broadly speaking, any organism that possesses these four features at any point during its life cycle is called a Chordate (a phylum of organisms that includes sea squirts, lancelets and vertebrates like ourselves).  Vertebrates are distinguished from other chordates by the presence of a stiff, segmented vertebral column that replaces the primitive notochord during embryonic development.

1.2.1 Tunicate Biology

Anatomical descriptions are unwieldy and uninteresting. Hence, I will try my level best to restrict my cursory overview of tunicate anatomy to as few sentences as possible. I will underline the essential facts that will play a major role in our story.

(From www.earthlife.net)

Most tunicates are immobile – anchored to the ocean floor by small root-like processes called villi*. They sport two lobed openings – one through which water enters the body cavity (the incurrent siphon or mouth) and another through which it exits the same (the excurrent siphon). Tunicates can lengthen, shorten, bend, close and open these two apertures with the aid of circular muscles.

The body of the tunicate can be divided into an upper (pharyngeal) and a lower (abdominal) region. The upper region contains a pharynx.The lower region contains the digestive and reproductive organs as well as a rudimentary heart.

Tunicates do not have blood vessels and the blood merely sloshes around in large sinuses and spaces within the body tissue. They have a high enough ratio of surface area to body volume to rely on the diffusion of carbon dioxide and oxygen through the skin for respiration.

(Wow. Violin solos can make even filter-feeding look intense.)

Tunicates are the only creatures in the animal kingdom that can produce cellulose (the substance found in the cell walls of plants). The epidermis (skin) of the tunicate secretes a tough and often opaque protective covering called the test (or “tunic” – hence the name).

A groove in the wall of the pharynx secretes mucus that helps entrap plankton.

In humans, the pharynx is situated in the upper neck, just beyond the mouth cavity. Unsurprisingly, the Tunicate pharynx is also located posterior to the mouth cavity. It (the Pharynx) is perforated by small slits or clefts (that are analogous to the gills of fish) The Pharynx is essentially an elongated elliptical sac contained within a larger chamber called the atrium.

Perhaps unexpectedly, the primary function of the gill slits of Tunicates is not respiration, but filtration. Food particles (primarily plankton) carried into the pharynx are strained from the water (as the water is forcefully expelled through the gill clefts) and then carried to the gut. Fecal matter is expelled from the body via the excurrent siphon. Tunicates are prodigious filter-feeders – they can filter hundreds of liters of water per day and clear out a significant portion of the bacteria (over 90%) contained within it.

The water, on the other hand, passes into the atrium immediately after filtration and then exits the body through the excurrent siphon. The beating of millions of sub cellular hair-like structures called cilia is responsible for maintaining a steady water current.

Tunicates also have a primitive muscular system of longitudinal (running from the incurrent siphon to the base) and circular muscles that help the tunicate change shape and open and close its siphons. A small collection of nerve cell bodies (formally called ganglia) is observed between the two siphons that may be equivalent to the vertebrate diencephalon. A neural gland is also present – this may be ancestral to the pituitary gland. Unlike sponges, Tunicates can actually respond to touch.

In many tunicate species, individuals may aggregate to form large colonies.

Tunicates are extremely common marine invertebrates, and not all of them are necessarily sedentary. Some are free-swimming. Some Sea salps, for example, use their excurrent siphons for jet propulsion!

1.2.2 The larvae

I fear that I may have driven away a considerable portion of my readership in the preceding paragraphs. Do not fret if you aren’t able to immediately internalize the relative positions and functions of all the anatomical referents I’ve been going on about for the last few hundred words. It will be adequate for our purposes if you can firmly entrench in your mind the idea that –

1)      Tunicates are, for the most part, little sac-like bottom-dwelling animals that feed on plankton.

2)      They are closely related to vertebrates

“But” the discerning reader might object, “You still haven’t shown us how little sponge-like creatures could conceivably turn into fish-like creatures!”

Indeed I haven’t. I have been sloppy in my organization and pacing. But perhaps this question will be adequately answered in the next few paragraphs.

Most Tunicates are hermaphrodites. They eject eggs and sperm into the sea water through the excurrent siphon. Fertilization takes place in the open waters.   Embryonic development takes place astonishingly quickly. Fully formed tadpole-like larvae hatch within 25* hours. It is (principally) the larval form of the tunicate and not the adult form that is of great interest to us.

The tunicate larva enjoys a free life of only 6 to 36* hours before attaching itself to a hard substrate and metamorphosing into the sessile adult form.  It uses three hair-like structures on the head to anchor itself to the soil.

As noted earlier in this post, the larvae possess a cartilaginous notochord that runs along the tail and part of the back. The tail alone constitutes 4/5ths* of the length of the animal. The animal is only 1-2 cm* in length to begin with. The body is covered in test. The front end of the nerve cord is expanded to form a cerebral vesicle (equivalent to the vertebrate brain). The head also bears an eye-spot (a very primitive eye than can sense changes in light intensity and direction) and an otolith (sensitive to linear acceleration and the pull of gravity – these functions are performed by a specialized inner ear in higher chordates).

During metamorphosis many of these intriguing features are lost (the tunicate even digests its own brain).

The similarities between tunicate larvae and vertebrates are striking. These similarities are recounted in 1.2.0.

It is possible, and not altogether uncommon amongst tunicates, for juvenile features to be retained by an organism into the adult phase by a process called neoteny (also called Paedomorphosis). Such individuals can still reach sexual maturity and reproduce. The first fish-like chordates may well have descended from neotenous tunicate larvae. Paedomorphosis or Neoteny can play a major role in evolution. From Wikipedia:

“Neoteny plays a role in evolution, as a means by which, over generations, a species can undergo a significant physical change. In such cases, a species’ neotenous form becomes its “normal” mature form, no longer dependent upon environmental triggers to inhibit maturity. The mechanism for this could be a mutation in or interactions between genes involved in maturation, changing their function to impede this process”

The step between tunicate larvae and antediluvian “boneless” fish-like creatures is not an altogether difficult one.

In fact, members of one tunicate class, namely the Larvacea, retain their muscular larval tail into adulthood and are free-swimming creatures. We will study another group of invertebrate chordates that are even more fish-like in appearance – even in the adult stages.

1.2.3 Why does the notochord/proto-backbone make evolutionary sense?
We must remember that the larval stage of tunicates is essentially a dispersal form. The tadpoles are incapable of even feeding. The larvae are equipped with sensory devices (related above) primarily to seek out suitable places to settle – not for locating food or escaping predation as in most modern vertebrates.  Perhaps tunicates that could disperse their larvae over a larger physical range were favored by natural selection, and hence enjoyed greater reproductive success. The notochord, in its earliest form, appears to have been an adaptation for efficient locomotion (amongst tunicate larvae anyway). Tunicate larvae locomote by throwing their bodies into a series of lateral curves or undulations (rather similar to how eels move). This sort of motion gives the body a considerable forward propulsive thrust (allowing the larvae to travel farther and faster). Without the presence of a rigid supporting structure like the notochord, the body would simply shorten or telescope when the blocks of muscle fibres involved in this sort of movement (collectively called myomeres) contracted. Thus, such locomotion would be fairly inefficient, if not impossible, without the notochord.

Terrestrial plants have come up with all sorts of wonderful and astonishing seed dispersal mechanisms – from hardy fluid-filled coconuts to feathery dandelion seeds. Perhaps our own backbones first evolved under the influence of similar diversifying “trends” – albeit in an aquatic environment and in a different kingdom of life.

The narrative I have reproduced above is not free of controversy – but I have done my best with the sources I have at hand and with my own knowledge.

I apologize if this post was excessively dry or technical. Despair not. With the tunicates safely out of our way, we will track back 550 million years into the past in the next post and explore our earliest origins in a lost Cambrian Ocean. We shall meet the savage Anomalacaris, the clumsy trilobite, the noble Pikaia and our own pater familias, Haikouichthys. Should be fckin awesome.

Beautiful.

Sources:

Ahlberg, Per Erik, ed. Major events in early vertebrate evolution. CRC Press, 2002.
Bone, Quentin, ed. The biology of pelagic tunicates. Oxford: Oxford University Press, 1998.
Berrill, Norman John. The origin of vertebrates. Clarendon Press, 1955.
Tunicates. (n. d.) . In Wikipedia. Retrieved March 14, 2010, from http://en.wikipedia.org/wiki/Tunicates

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