Difficult-to-Classify Marine Animals: Sea Squirts, Phoronids, Bryzoans and Brachiopods

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Sea squirt in Komodo

Sea squirt (Scientific name: Ascidiacea) are saclike creatures that spend most of their life span attached to rocks, coral reefs and wharf piles, Officially known as tunicates, they are members of the phylum Chordata. Although they are very simple life forms but they are believed to be the ancestors to the world's most sophisticated life forms: vertebrates. The evidence is a primitive proto-backbone found in the sea squirt larvae.

Sea squirts, and all tube tunicates, are thought to occupy a special evolutionary position because they contain a notochord (a sort of backbone-like cartilaginous skeletal rod) and a branchial basket (a cartilaginous structure supporting the gills in lower vertebrates). — structures found mainly in vertebrates. Linnaeus first categorised sea squirts as a kind of mollusk, Alexander Kovalevsky found a tadpole-like larval stage during development that shows similarity to vertebrates. Recent molecular phylogenetic studies as well as phylogenomic studies support that sea squirts are the closest invertebrate relatives of vertebrates. A full genome was sequenced using a specimen from Half Moon Bay in California. It revealed a very small genome size, less than 1/20 of the human genome, but had a gene corresponding to almost every family of genes in vertebrates. [Source: Ana Alcaraz, Animal Diversity Web (ADW)]

Sea squirts are found throughout the world, usually in silty conditions at depths between the sea surface and 500 meters (1640 feet). Most are found near rocky shores and estuaries, where the tide of the ocean meets a river current. They are often found growing in great numbers on man-made structures such as pilings, piers, and even in marine aquariums. Sea squirts settle within the metal plumbing pipes and filters of aquariums. This does not occur when these structures are made of plastic. Sea squirts are also categorized a "common fouling organisms" for piling up on buoys, pier-piles, and hulls Some sea squirts look like World War II sea mines. Those found on reefs can be extraordinarily colorful. The life span of most sea squirts is about one year.

In Korea, sea squirts are eaten raw and featured in a number of dishes. Yondelis is an anti-cancer agent derived from Didemin B, which in turn is derived from Caribbean sea squirts. It works as an inhibitory drug in the chemotherapy treatment of sarcomas and bone tumors and is being tested on patients with breast cancer. Scientists are experimenting with plasmalogen, another substance derived from sea squirts, as a tool in combating Alzheimer’s disease. Sea squirts are not endangered. They have no special status on International Union for Conservation of Nature (IUCN) Red List and for the Convention on International Trade in Endangered Species (CITES)

Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) coris.noaa.gov ; International Coral Reef Initiative icriforum.org ; Coral Reef Alliance coral.org ; Global Coral reef Alliance globalcoral.org ; Global Coral Reef Monitoring Network gcrmn.net

Sea Squirt Characteristics

Sea squirts have no tentacles. Instead they have two openings which are connected by a U-shaped tube. The entire structure is covered by jelly. Under water it is dilated and beautiful. When exposed by low tide they become blobs of jelly. When touched they shoot streams of water, hence their name.

According to Animal Diversity Web (ADW): All the animals in the family Cionidae are called tunicates because they have a "tunic" of cellulose-like substance that covers the body. In this tunic are scattered cells, nerves, and blood vessels. The animals in the genus Ciona are known for their soft tunics and flexible bodies because the upper part of their bodies can be drawn into the lower part, "like the finger of a glove". A branching vascular system is interlaced within the entire tunic, which makes up about 60 percent of the animal's weight. [Source: Ana Alcaraz, Animal Diversity Web (ADW)]

Sea squirts in a Korean market
Sea squirts grow to be about 12 centimeters (5 inches) high and have a simple and elongated shape. They are a sessile (fixed in one place), usually attached to a substrate such as seeweed, solitary, and non-colonial organisms. They are greenish, translucent, and tubular with terminal inhalent and sub-terminal exhalent siphons (openings). The inhalent siphon is surrounded by eight distinct lobes and the exhalent siphon by six; the lobes are interposed with red or orange pigment spots. The retractor muscles, gut, sex organs, and large filter-feeding and respiratory pharynx can sometimes be seen through the body wall.

The pharynx has a ring of tentacles at the beginning that prevents large objects from entering it. It then gets larger, and the walls contain many gill slits lined with cilia. The sweeping movement of these cilia sets up the current circulation of water from the pharynx to the alimentary canal and back out to the exterior. An organ known as the endostyle lies on the floor of the branchial chamber; it is believed to be the precursor to the thyroid gland. The endostyle secretes mucus that traps food particles, and then the cilia lining the endostyle pass the mucus to the dorsal midline of the pharynx "where it is rolled into a mucus rope" (Larousse 1967). The rope is then passed to the oesophagus, stomach, and intestine, and faecal pellets are discharged through the atrial opening. These animals contain few blood vessels, no capillaries, and the circulatory system is made up of haemocoelic cavities. The neural gland contains the gonatropic substances and is thought to match the pituitary gland in vertebrates.

Sea Squirt Feeding

Sea squirts are filter feeders. They draw water through one opening, pass it through a bag of jelly with slits and then expel it out the other opening. Food particles stick to the wall and are pushed with silica to a primitive gut. In some species the bag of jelly is pink or gold. In other species it is transparent.

According to Animal Diversity Web (ADW): Sea squirts feed mainly on fine detrital particles and phytoplankton, During the cirulation of water through the large gill basket, food particles are taken from the water and the endostyle secretes mucus to trap the food. It is then passed to the dorsal midline of the pharynx where it is rolled into a mucus rope and passed to the stomach. From here it passes to the intestine, and the faecal pellets formed go from the anus to the atrial opening where they are expelled from the body. [Source: Ana Alcaraz, Animal Diversity Web (ADW)]

When feeding, the Sea squirts are fully stretched and the siphons are visible; but when they are resting, these animals withdraw their siphons. The flexibility of their bodies helps to do this easily..

Sea Squirt Reproduction and Development

Sea squirts begin life as tadpole-like, two-millimeter-long larvae. After a few hours or a couple of days, the larvae goes through a strange metamorphosis. First it glues three toes on its head to a hard surface. Then it tail and nervous system dissolve and it larval organs break down and are replaced by adult organs, and an entirely different animal emerges.

Sea squirts are hermaphroditic and release sperm and eggs through the exhalent siphon. Fertilization occurs at sea, and a tadpole-like larva is formed about 25 hours later. The larva lasts about 36 hours, depending on the temperature of the area, after which it settles and metamorphoses into an adult. [Source: Ana Alcaraz, Animal Diversity Web (ADW)]

Sea squirt larvae contain a notochord — within its muscular tail that is lost at metamorphosis — as well as , a dorsal nerve cord, a brain and sense-organs. In its life-history, the tunicate shows retrogressive evolution because the larva contains more features similar to the vertebrates than the adult does.

The eggs and sperm of sea squirts mature at specific times. Chemosensory organs in their bodies sense the presence of species-specific sperm in the water and thus discharge the eggs. Self-fertilization is prevented by self-sterilization and by having the ova and sperm in the same animal mature at different times


Can Eating Sea Squirts Reverse Aging?

A study published in in May 2022, said that eating sea squirts may be able to reverse signs of aging. The research used mouse models and was carried out by scientists from Xi’an Jiaotong-Liverpool University, Stanford University, Shanghai Jiao tong University, and the University of Chinese Academy of Sciences. BGR reported: These strange sea creatures contain a substance called plasmalogens. Plasmalogens are vital to the processes that our body uses. These substances are normally found in our heart, brain, and immune cells, and they occur naturally. However, as we age, our body starts to carry fewer plasmalogens. The loss of these vital substances is characteristic of a number of neurodegenerative diseases. If adding more to your body can reverse the signs of aging, it could open the door for new supplements. [Source: Joshua Hawkins, BGR, May 23, 2022]

But just how beneficial can it be to add plasmalogens to your diet? The researchers found that plasmalogens were able to stop cognitive decline. The mice who were fed the supplement experienced a higher number and quality of synapses. Synapses are essentially the connections between neurons. Aged mice who hadn’t been fed the supplements showed fewer and lower quality synapses. Additionally, aged mice fed with the plasmalogens grew new black hair that was thicker and glossier than that of aged mice who didn’t receive the supplements.

To test just how much these substances can affect the aged brain, the scientists put the mice through a Morris water maze. This test uses a pool of water that contains a single platform as a resting area. Typically, mice aren’t fans of swimming. As such, they usually remember where the platform is and swim directly to it. However, the researchers found that older mice struggled to find the platform despite five days of training. However, when given plasmalogen supplements in their diet, the aged mice behaved more like younger mice. As such, it seems that these vital substances can indeed help reverse the signs of aging, even in the brain. To test this further, the researchers inspected the brains of mice with and without the extra plasmalogens.


Brachiopods are shelled marine animals with long, fleshy stalks that live in burrows on the seafloor. They can be found in reefs, filter-feeding from the water around them. At first glance, they look like clams. They are actually quite different from clams in their anatomy, and they are not closely related to the molluscs. They are lophophorates, and so are related to the Bryozoa and Phoronida.

Brachiopods are regarded as living fossils Alice Clement of Flinders University wrote: Brachiopods living today, such as Lingula, look more or less the same as their Cambrian counterparts from about 500 million years ago! They are considered the oldest known animal (genus) that still contains living representatives. In The Origin of Species, Darwin noted “some of the most ancient...animals as...Nautilus, Lingula, etc., do not differ much from living species”. It’s these observations that led him to propose the term “living fossil”. Lingula is a brachiopod genus. [Source:Alice Clement, Research Associate in the College of Science and Engineering, Flinders University, The Conversation October 10, 2022]

Brachiopoda occupy their own phylum. They have hard "valves" (shells) on the upper and lower surfaces as opposed to left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major categories of brachiopods have traditionally been recognized, articulate and inarticulate brachiopods. The word "articulate" is used to describe the tooth-and-groove structures of the valve-hinge which is present in the articulate group, and absent from the inarticulate group. [Source: Wikipedia]


parts of a Phoronid

Phoronids are a small phylum of marine animals that filter-feed with a lophophore (a "crown" of tentacles), and build upright tubes of chitin to support and protect their soft bodies. Sometimes called horseshoe worms, they live in most of the oceans and seas, including the Arctic Ocean but excluding the Antarctic Ocean, at depths between the intertidal zone and 400 meters (1300 feet). Most adult phoronids are two centimeters long and about 1.5 millimeters wide. The largest are 50 centimeters long. [Source: Wikipedia]

Phoronids don’t have a brain. There is a nervous center between the mouth and anus, and a nerve ring at the base of the lophophore. The ring supplies nerves to the tentacles and, just under the skin, to the body-wall muscles. Phoronis ovalis has two nerve trunks under the skin, whereas other species have one. The trunks have giant axons (nerves that transmit signals very fast) which co-ordinate the retraction of the body when danger threatens.

The bottom end of the body is an ampulla (a flask-like swelling), which anchors the animal in the tube and enables it to retract its body very quickly when threatened. When the lophophore is extended at the top of the body, cilia (little hairs) on the sides of the tentacles draw food particles to the mouth, which is inside and slightly to one side of the base of the lophophore. Unwanted material can be excluded by closing a lid above the mouth or be rejected by the tentacles, whose cilia can switch into reverse. The food then moves down to the stomach, which is in the ampulla. Solid wastes are moved up the intestine and out through the anus, which is outside and slightly below the lophophore.

Except for retracting the body into the tube, phoronids have limited and slow movement: partial emerging from the tube; bending the body when extended; and the lophophore's flicking of food into the mouth. A blood vessel leads up the middle of the body from the stomach to a circular vessel at the base of the lophophore, and from there a single blind vessel runs up each tentacle. A pair of blood vessels near the body wall lead downward from the lophophore ring to the stomach and also to blind branches throughout the body.

There is no heart, but the major vessels can contract in waves to move the blood. Phoronids do not ventilate their trunks with oxygenated water, but rely on respiration through the lophophore. The blood contains hemoglobin, which is unusual in such small animals and seems to be an adaptation to anoxic and hypoxic environments. The blood of Phoronis architecta carries twice as much oxygen as a human of the same weight. Two metanephridia filter the body fluid, returning any useful products and dumping the remaining soluble wastes through a pair of pores beside the anus.

All phoronids reproduce sexually from spring to autumn. The eggs of most species form free-swimming actinotroch larvae, which feed on plankton. An actinotroch settles to the seabed after about 20 days and then undergoes a radical change in 30 minutes: the larval tentacles are replaced by the adult lophophore; the anus moves from the bottom to just outside the lophophore; and this changes the gut from upright to a U-bend, with the stomach at the bottom of the body. One species forms a "slug-like" larva, and the larvae of a few species are not known. Phoronids live for about one year. One species builds colonies by budding or by splitting into top and bottom sections.

Some species live separately, in vertical tubes embedded in soft sediment, while others form tangled masses buried in or encrusting rocks and shells. Species able to bore into materials like limestone and dead corals do so by chemical secretions. In some habitats populations of phoronids reach tens of thousand of individuals per square meter. The actinotroch larvae are familiar among plankton, and sometimes account for a significant proportion of the zooplankton biomass. Predators include fish, gastropods (snails), and nematodes (tiny roundworms).


Bryozoa are a phylum of simple, aquatic invertebrate animals, nearly all of which live in sedentary colonies. Typically about 0.5 millimeters (1∕64 in) long, they too have lophophore, a "crown" of tentacles used for filter feeding. Most marine bryozoans live in tropical waters, but a few are found in oceanic trenches and polar waters. The bryozoans are classified as the marine bryozoans (Stenolaemata), freshwater bryozoans (Phylactolaemata), and mostly-marine bryozoans (Gymnolaemata), a few members of which prefer brackish water. 5,869 living species are known.At least two genera are solitary (Aethozooides and Monobryozoon); the rest are colonial. [Source: Wikipedia]

Bryzoa illustration from the 1900s

Colonies take a variety of forms, including fans, bushes and sheets. Single animals, called zooids, live throughout the colony and are not fully independent. These individuals can have unique and diverse functions. All colonies have "autozooids", which are responsible for feeding, excretion, and supplying nutrients to the colony through diverse channels. Some classes have specialist zooids like hatcheries for fertilized eggs, colonial defence structures, and root-like attachment structures. Cheilostomata is the most diverse order of bryozoan, possibly because its members have the widest range of specialist zooids. They have mineralized exoskeletons and form single-layered sheets which encrust over surfaces, and some colonies can creep very slowly by using spiny defensive zooids as legs.

Each zooid consists of a "cystid", which provides the body wall and produces the exoskeleton, and a "polypide", which holds the organs. Zooids have no special excretory organs, and autozooids' polypides are scrapped when they become overloaded with waste products; usually the body wall then grows a replacement polypide. Their gut is U-shaped, with the mouth inside the crown of tentacles and the anus outside it. Zooids of all the freshwater species are simultaneous hermaphrodites. Although those of many marine species function first as males and then as females, their colonies always contain a combination of zooids that are in their male and female stages. All species emit sperm into the water. Some also release ova into the water, while others capture sperm via their tentacles to fertilize their ova internally. In some species the larvae have large yolks, go to feed, and quickly settle on a surface. Others produce larvae that have little yolk but swim and feed for a few days before settling. After settling, all larvae undergo a radical metamorphosis that destroys and rebuilds almost all the internal tissues. Freshwater species also produce statoblasts that lie dormant until conditions are favorable, which enables a colony's lineage to survive even if severe conditions kill the mother colony.

Mineralized skeletons of bryozoans first appear in rocks from the Early Ordovician period (485 million to 444 million years ago), making it the last major phylum to appear in the fossil record. This has led researchers to suspect that bryozoans arose earlier but were initially unmineralized, and may have differed significantly from fossilized and modern forms. Predators of modern marine bryozoans include sea slugs (nudibranchs), fish, sea urchins, pycnogonids (sea spiders), crustaceans, mites and starfish.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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