Sea squirts, also known as ascidians, are a group of marine vertebrates that belong to the class Ascidiacea within the phylum Tunicata. These animals, which are typically sessile and potato-shaped, possess a tough outer covering called a tunic made of a polysaccharide. With around 2,300 species, sea squirts are primarily found in shallow waters across the globe.
They are characterized by two small openings called siphons through which water flows in and out of their bodies, enabling them to filter feed. During their larval phase, sea squirts are free-swimming but become firmly attached to substrates like rocks and shells as adults. These creatures can be solitary, forming clumped communities, or part of large colonies of many small individuals called zooids.
They usually range between 0.2 to 4 in (0.5 to 10 cm) in size.
Their bodies are typically rounded or cylindrical, with their lower ends pitted, ridged, or bearing root-like extensions. A thick, vascular external tunic made of polysaccharide (a characteristic feature of all tunicates) covers the body surface. In some colonial species, the tunics are fused.
The upper part of a sea squirt features two siphons used for filter feeding. They can spurt water from these siphons when removed from the water, thus their name ‘sea squirts.’
The pharyngeal region houses the ciliated pharynx with numerous perforations or stigmata. It also includes a long, ciliated groove called an endostyle, homologous to the vertebrate thyroid, and a long ridge on the opposite side. Water is drawn in through the large buccal siphon, passes through the pharynx, and exits via the smaller atrial siphon into the atrium, which surrounds the pharynx.
The abdominal region contains most internal organs, while the postabdominal region includes the heart and gonads. Some species, such as the sea peach (Halocynthia aurantium), lack distinct abdominal and postabdominal regions, with their organs located more anteriorly.
Sea squirts have curved tubular hearts, with each end opening into a single vessel. One vessel is connected to the endostyle, and the other is attached to the dorsal surface of the pharynx. Both vessels are interconnected by a network of sinuses that facilitate blood flow throughout the body.
A distinctive feature of sea squirts is their heart’s alternating blood flow direction, switching every three to four minutes. This process is driven by two excitatory areas located at each end of the heart, which sequentially become dominant to push blood through the ventral and dorsal vessels.
The blood contains four types of cells: lymphocytes, phagocytic amoebocytes, nephrocytes, and morula cells. Nephrocytes collect waste materials like uric acid, which accumulate in renal vesicles near the digestive tract. Morula cells contribute to forming the tunic and are often found within it. These cells can contain iron-based hemoglobin, giving the blood a red color, or vanadium-based hemovanadin, resulting in a green hue. In the latter case, the cells are also known as vanadocytes.
They facilitate gas exchange with their surroundings using gill slits, also known as pharyngeal stigmata. Water enters through the mouth and passes through the pharynx, then flows through the mucus-laden gill slits into the atrium, and finally exits via the atrial siphon.
The digestive tract begins with the pharynx, where mucus produced by the endostyle bathes food particles. This mucus moves across the pharyngeal surface, trapping planktonic food, and collects in the dorsal ridge. From there, the food passes into the esophageal opening at the base of the pharynx.
The esophagus leads to the stomach, where digestive enzymes are secreted. The intestine runs upwards parallel to the esophagus, ending in a short rectum and anus, which open into the cloaca below the atrial siphon. In colonial species, the zooids may share a common cloacal aperture.
Nephrocytes in the blood filter waste materials, such as uric acid, and deposit them in renal vesicles near the digestive tract. In contrast, ammonia is directly extracted from the blood through the pharyngeal walls and expelled via the atrial siphon.
The nervous system of sea squirts is characterized by a nerve plate that rolls up to form a long neural tube. This tube includes the caudal nerve cord, sensory vesicle, tail or visceral ganglion, and neck. Although sea squirts lack true brains, they have a large ganglion suspended in the connective fluid between the two siphons, which distributes a network of nerves to innervate the entire body.
Beneath this ganglion lies an exocrine gland derived from the neural tube, often considered homologous to the vertebrate spinal cord. While sea squirts lack specialized organs, they possess receptors for touch, chemoreception, and vision embedded in their body wall.
Based on morphological evidence, sea squirts were initially considered a sister group to the classes Thaliacea and Appendicularia within the phylum Tunicata. However, molecular evidence suggests they could also be polyphyletic.
Presently, around 2,300 species of sea squirts are classified under three orders.
Since they are soft-bodied vertebrates, sea squirts have scanty fossil records. The earliest undisputed sea squirt is Shankouclava shankouense from the Lower Cambrian Period. Two other genera, Ausia and Burykhia from Namibia and Russia, are also considered sea squirts.
Fossil records indicate that sea squirts were prevalent during the Lower Jurassic and Tertiary Periods. Members of the genus Cystodytes from the Pliocene and Eocene Periods have been unearthed in France.
Sea squirts inhabit all seas, from intertidal zones to deep waters. They are predominantly sessile and attach themselves to structures such as piers, rocks, large seashells, ship hulls, and even the backs of large crabs.
When near the shore, sea squirts primarily feed on detritus (dead plants and animals), while at greater depths, they filter out plankton from the water. They feed by drawing water through their oral siphon, passing it through mucus-covered gill slits into a chamber called the atrium, and then expelling it through the atrial siphon.
Sea squirts are preyed upon by various marine animals, including nudibranchs, flatworms, mollusks, rock crabs, sea stars, sharks, skates, birds, and sea otters. Additionally, in regions such as Japan, Korea, Chile, and Europe, humans also consume these vertebrates.
Almost all adult sea squirts are hermaphrodites, possessing both male and female organs within a single individual. Their gonads, comprising testes and ovaries, open via a single duct into the cloaca. Sea squirts exhibit two main reproductive strategies, differing between solitary and colonial species.
Multiple eggs are released from the atrial siphons and are externally fertilized by sperm released simultaneously by other individuals. Within 12 hours to a few days, the fertilized eggs develop into free-swimming, tadpole-like larvae.
These larvae typically have long tails, though members of the Molgulidae family develop directly into juveniles without a tailed larval stage. The larvae take around 36 hours to find and settle on a suitable substrate, using their receptors to search for a suitable surface and secreting an adhesive from the papillae (finger-like nervous projections) on their anterior end to attach themselves. This attachment triggers an irreversible metamorphosis, developing into a juvenile.
During development, the tunic and notochord form early, with the notochord consisting of a row of 40 cells. Concurrently, the nerve tube develops into the cerebral ganglion. The siphons orient themselves to optimize water flow into the body. Over time, the larval tail and fins are lost, resulting in a less advanced adult form than the larva, a process known as retrogressive metamorphosis.
Sea squirts reproduce both sexually and asexually. In colonial species, each member, known as a zooid, is genetically identical to the others within the colony.
Sexually reproducing sea squirts can be either broadcast spawners with long-range dispersal or philopatric spawners with extremely short-range dispersal.
In broadcast spawning, sperm and ova are released into the water column, where external fertilization occurs near the parent colonies. The resulting zygotes develop into microscopic larvae that are carried miles away by ocean currents.
In philopatric spawners, sperm from a neighboring colony or another zooid within the same colony enters the atrial siphon to fertilize the ovum internally within the atrium. The zygotes develop into macroscopic tadpole-like larvae, which exit through the atrial siphon and settle within a few meters of the parent colony.
Fertilization
During fertilization, the sperm penetrates the egg’s layer of follicular cells and moves through the perivitelline space to reach the egg plasma membrane. Upon entry, the egg releases the enzyme glycosidase into the seawater to prevent further sperm entry, a process known as polyspermy prevention.
As fertilization occurs, many free calcium ions are released in the egg cytoplasm. This temporary increase in calcium induces physiological and structural changes in the egg. The cytoplasm undergoes a dramatic rearrangement called ooplasmic segregation, which determines the axial patterning of the embryo.
Different families of sea squirts utilize distinct strategies for asexual reproduction. In the skeleton panda sea squirt (family Clavelinidae), colony members are interconnected by root-like projections known as stolons. Buds containing food storage cells sometimes grow within these stolons and later separate to form new adults. In the family Didemnidae, an individual often splits into two, each half developing necessary structures: the pharynx in one half develops a new digestive tract, while the original digestive tract in the other half grows a new pharynx.
In some species, the postabdomen elongates and breaks into a string of separate buds, forming a new colony. The abdomen can also break into germinal tissues that develop into new individuals.
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