INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION DEUTEROSTOMES (DEUTEROSTOMIA)

PHYLUM ECHINODERMATA

CLASS STARFISH (ASTEROIDEA)

Inhabitants of seas and oceans, typically absent in waters with a salinity below 30 . Only a single species, Marthasterias glacialis, lives in the Black Sea near the Bosphorus. Over 300 fossil and about 1.5 thousand extant species are known.

Body sizes range from 1 to 80 cm or more. The Class gets its name from its characteristic star-shaped body form. The main morphological feature of starfish is the presence of radial rays into which

the body cavity and Internal Organs extend. Starfish may be flattened, convex, or, exceptionally, rounded without rays (Fig. 441). They are often brightly colored in A wide variety of hues, sometimes mottled.

Fig. 441. Starfish: a - Acanthaster planci; b - Pteraster obscurus; c - Asterias forbesi

Anatomy. The body of starfish is flattened along the oral-aboral axis, comprising a central disk and radial rays, or arms. Starfish always face the substrate with their oral side. A pentamerous (five-rayed) Structure is typical, though species with six or more (up to 50) rays are quite common.

Integument. The body is covered by a thin cuticle, beneath which lies a single-layered epithelium with glandular Cells. The bulk of the body wall consists of Connective Tissue, within the thickness of which calcareous skeletal plates and Muscle fibers are embedded.

The Skeleton of starfish does not form a solid shell, but rather consists of numerous plates interconnected by connective tissue and Muscles. The Skeleton of the oral side is more strongly developed than that of the aboral side. Along the margin of each ray on the oral side run one or two rows of marginal plates, medial to them lies a row of adambulacral plates, and flanking the midline are two rows of ambulacral plates angled toward each other to form a gabled roof over the ambulacral groove. Each ambulacral plate features a small notch through which the canal of the ambulacral tube FOOT passes. The aboral skeleton most commonly consists of narrow plates forming an irregular meshwork.

Both oral and aboral skeletal plates bear numerous skeletal appendages on their surface in the form of needles and spines. Spines may be simple or take the form of so-called paxillae—calcareous pillars with tiny spinules and needles at the apex. Some spines are modified into pedicellariae of various shapes (Fig. 442), which perform sanitary and protective Functions, much like those in sea urchins.

Fig. 442. Paxillae (a) and pedicellariae (b) of starfish (from Belyaev)

Ambulacral System.

The madreporite is located on the aboral side of the body (it stands out among other skeletal plates by its larger size and color); the stone canal descends to the oral side, where it empties into the ring (circumoral) canal. Small Tiedemann's bodies are arranged in pairs on top of the ring canal, which are believed to function as Lymph Nodes AND produce amoebocytes. Polian vesicles may empty into the ring canal (sometimes absent). Radial canals extend to the tip of each ray along the floor of the ambulacral grooves. Tube feet have well-developed suckers; In addition to locomotion, tube feet perform a respiratory function, and those located at the tips of the rays also serve a tactile function. Suckers are absent on the latter (Fig. 428).

The Digestive System is straight and very short. The Mouth opening is surrounded by a soft peristomial membrane, allowing it to stretch widely. The mouth leads to a short Esophagus, which transitions into a Stomach divided by a constriction into two sections. The first—the cardiac stomach—has five radial pouches; from the second—the pyloric stomach—five radial tubular outgrowths extend into the rays, where they fork to form paired channels of branched digestive glands, sometimes referred to as hepatic caeca (Fig. 443). They produce Enzymes that digest food. Absorption of nutrients also takes place here. The pyloric stomach continues into a short intestine, from which two blind outgrowths extend; their function is not yet known. This is followed by a barely noticeable rectum, which ends in the center of the aboral surface with a microscopic anus. In some starfish, the hindgut and anus are not developed. Undigested food residues in all starfish are expelled outwards through the mouth.

Fig. 443. Fragment of dissection of Asterias rubens (aboral view) (from Ivanov):

1 - Skin region with anus; 2 - rectal glands; 3 - pyloric region of The Stomach; 4 - cardiac region of the stomach; 5 - gonad;

6 - hepatic caeca; 7 - marginal plates; 8 - adambulacral plates; 9 - genital stolon; 10 - hindgut;

11 - genital duct; 12 - wall of the axial sinus; 13 - area of skin with the madreporite;

14 - stomach retractor muscles; 15 - stone canal

Most sea stars are active predators, feeding on various Mollusks, crustaceans, Cnidarians, Sponges, other Echinoderms, fish, or carrion. Some species are specialists, feeding exclusively on specific types of animals. Prey is captured and bivalve shells are forced open using the tube feet located on flexible arms. When attacking large prey, a sea star everts its cardiac stomach and envelops the victim, initiating Digestion externally. This process is completed inside the star's body after the stomach, along with the semi-digested food, is retracted. However, some sea stars are strict or partial detritivores. Additionally, epidermal Cells of the skin are known to absorb dissolved organic matter from the Water, which is subsequently assimilated by the digestive system.

The lacunae of the Circulatory system are situated within the septa of the perihemal system. This system includes the Pericardium (a coelomic cavity that is part of the axial complex); its pulsations drive fluid through the circulatory lacunae of the axial organ.

Sea stars respire using dermal papulae (skin gills)—numerous thin-walled evaginations of the body wall enclosing extensions of the coelom. Oxygen diffuses directly through the skin into the coelomic fluid. Gas exchange also occurs across other thin-walled body regions, including the tube feet.

The Nervous system of sea stars is similar to that of other echinoderms. Despite having a relatively primitive nervous system lacking cerebral ganglia, it has been experimentally demonstrated that conditioned Reflexes can be established in certain species.

Sense Organs. Most sea stars possess primitive ocelli resembling pigment-cup eyes, located at the tip of each arm near the base of modified tentacular tube feet.

Reproductive System. Most sea stars are dioecious, although Various Forms of Hermaphroditism are known. For example, in Marthasterias glacialis, alongside dioecious individuals, specimens with mosaic hermaphroditic Gonads occur. In Asterina gibbosa, the gonads of young individuals in certain populations produce sperm, while older individuals produce eggs. Sexual Dimorphism is generally absent, though during the breeding season, individuals of opposite sexes may occasionally differ sharply in size and coloration. Gonads are arranged in pairs in each arm, starting from its base, and appear grapelike in mature individuals.

Reproduction. The fecundity of sea stars is very high and can reach up to 200 million eggs. Fertilization is external.

Development may proceed via a complex metamorphosis or directly. In most species, the fertilized egg develops into a simple, ciliated larva by the blastula or gastrula stage. Larvae developing from yolk-poor eggs undergo complex metamorphosis, passing through a transient dipleurula stage, which in turn develops into a bipinnaria. Later, three cylindrical appendages—brachiolaria arms—with adhesive disks appear on the preoral lobe of the bipinnaria, transforming it into a brachiolaria (Fig. 444). The brachiolaria settles and attaches to various submerged objects, completing its metamorphosis into a tiny sea star, during which part of the larval body undergoes reduction.

Fig. 444. Larvae of Asterias rubens (after Ivanova-Kazas): a - bipinnaria; b - brachiolaria: 1 - sucker; 2 - Formation of the sea star body

Larvae developing from yolk-rich eggs transform directly into a miniature sea star. Such species often exhibit parental care. For instance, in Representatives of the genus Henricia, common in temperate and cold marine waters of both hemispheres, the embryos are united into a cluster by a single strand and carried by the sea star on its oral (lower) side. In members of the family Pterasteridae, which predominantly inhabit great depths, embryonic development takes place within a specialized brood chamber.

Alongside sexual reproduction, some sea stars are known to reproduce asexually through binary fission of the disk across the interradius or via arm autotomy followed by the regeneration of the lost parts. This capacity for division is closely linked to their remarkable ability to regenerate an entire Organism from a single arm or even a fragment of one.

Most sea stars are predators that act as natural sanitary agents in marine biocenoses, as they primarily prey on weakened animals. However, they play a negative role from a human perspective. Sea stars cause significant economic damage by destroying oysters and other commercially valuable mollusks, including aquaculture species. In recent years, it has been definitively established that sea stars are fierce food competitors for many commercially harvested species: approximately 80 % of all benthic animals consumed by these commercial species are also eaten by sea stars.

During the 1960s, a catastrophic population explosion of the large (40–50 cm in diameter) multi-armed crown-of-thorns starfish (Acanthaster planci), which feeds on coral polyps, was recorded on many coral reefs in the western Pacific Ocean. This posed a severe threat to the destruction of coral reefs, including the Great Barrier Reef off the coast of Australia. To combat this, divers controlled the acontiate populations by injecting formalin into the stars. By the 1990s, the population numbers of these sea stars had declined to normal levels.