INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION SPIRALIA (SPIRALIA)

PHYLUM ANNELIDS (ANNELIDA)

SUBPHYLUM ACLITELLATA (ACLITEIJATA)

CLASS POLYCHAETES (POLYCHAETA)

SUBCLASS SEDENTARY POLYCHAETES (SEDENTARIA)

Sedentary polychaetes inhabit marine sediments and various submerged objects. As a rule, their epidermal glands secrete temporary or permanent protective tubes made of organic matter, often heavily impregnated with calcium carbonate.

The prostomium in these worms is poorly developed or completely reduced, with its appendages modified into tactile tentacles or gills that protrude from the tube but can be retracted instantly. Numerous simple eyes are located on the tentacles. The body is typically heteronomous, consisting of two or three distinct regions whose segments differ significantly from one another.

Two main ecological life forms are distinguished among them: burrowers, which feed primarily on organic matter from the sediment, and tube-dwellers, which feed on fine suspended particles captured from the Water using their tentacular apparatus. Among the burrowers, lugworms are the most well-known—large worms reaching up to 30 cm in length. For instance, Arenicola marina (Fig. 176, a) inhabits the sandy intertidal Zones of the White and Barents Seas, while A. branchialis is found on the sandy shoals of the Black Sea. The body of the lugworm is divided into three sharply distinct regions: an anterior region with small parapodia but lacking gills, a middle region with parapodia bearing gills, and a posterior region devoid of any appendages. This serves as a striking example of heteronomous metamerism. The lugworm constructs a U-shaped burrow in the sand, with walls reinforced by dermal gland secretions. The coelom in lugworms possesses transverse partitions (septa) only in the anterior part of the body, remaining continuous throughout the rest. This drives a hydraulic mode of locomotion, wherein coelomic fluid is propelled by muscular action from one end of the body to the other.

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Fig. 176. Sedentary polychaetes (after Dogiel): a, b, c - general appearance of the lugworm Arenicola marina, Spirorbis, and Serpula in calcareous tubes, respectively: 1 - caudal region; 2 - gills; 3 - notopodium; 4 - anterior segments lacking parapodia; 5 - peristomium;

6 - prostomium; 7 - buccal region; 8 - Pharynx; 9 - neuropodium; 10 - anus; 11 - tentacles; 12 — operculum

Tube-dwelling polychaetes are characterized by numerous specialized appendages located at the anterior end of the body protruding from the tube, functioning as feeding, respiratory, or defensive structures. These are most often modified palps or sometimes antennae of the first segment (Fig. 176, b-c). The portion of the body housed within the tube is divided into two regions: an anterior, more muscular section covered with glandular shields involved in tube construction, bearing more or less developed parapodia; and a posterior section with reduced parapodia, which primarily performs reproductive Functions.

The tubes of sedentary polychaetes often carpet the entire seabed at shallow depths. Frequently, they fuse together to form massive reef-like structures. For example, the tubes of Sabellaria alveolata form a continuous blanket over coastal rocks off the shores of southern England, creating formations resembling coral reefs. Other species, such as those from the family Serpulidae (Fig. 176, c), occur among coral reefs, where they form robust conglomerates

of calcareous tubes comprising hundreds of individuals. These are among the most aesthetically striking polychaetes, featuring brilliantly colored tentacles.

Polychaetes play a vital role in marine ecosystems. Above all, they serve as a high-calorie food source for numerous aquatic animals, including commercially valuable fish species. In this context, It is worth noting that between 1939 and 1941, the acclimatization of Nereis succinea was initiated in the Caspian Sea, where approximately 70,000 individuals of this species were introduced from the Azov Sea. They rapidly colonized new habitats and now rank third in the total biomass of the Caspian bottom fauna, trailing only Mollusks and crustaceans. They are consumed by all benthic fish species, while for sturgeons (Acipenser and Huso species), Nereis has become a primary and occasionally exclusive food source.

Polychaetes actively participate in The breakdown of organic matter within biocoenoses, facilitating biogeochemical nutrient cycling. Detritus-rich silt accumulates on the sea floor. By utilizing it as food, detritivorous polychaetes actively integrate detritus into the nutrient cycle via trophic webs. As suspension feeders, sedentary polychaetes also contribute to water purification by removing suspended particulate and organic matter.

Certain polychaetes (such as those of the family Serpulidae), alongside other fouling organisms, colonize various underwater structures and ship hulls, causing substantial economic losses to shipping and maritime navigation.

Certain species are consumed by humans, most notably the aforementioned palolo worm, which inhabits the coral reef crevices of the Fiji and Samoa islands in the Pacific Ocean. During the reproductive season, the epitokous region of the body, laden with Gametes, detaches simultaneously across virtually all individuals in the population. This mass swarming of palolo worms occurs on specific days of the final quarter of the lunar cycle, which are well known to the local inhabitants. They harvest the worms in vast quantities, enjoying them as a highly nutritious and delicious delicacy.