Gnathiferans and Smaller Lophotrochozoans

1. Gnathiferans and Smaller Lophotrochozoans Chapter 15

2. Pseudocoelomates • Pseudocoelomates have a body cavity (the pseudocoel) between the gut (derived from endoderm) and body wall (derived from mesoderm). • Derived from the blastocoel.

3. Advantages of a Body Cavity • A body cavity, pseudocoel or coelom, has several advantages. It provides: • Greater freedom of movement. • Space for development of organ systems. • A simple means for circulation of materials around the body. • Storage place for waste products. • A hydrostatic organ.

4. Common Features of Pseudocoelomates • All have a body wall of epidermis, a dermis, and muscles surrounding the pseudocoel. • Almost all have a complete digestive tract. • The epidermis of many secretes a nonliving cuticle with bristles & spines.

5. Pseudocoelomates • Pseudocoelomates do not form a clade. • Some are part of superphylum Lophotrochozoa, others are in superphylum Ecdysozoa. • All share the pseudocoelomate body plan.

6. Lophotrochozoa (10 Phyla) • Ancestors possessed complex cuticular jaws – Clade Gnathifera • Gnathostomulida • Micrognathozoa • Rotifera • Acanthocephala • 6 other lophotrochozoan phyla • Gastrotricha • Tiny aquatic animals that may be closely related to gnathiferans • Molecular characteristics place the following with Lophotrochozoa • Cycliophora • Entoprocta • Ectoprocta • Brachiopoda • Phoronida

8. Clade Gnathifera • Possess small cuticular jaws with a homologous microstructure. • Numbers of pairs of jaws vary. • Gnathostomulida, Micrognathozoa, and Rotifera are tiny, free-living, aquatic animals. • Acanthocephalans are worm-like endoparasites living as adults in fish or other vertebrates.

9. Clade Gnathifera • Rotifera and Acanthocephala • Presumed sister taxa. • Form a clade called Syndermata. • Have eutelic syncytial epidermis. • Constant number of nuclei. • Grouping is controversial.

10. Phylum Gnathostomulida • Phylum Gnathostomulida includes the jaw worms. • Very small - <2mm. • Live in interstitial spaces of fine coastal sediments. • Can endure low O2.

11. Phylum Gnathostomulida • Feed by scraping bacteria and fungi from the substratum with a pair of jaws on the pharynx. • Acoelomate • Sexual stages include males, females, and hermaphrodites. • Fertilization is internal.

12. Phylum Micrognathozoa • Micrognathozoans are tiny animals that live interstitially (between sand grains). • Body consists of a two-part head, a thorax, and abdomen with short tail. • Move using cilia and have a unique ventral ciliary adhesive pad that produces glue. • Three pairs of complex jaws.

13. Phylum Micrognathozoa • Simple gut • Anus opens to outside only periodically. • Reproductive system is not well understood: • Only female reproductive organs have been identified. • May reproduce parthenogenetically. • Cleavage and subsequent development have not been studied.

14. Phylum Rotifera • Members of the phylum Rotifera are pseudocoelomate and have three embryonic germ layers (triploblastic). • Complete digestive system.

15. Phylum Rotifera • Dioecious (separate sexes) but some species parthenogenetic (females produce diploid eggs). • Some are parthenogenetic during part of the year, depending on environmental conditions. • Thick shelled eggs that can withstand harsh conditions are sometimes produced.

16. Phylum Rotifera • Rotifers have a ciliated crown, the corona, that is characteristic of the phylum.

17. Phylum Rotifera • Rotifers come in a wide range of colors and shapes. • Shapes often correspond to lifestyle (floaters, swimmers, sessile). • They may be individual or colonial. • Mostly freshwater. • Benthic and pelagic forms.

18. Phylum Acanthocephala • All spiny-headed worms (Phylum Acanthocephala) are parasites in the intestines of vertebrates. • Over 1100 species known. • Occur worldwide and parasitize fish, birds, and mammals. • Larvae develop in crustaceans or insects.

19. Phylum Acanthocephala • Proboscis has rows of recurved spines that penetrate and may rupture host intestines. • Proboscis with hooks can be inverted into a proboscis receptacle by retractor muscles.

20. Phylum Acanthocephala • Body somewhat flattened. • About 80% of tegument is a lacunar system of fluid-filled canals that may distribute nutrients and remove wastes from muscles. • No heart - function provided by lacunar fluid. • Both longitudinal and circular body wall muscles are present.

21. Phylum Acanthocephala • No respiratory system. • Protonephridia with flame cells, if present, perform excretory functions. • Nutrients are absorbed across the tegument, which bears some enzymes - no digestive tract. • Dioecious • No species normally parasitizes humans.

22. Phylum Acanthocephala • Acanthocephalans penetrate the intestinal wall with spiny proboscis. • Remarkably little inflammation on host wall, but pain of infection is intense. • Larval acanthors burrow through beetle intestine. • Develop into juvenile cystacanths in the insect hemocoel. • Pigs become infected by eating grubs.

23. Phylum Gastrotricha • Gastrotrichs appear similar to rotifers, but without the ciliated corona and have a bristly looking body. • Members of the phylum Gastrotricha are pseudocoelomate and have three embryonic germ layers (triploblastic). • Complete digestive system. • Hermaphroditic or parthenogenetic.

24. Phylum Entoprocta • About 150 species in the phylum Entoprocta occur worldwide. • Usually in marine environments. • Less than 5 mm long and mostly microscopic, resembling hydroid cnidarians.

25. Phylum Entoprocta • Urnatella gracilis is acommon freshwater species in North America. • Body or calyx is cup shaped and bears a circular crown of ciliated tentacles. • Attaches by a stalk with adhesive glands.

26. Phylum Entoprocta • Tentacles (3-30) and stalk are continuations of the body wall. • Tentacles on lateral and inner surfaces can roll inward but cannot be retracted into the calyx. • Gut is U-shaped with both mouth and anus opening within the circle of tentacles.

27. Phylum Entoprocta • Long cilia on sides generate current bringing in particles. • Short cilia on inner surfaces capture food and direct it to mouth. • Pair of protonephridia embedded in gelatinous parenchyma. • Well-developed nerve ganglion on the ventral side of stomach. • No circulatory or respiratory organs.

28. Phylum Entoprocta • Some are monoecious, some dioecious, and some first produce sperm and later eggs. • Fertilized eggs develop in a brood pouch. • Modified spiral cleavage leads to trochophore-like larva.

29. Lophophorates • Phylum Ectoprocta • Phylum Brachiopoda • Phylum Phoronida • Belong within the lophotrochozoan subgroup of protostomes. • Grouping very controversial. • Evidence comes from sequence analysis of genes encoding small-subunit ribosomal RNA. • Some aspects of development place these taxa within Deuterostomia.

30. Lophophorates • Lophophorates possess a mesoderm-lined coelom. • Tripartite coelom – coelom is divided into three parts: protocoel, mesocoel, metacoel.

31. Lophophorates • Members of these 3 taxa possess a feeding device called a lophophore. • Unique arrangement of ciliated tentacles borne on a ridge or fold on the body wall. • Tentacles are hollow and contain an extension of the mesocoel.

32. Lophophorates • Ciliated tentacles are also a respiratory device permitting gas exchange between surrounding water and internal coelomic fluid. • Gut is U-shaped: • Mouth opens inside the lophophore ring, and the anus opens outside the ring.

33. Phylum Ectoprocta • Phylum Ectoprocta contains aquatic animals that often encrust hard surfaces (bryozoans). • Approximately 4500 living species. • Inhabit both shallow freshwater and marine habitats. • Most are sessile, some slide slowly, and others crawl actively across surfaces. • Mostly colony builders. • Each member is less than 0.5 mm in length and is called a zooid.

34. Phylum Ectoprocta • Zooids feed by extending lophophores into surrounding water to collect tiny particles • Zooids secrete exoskeleton in which they live in.

35. Phylum Ectoprocta • Exoskeleton may be gelatinous, chitinous, or stiffened with calcium and possibly impregnated with sand. • Shape may be boxlike, vaselike, oval, or tubular. • Some colonies form limy encrustations on seaweed, shells, and rocks. • Others form fuzzy or shrubby growths or erect branching colonies. • Freshwater colonies may form mosslike colonies on stems of plants or on rocks.

36. Phylum Ectoprocta • To feed, the lophophore is extended and tentacles spread out into a funnel. • Cilia on tentacles draw water into funnel. • Food particles caught by cilia in the funnel are drawn into the mouth. • Digestion begins extracellularly in the stomach and is completed intracellularly within the intestine.

37. Phylum Ectoprocta • Respiratory, vascular, and excretory organs absent. • Gas exchange is through body surface. • Ganglionic mass and a nerve ring around the pharynx. • No sense organs.

38. Phylum Ectoprocta • Reproduction - most hermaphroditic. • Some species shed eggs into seawater, but most brood their eggs. • Brooding occurs within coelom and some have an external chamber called an ovicell. • Sometimes embryos proliferate asexually from the initial embryo. • Cleavage is radial but mosaic. • Larva of nonbrooding species have a functional gut and swim for a few months before settling.

39. Phylum Ectoprocta • Larva of brooding species do not feed and settle after a brief free-swimming existence. • Attach to substratum by secretions from an adhesive sac, then metamorphose to adult form. • New colonies begin from this single metamorphosed primary zooid, called an ancestrula. • Ancestrula undergoes asexual budding to produce many zooids of a colony.

40. Phylum Ectoprocta • Freshwater ectoprocts undergo budding that produces statoblasts. • Hard, resistant capsules containing a mass of germinative cells.

41. Phylum Brachiopoda • Brachiopods appear similar to bivalve molluscs because they have two calcareous shell valves secreted by a mantle. • Dorsal/ventral instead of left/right. • Pedicel – a fleshy stalk used for attachment.

42. Phylum Brachiopoda • Brachiopods are an ancient group they were prolific during the Paleozoic and Mesozoic eras. • One living species, Lingula, is considered to be a living fossil since it has changed little since the Ordovician (505 mya).

43. Phylum Brachiopoda • Characteristics of both protostomes & deuterostomes: • Cleavage is radial (deuterostome) • Coelom formation enterocoelous at least in some brachiopods. (deuterostome) • The relationship of the blastopore to the mouth is uncertain.

44. Phylum Phoronida • Species in the phylum Phoronida are small wormlike animals. • Secrete tubes to live in. • Tentacles of the lophophore are extended for feeding. • U-shaped digestive tract.

45. Phylum Phoronida • Characteristics of both protostomes & deuterostomes: • Blastopore becomes mouth (protostome). • Cleavage is radial (deuterostome). • Coelom formation – highly modified enterocoelous (deuterostome).

46. Phylogeny • Analysis of rRNA gene sequencing suggests that after the ancestral deuterostome diverged from ancestral protostomes: • Protostomes split into two large groups: • Ecdysozoa that molt. • Lophotrochozoa that exhibit lophophore feeding and trochophore-like larvae. • Most lophotrochozoans share some developmental features: • Spiral mosaic cleavage and formation of mouth from embryonic blastopore. • No common body plan.

47. Phylogeny • Lophotrochozoan protostomes are a heterogeneous group for which evolutionary branching order remains to be determined. • DNA sequence analysis has led to the conclusion that acanthocephalans are highly derived rotifers. • Sequences put Acanthocephala and Rotifera together as clade Syndermata, sharing a eutelic syncytial epidermis. • Syndermata is placed with Micrognathozoa and Gnathostomulida in clade Gnathifera. • Controversial placement of Cycliophora, Gastrotricha, and Platyhelminthes close to Gnathifera.

48. Phylogeny • Entoprocts and ectoprocts were once considered both Bryozoa, but ectoprocts have been broken out as true coelomate animals. • Sequence analysis now places them both in the lophotrochozoan phyla.

49. Phylogeny • Ectoprocts, brachiopods, and phoronidsshare a lophophore and a tripartite coelom. • Other features are mixtures of developmental traits from both protostomes and deuterostomes. • Debate continues on whether the lophophorates form a clade or whether the group members, either individually or collectively belong within Protostomia or Deuterostomia.