11 The Diversity of Benthic Invertebrates

1. 11 The Diversity of Benthic Invertebrates Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton

2. Introduction to Overall Invertebrate Diversity • Invertebrates have basic symmetry difference of individuals, radial versus bilateral symmetry • Radial symmetry (e.g., jellyfish, corals) involves similar form with respect to a center point in all directions • Bilateral symmetry involves a plane of symmetry, with similar form on either side of the plane; also involves an anterior and posterior (e.g., worms, lobsters)

3. Form Diversity

4. Other Major Features • Cleavage: symmetric (radial cleavage) in deuterostome phyla (echinoderms, chordates) • Cleavage: asymmetric (spiral cleavage) in protostome phyla (flatworms, annelids, mollusks, and arthropods)

5. Other Major Features • Cell layers: two layer in Cnidaria, but most phyla have three layers (ectoderm, mesoderm, endoderm); most phyla have structures of several cell layers that perform functions (e.g., digestive gland)- organs • Coelom: cavity that develops in embryo within mesoderm; found in several phyla (e.g., arthropods, molluscs) but may have evolved more than once • Gut - may be absent (Protista), blind (as in corals), or complete with mouth and anus

7. Protostomes and Deuterstomes • Deuterostomes: pattern of invagination of the early ball of cells, known as the blastula. The blastula invaginates, leading to the gastrula stage, and the opening formed to the outside eventually forms the anus. The mouth forms elsewhere on the embryo. • Protostomes: blastopore divides to form both mouth and anus

8. Relationships of Phyla P denotes protostome branch, D the deuterostome branch. Brachiopods, Bryozoa and Phoronida (the lophophorates) are allied with the protostomes bases on DNA sequencing evidence

9. Protista (kingdom) • Grade of construction: single autonomous cells • Symmetry: variable • Type of gut: none • Type of body cavity other than gut: none • Segmentation: none • Circulatory system: none • Nervous system: none • Excretion: diffusion from cell surface

10. Protista (kingdom) • Amoeboid forms: Foraminifera (calcium carbonate test with chambers), Radiolaria (silica test); feed with pseudopodia that engulf bacteria • Ciliates: elongate, mouth, propelled with outer coating of cilia, some have ciliated mouth and feed on particles, others engulf bacteria smaller protistans in mouth • Flagellates: elongate, propelled with fewer longer flagellae, similar feeding range to ciliates

11. Benthic protists: (a) ciliate and (b) foraminiferan

12. Phylum Porifera • Grade of construction: cellular, with no distinct tissues or organs • Symmetry: variable but usually asymmetrical or radial • Type of gut: none • Type of body cavity other than gut: none • Segmentation: none; circulatory system: none; nervous system: none • Excretion: diffusion from cell surface • Flagellated cells called choanocytes drive water through pores and cavities • Main features: cellular in organization, use collar cells to move particles into particle capture chambers, organic skeleton of spongin, with silica elements

13. Phylum Porifera • Sponges feed on bacteria, trapped by flagellae of choanocytes

14. Phylum Cnidaria(benthic colonial forms, sea anemones, corals, sea whips) • Grade of construction: two tissue layers • Symmetry: radial • Type of gut: blind gut • Type of body cavity other than gut: none • Circulatory system: none • Nervous system: network of nerve cells • Excretion: diffusion from cell surface • Main Features: two basic stages—sessile polyp and swimming medusa, both tentaculate; use specialized stinging cells to capture prey

15. Cnidaria- divided into Hydrazoa, Scyphozoa (true jellyfish), Anthozoa (corals, anemones, sea whips) Basic body plans of a cnidarian, including the polyp (left) and medusa (right) stages

16. Cnidaria (Hydrozoa) Life cycle of a hydrazoan cnidarian

17. Cnidaria - Anthozoa Anemones capture prey from zooplankton to larger invertebrates and even fish

18. Anthozoa - corals

19. Anthozoa - corals

20. Anthozoa - corals

21. Platyhelminthes - Flatworms • Grade of construction: organs derived from three tissue layers • Symmetry: bilateral • Type of gut: blind • Type of body cavity • Other than gut: none • Circulatory system: none • Nervous system: small bundles of nerves (ganglia), two ventral nerve cords • Excretion: excretory organs in many species • Main Features: bilaterally symmetrical, flattened free-living worms, often with tubular pharynx to gather food; also, many parasitic species

22. Platyhelminthes - Flatworms A free-living flatworm, Prosterceraeus vittatus, which lives in northern European waters

23. Nemertea - Ribbon worms • Grade of construction:organs derived from three tissue layers • Symmetry: bilateral • Type of gut: complete, with anus • Type of body cavity other than gut: rhynchocoel surrounding proboscis • Segmentation: absent; circulatory system: present • Nervous system: small bundles of nerves (ganglia), two nerve cords • Excretion: excretory organs in many species • Main Features: elongate free-living carnivorous worms, carnivorous, using barbed proboscis to kill prey

24. Nemertea

25. Nematoda - Roundworms • Grade of construction: organs derived from three tissue layers • Symmetry: bilateral • Type of gut: complete • Type of body cavity other than gut: pseudocoel • Segmentation: absent • Circulatory system: present • Nervous system: small bundles of nerves (ganglia), two nerve cords • Excretion: special excretory cells • Main Features: small free-living and parasitic worms with only longitudinal muscles that work against hydraulic skeleton, circular in cross section

26. Nematoda • Nematodes: tend to be small, < 1 mm, diverse feeding, deposit feeders, some predators, dense in soft sediments

27. Annelida - polychaetes, oligochaetes, leeches • Grade of construction: organs derived from three tissue layers • Symmetry: bilateral • Gut: complete with anus • Type of body cavity other than gut: coelom; segmentation: present • Circulatory system: closed system • Nervous system: brain, with nerve cords and bundles (ganglia) • Excretion: excretory organs in most segments • Main features: segmented worms, with great diversity of head and locomotory appendages, move with opposing circular and longitudinal muscles in each segment

28. Polychaetes • Locomotion based on longitudinal and circular muscles working in opposition • Each segment has a pair of locomotory appendages - parapodia

29. Polychaetes • Diverse forms, free living and in tubes • Diverse feeding types: deposit feeders, carnivores, herbivores • Size varies from several cm to a few mm long • Most have swimming lecithotrophic or planktotrophic larvae

30. Polychaetes (a) Free-living Aphrodite, (b) herbivorous Nereis vexillosa, (c) Harmothoe, (d) deposit-feeding Pectinaria, and (e) Onuphis

31. Oligochaetes • Free-living, includes earthworms in soils, numerous worm-like species in marine sediments • Lack parapodia, each segment has projecting chitin hairs • In marine sediments generally small, a few mm in length • Usually are hermaphroditic, non-selfing, young released in cocoons, non-swimming

32. Oligochaetes Marine and estuarine oligochaete Paranais litoralis, that usually reproduces by fragmentation

33. Mollusca • Grade of construction: organs derived from three tissue layers • Symmetry: bilateral • Type of gut: complete, with anus • Type of body cavity other than gut: coelom • Circulatory system: usually open to large coelomic cavity • Nervous system: brain, with nerve cords and bundles (ganglia), brain very well developed in squids and relatives • Excretion: excretory organs • Main Features: most have a head-foot complex, typically externally shelled, mantle secretes shell; respires with ctenidium; includes Monoplacophora, Gastropoda (snails), Polyplacophora (chitons), Bivalvia, Cephalopoda (octopods and cuttlefish), Scaphopoda (tusk shells)

34. Bivalvia • Two (usually) equally sized valves, secreted by mantle, which forms a cavity that encloses most of body • Ctenidium used for respiration, collection of food particles in most species • Water enters and leaves mantle cavity through a siphon system • Diverse life habits, infaunal, epifaunal, foot used in most species for hydromechanical burrowing, mussels secrete byssal threads for attachment to rocks and vegetation

35. Bivalvia (a) mussel Crenimytilus, (b) suspension-feeding clam Mactra,(c) deposit-feeding clam Macoma, (d) scallop Pecten, and (e) boring bivalve Bankia

36. Gastropoda • Most have a cap-shaped or coiled shell, body also twisted (torsion) during development, some lack shells (often have poisonous compounds for defense against predators) • Feed by means of buccal (mouth) apparatus with teeth (radula) that scrape against food, such as microalgae; Conus with specialized teeth for injecting poison • Locomotion usually on a flattened muscular foot often with operculum attached that can close off and protect shell

37. Gastropoda (a) limpet Notoacmea, (b) whelk Neptunea, (c) rocky-shore snail Tegula, (d) nudibranch Tridachia, a shell-less gastropod, and (e) carnivorous Conus

38. Gastropoda Radular teeth of the snail Lacuna variegata

39. Polyplacophora - Chitons • Flattened, have 8 protecting interlocking plates • Feed with a head with radula • Herbivorous, feeding on seaweeds, coralline algae, common on rocky shores and rocky reefs

40. Polyplacophora - chitons West coast intertidal chiton Catherina tunicata

41. Cephalopoda • Benthic demersal forms include octopus and cuttlefish, complex behavior, good vision, complex eyes • Carnivorous, arms with suckers that can seize prey • Swim, but octopus move along bottom with arms, cuttlefish swim along bottom • All have photophores with ability to change color, octopus produce ink, useful in fooling prey and predators

42. Cephalopoda

43. Arthropoda • Grade of construction: organs derived from three tissue layers; segmentation • Symmetry: bilateral • Type of gut: complete with anus • Type of body cavity other than gut: coelom • Circulatory system: usually open to large coelomic cavity • Nervous system: brain, with nerve cords and bundles (ganglia), compound eyes; antennae common; compound and single eyes • Excretion: excretory organs • Main features: external chiton cuticle skeleton, segmentation, grow by periodic molting, jointed appendages; includes horseshoe crabs, shrimp, crabs, sow bugs, insects

44. Arthropoda • Trilobita (extinct) • Chelicerata (includes horseshoe crabs) • Crustacea (crabs, barnacles, lobsters, others)

45. Arthropoda - Chelicerata • Terrestrial spiders and scorpions, horseshoe crabs, pycnogonids (sea spiders) • Body in two major sections, first bearing chelicerae or claws, Merostomata (include horseshoe crabs) use these and another pair of appendages to handle food and pass to mouth. • Pycnogonids have 4 pairs of walking legs and a sucking structure protruding from the head, feeds on colonial invertebrates

46. Arthropoda - Chelicerata Horseshoe crab, Limulus polyphemus

47. Arthropoda - Crustacea • Head with 2 pairs of antennae and specialized mouthpart appendages • Feeding and/or walking appendages on thorax (midsection) • Posterior abdomen may have appendages used for respiration (shrimp) or swimming, or abdomen reflected forward over thorax (crabs) • Different groups have varying combinations of appendage specializations • Great range of feeding adaptations: crabs are largely carnivores, barnacles are suspension feeders, amphipods are detritus feeders

48. Arthropoda - Crustacea Some of the diversity of benthic crustaceans: (a) acorn barnacle, (b) stalked barnacle, (c) harpacticoid copepod, (d) hermit crab (without shell), (e) mantis shrimp or stomatopod,(f) isopod, (g) decapod crab, (h) mysid shrimp, (i) cumacean, and (j) amphipod

49. Lophophorate Phyla • Share a feeding organ, lophophore that is shaped like a loop and ciliated • Early embryology suggested that these were deuterostomes, but molecular evidence allies them with protostomes. • The presence of a lophophore is the evidence for uniting these phyla but they may be distantly related

50. Lophophorate Phyla (a) a phoronid worm, (b) an inarticulate brachiopod, and (c) opened articulated brachiopod, showing lophophore feeding organ (green)