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Page 1 of 11 Biology 1B – Animal Diversity and Evolutionary Principles Spring 2011 Figure source: Brusca and Brusca, 2003. Invertebrates, second edition. Sinauer Associates, Inc. LOPHOTROCHOZOA: LOPHOPHORA AND ANNELIDA Let's take a closer look at the word "Lophotrochozoa." The first part "lopho-" refers to the circular or horseshoe-shaped ring of coelomate, ciliated feeding tentacles, the lophophore, possessed by a clade of animals called the Lophophora. We've already seen how the "trocho-" part of the word refers to the trochophore larva of molluscs and annelids. This larva bears one or more bands of cilia, called trochs, which it uses to swim and to feed. Lophophoran subclades (formerly phyla): Phoronida – unsegmented worms, live in tubes in sand Brachiopoda – lamp shells Bryozoa – colonial animals Trochozoan subclades (formerly phyla and classes): Phylum Annelida Polychaeta – marine segmented worms Oligochaeta – earthworms Hirudinea – leeches Pogonophorans – deep-sea beard worms Echiurans – spoon worms Sipunculans – peanut worms Phylum Mollusca (covered in a separate lab) Page 2 of 11 Lophophora: Bryozoans The Bryozoa are the only entirely colonial phylum of animals. You will (hopefully!) recall that we have already encountered the phenomenon of coloniality when we studied the cnidarian hydrozoans. Is this an example of convergence or divergence? As in the hydrozoans, bryozoan colonies come in a variety of shapes and colors, from encrusting sheets to upright foliaceous forms. Some grow only on specific surfaces, such as kelp blades, and others grow just about everywhere. Although most of the ~4000 bryozoan species are marine, there are some freshwater species. A bryozoan colony is made up of interconnected units called zooids. Each zooid lives in its own tiny exoskeleton and has its own lophophore, which it extends for suspension feeding. As in the hydrozoans, some bryozoan colonies exhibit polymorphism, where the zooids become specialized for separate functions such as feeding, reproduction, and defense. Page 3 of 11 Lophophora: Brachiopods The extant brachiopods are a mere fraction of the brachiopod diversity that flourished in Paleozoic seas. The endPermian mass extinction (~250 mya) slammed the brachiopods, which had until then been the dominant bivalved invertebrates. So, how would you distinguish between a bivalved mollusc and a brachiopod? The easiest difference is in the shells. Molluscan bivalves have two shells, left and right, and generally look the same. Brachiopods have two shells but they are dorsal and ventral, and are unequal in size, with the ventral shell generally being smaller. The entire body is enclosed within the dorsal and ventral shells, which open just a tiny bit to allow water to flow over the lophophore. The lophophore itself is curled up and may be supported by an armlike structure called the brachium. Page 4 of 11 LOPHOTROCHOZOA: Phylum Annelida (the segmented worms) The Phylum Annelida has traditionally been the phylum of segmented worms, best represented in the sea by members of the Class Polychaeta and on land by the Classes Oligochaeta (earthworms) and Hirudinea (leeches). However, recent molecular studies indicate that the group of (mostly) wormlike creatures called annelids might be more properly categorized into two other subclades, the Errantia and the Sedentaria. In this scheme, the Sedentaria comprises the oligochaetes, hirudineans, echiurans, and some polychaetes, while the remaining polychaetes make up the Errantia. The good thing about the names Errantia and Sedentaria is that for the most part they correctly describe the animals: the errant worms generally crawl around, and sedentary ones don't. In errant polychaetes segmentation is usually visible externally as a series of annulations in the body wall, each of which bears a pair of lateral parapodia with setae. Sedentary polychaetes, which don't move around as much as their errant relatives, often have reduced parapodia and external segmentation. Some tube-dwelling polychaetes, namely Chaetopterus, demonstrate what appears to be a form of serial homology, having a heteronomous body that is divided into distinct functional regions. We will revisit the concept of serial homology when we discuss arthropods. Page 5 of 11 Page 6 of 11 Page 7 of 11 Here are some photos of tube-dwelling polychaetes. Are any of them familiar to you? Page 8 of 11 Errant polychaetes tend to be predators or scavengers (or both). Their roaming lifestyle allows them many opportunities to encounter prey, and some of them are armed with elaborate weapons to subdue prey. Some polychaetes have an eversible pharynx that can be shot out of the mouth at prey and is armed with either one or two pairs of chitinous jaws that can be large enough to take a chunk of flesh out of your hand. Living in a tube severely limits an animal's ability to forage actively for live prey, so tube-dwelling polychaetes tend to be suspension feeders or deposit feeders. Suspension feeders feed on particles in the water column, either passively (catching particles that fall on them) or actively (using metabolic energy to move water through a filtering apparatus of some sort); the latter are called filter feeders. The figure below diagrams suspension feeding in two polychaetes. Page 9 of 11 Deposit feeding polychaetes collect organic matter that settles onto the substrate around them, and most of them live in burrows. Active deposit feeders actually ingest the surrounding substratum and digest the associated organic materials; much of their fecal matter consists of sediment that passes unprocessed through the digestive system. Selective deposit feeders are able to sort the desired organic matter from the indigestible sediments before ingesting anything. Page 10 of 11 LOPHOTROCHOZOA: Annelida (Subclade Oligochaeta) The oligochaetes (earthworms and kin) live mostly on land or in fresh water. Earthworms are important members of the underground community. They aerate soils, and thus prevent compaction, and drive the decomposition of buried plant material. Their castings (worm poop) make great organic fertilizers. Australia is a laboratory of natural selection, and some mighty odd animals have evolved (and been driven extinct by humans) there. One example is the giant Australian earthworm, Megascolides australis. It routinely grows to lengths of 2 to 3 meters! Earthworms are hermaphrodites, possessing functional ovaries and testes at the same time. When the worms mate, they line up anti-parallel (what does that mean?) and exchange sperm to fertilize their own eggs. Then each worm goes off to deposit its fertilized eggs in a cocoon in the ground, where they hatch into little wormlets. Page 11 of 11 LOPHOTROCHOZOA: Annelida (Subclade Hirudinea) Leeches are highly derived annelids, lacking both external segmentation and bristles. They are a sister group to the Oligochaeta. Although many of them are free-living predators, the more notorious ones are parasites that feed on the blood of vertebrates. They live in freshwater ponds and streams. The parasitic leeches can detect a warm-blooded animal and home in on it. Their mouth contains a set of three jaws to pierce the skin of their prey. They also have a very complex saliva that contains: (1) an anaesthetic, to keep the prey from reacting to the bite; (2) a blood vessel dilator, and (3) an anticoagulant to keep the blood flowing. Although they seem gross to us, leeches have been used medicinally for several centuries, and have even been incorporated into modern medicine. They can help reduce swelling after injury or surgery, and are most helpful when surgeons are attaching lost limbs or digits.