Polychaete, sipunculan and mollusk larvae
Polychaete larvae often broadcast their eggs into seawater and are fertilized synchronously as in the Palolo worm. Other like the serpulids, brood their eggs within their tubes. Most polychaetes form a a trochophore larva as shown below. Depending on the amount of yolk, the trochophore may be lecithotrophic or may develop a digestive system and become planktotrophic. In many, the trochophore is passed in the egg and the hatching form is a more advanced trochophore that has some segementation and setae. These are referred to as metatrochophore as shown below (right).
Planula larvae
http://www.biology.ualberta.ca/courses.hp/zool250/Labs/
Lab04/Aurelia-planula+SEM.gif
The larval form typical of corals and cnidarians in general is called a planula. In the case of broadast spawners, the larvae are formed in the water. A moderately-sized colony may produce up to several thousand planulae per year. After floating at the surface for some time, the planulae swim back down to the bottom, where, if conditions are favorable, they will settle and begin a new colony. In most species, the larvae settle within two days, although some will swim for up to three weeks, and in one known instance, two months. In contrast, some coral species brood planulae within the polyps after internal fertilization. While spawning is associated with high numbers of eggs and planulae, brooding results in fewer, larger and better-developed planulae. Once the planulae settle, they metamorphose into polyps and begin to form colonies which increase in size asexually by budding. The new colony becomes sexually mature at a minimum size, depending on the species. Some massive species reach sexual maturity when polyps grow to about 10 cm in diameter, which
Hermaphroditic spawning and coral life cycle
http://www.aims.gov.au/pages/reflib/bigbank/pages/bb-09e.html
References: Veron, J. 1995. Corals in Space and Time.
http://www.coris.noaa.gov/about/biology/biology.html
www.microscopy-uk.org.uk/micropolitan/ marine/larva/frame7.html
Annelids
The Annelida is a phylum of soft-bodied, bilaterally symmetrical, segmented animals, known as worms. Over 12,000 known species are grouped in three classes: the earthworms and freshwater worms (Oligochaeta), the leeches (Hirudinea), and the marine worms (Polychaeta). Annelids are found throughout the world, from deep ocean bottoms to high mountain glaciers. They live in protected habitats such as mud, sand, and rock crevices, and in and among other invertebrate animals, such as sponges. Many live in tubes they secrete around themselves.
The vast majority of the more than 8,000 known species of polychaete worms are marine.. They are abundant from the intertidal zone to depths of over 5,000 m. The polychaetes, so named because of the numerous setae (chaetae) they bear, range in length from less than 18 in. to more than 9 ft (2 mm to 3 m), but most are from 2 to 4 in. (5-10 cm) long. Their colors are often brilliant, and some species are iridescent. The class has usually been divided on the basis of mode of existence into two groups, the Errantia and the Sedentaria.
Errant polychates
The most easily distinguished feature of errant polychates is their well developed head (prostomium) composed of eyes, tentacles and palps or jaws in some species. Each segment of the body displays extensions called parapodia, and these are often associated with many setae (chetae).
The digestive tract of errant polychaetes is straight, extending from the mouth to the anus. The may be modified according to the type of diet, which runs the gamut from detritivore to carnivore. Respiration is entirely through the body wall in some polychaetes, and partially so in most. Many species have thin-walled extensions of the body surface, i.e., gills, used for gas exchange; most commonly the gills are extensions of the parapodia. Errant polychaetes typically have a pair of excretory tubes (nephridia) in each segment.
A bristleworm, so-called because of the distinctive and numerous white setae extending from the parapodia.
The tentacles of feather-duster worms are used for respiratory exchange as well as for feeding.
Sedentary Polychaetes
Sedentary polychaetes are usually adapted to living permanently in tubes or burrows; some attach themselves to rocks or piers. Many sedentary polychaetes, like the lugworm, Arenicola, live in u-shaped burrows in sand or mud. The majority, however, are tube builders open only at the oral end.
Many are permanently attached to the tube and the worm cannot leave it. Therefore, they have modifications not found in errant polychaetes, including a u-shaped gut with a single pair of nephridia that open below the mouth to insure that wastes will be washed away and not foul the tube. The parapodia are reduced in the sedentary polychaetes, and the setae of many tube-dwelling forms are hooked to help the worm hold itself to the wall of its tube.
Tubes of different species vary greatly in their composition and structure. They may be composed of sand, shell, or other particles held together with mucus, or made entirely of organic substances secreted by the worm that harden on contact with water. One of these is the family Sabellariidae which makes tubes out of plate-like sand grains. When they are well-developed, “worm reefs” may form (see attached reading on this family).
Sedentary polychaetes often have greatly modified head regions for specialized feeding habits. Many are adapted for feeding on organic matter deposited on the ocean floor. Other worms have feeding tentacles that extend from the tube opening and creep along the mud or sand, picking up organic deposits. These are indirect deposit feeders as opposed to some errant polychaetes (direct deposit feeders) that consume sediment directly by swallowing it.
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Still other Sedentaria are filter feeders: the beautiful feather-duster worms (Serpulidae) and parchment tube (Sabellidae) worms have a prostomium composed of a crown of feathery, ciliated radioles that extend from the tube. Organic particles are trapped on the radioles and driven by cilia in a groove where they are sorted (larger particles are rejected) and then accepted by ciliated tracts around the mouth. Particles in suspension are captured by cilia using the radioles as a filtration system, so these organisms are filter feeders.
Sabellid worms construct parchment-like tubes using a protein secretory gland on the pharynx. The radioles are cup-like rather than sprialled.
Polychaete Reproduction
Most polychaetes reproduce sexually, and the sexes are separate. Sex cells develop from masses of tissue in the metameres and leave by way of tubules or by rupture of the body wall. In most cases fertilization of the eggs by sperm occurs externally in seawater and results in the formation of free-swimming larvae. Variations include internal fertilization, laying of egg masses that are attached to objects with mucus, and brooding of developing eggs in the worm's body. Some errant polychaetes, including the palolo worm, undergo extreme changes in appearance and become active swimmers at the time of year that the sex cells mature; males and females swarm to the surface of the sea to spawn. In some of these species the portion of the body containing the sex cells breaks free and engages in swarming and spawning, leaving the asexual portion behind to regenerate its lost parts. Swarming generally occurs at night and is correlated with particular phases of the moon. The most famous swarming polychaete is the tropical palolo worm, a name sometimes applied to all swarming polychaetes.
Palolo swarming
Palolo worms are a group of reef-dwelling polychates that are circumtropical in their distribution. The Samoan palolo (Eunice viridis) lives in shallow coral reefs throughout the south central Pacific. It is about 12 inches long and lives in burrows dug into the coral pavement on the outer reef flat. When fertile, the worm is composed of two distinct sections (see drawing). The sterile, front section containing the eyes, mouth, look very much like a typical errant polychaete and is called the atoke. This is followed by an odd-shaped string of segments called the epitoke that contain gametes colored blue-green in females and tan in males. Each of the epitoke segments bear a single, tiny eyespot that can detect light.
Once or twice a year, when the epitokes are mature, the palolo will back out of their burrows and release the epitoke section at night. They will then swarm to the surface of the sea in large numbers and will twirl around in the water looking like dancing spaghetti. Around daybreak, the segments dissolve and release the eggs and sperm that they contain. Left to themselves, the fertilized eggs hatch into small larvae that drift with the plankton until settling on a coral reef as a juvenile.
swarming epitokes in situ
However, Samoans and other Polynesian groups are aware of the spawning periodicity and eagerly await these nights. With lanterns in hand to attract them, the islanders scoop up large amounts of this delicacy along the shoreline with hand nets. This gift from the sea was traditionally greeted with necklaces made from the fragrant moso’oi flower and the night of the palolo was and still remains a happy time of celebration. The epitokes are enjoyed raw or fried with butter, onions or eggs, or spread on toast. Its fishy flavor is said to be an acquired taste. This reproductive phenomenon is well known in
Epitoke swarming is a classic example of the coordinated mass spawning of a simple marine organism. The worms emerge from their burrows during a specific phase of the moon, but the actual date is a bit complicated. The swarms occur on the evenings of the last quarter moon of spring or early summer. In
Freshly caught epitokes
Please pass the ketchup.
Trochophore larva