title used in learnabout sections of A SNAIL'S ODYSSEY
  Reproduction
   
  Polychaetes as a group have a broad spectrum of reproductive modes. Within tubeworms there are species that brood inside of tube, considered here, and SPECIES THAT BROADCAST GAMETES, SPECIES THAT BROOD OUTSIDE OF TUBE, and SPECIES THAT ARE POECILOGONOUS, considered in other sections. There is also a general section on LARVAE.
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  Species that brood inside of tube
   
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Research study 1
 

drawing of several egg capsules of the spionid worm Pseuodopolydora paucibranchiatadrawing of pre-hatching stage of spionid worm Pseudopolydora paucibranchiataThe spionid worms Pseudopolydora paucibranchiata and P. kempi lay eggs in capsules and brood them within their tubes until they hatch as juveniles.  In tidal flats of California bays and estuaries both species appear to reproduce most of the year. Both species construct tubes of silt and/or fine sand. However, the 2 species differ markedly in their pattern of development.  All eggs in P. paucibranchiata are fertilised before being encapsulated. Each capsule is about 0.6mm in length and contains 35-50 eggs, each 100um in diameter. Capsules are deposited in strings attached by cytoplasmic extensions to the inner wall of the tube, and there are about 8 capsules per string (see drawing upper Left). The eggs hatch to a 3-setiger stage, and settlement occurs at the 13-17 setiger stage (developmental stages shown only for P. paucibranchiata). 

drawing of pelagic 5-setiger larval stage of the spionid worm Pseudopolydora paucibranchiatadrawing of pelagic 13-setiger larval stage of the spionid worm Pseudopolydora paucibranchiataIn comparison, only a small portion of the encapsulated eggs in P. kempi are fertilised, and the unfertilised ones fragment into separate yolk granules which are eaten by the developing embryos.  The worms remain in the capsule until they have about 15 setigers.  Planktonic life is shorter in P. kempi than in P. paucibranchiataBlake & Woodwick 1975 Biol Bull 149: 109.

NOTE  setiger = segment

 
Research study 2
 

photograph of spaghetti worm Thelepus crispus with tentacles spreadThe spaghetti worm Thelepus crispus (28cm length) broods its eggs inside its tube.  The tube is constructed of coarse sand and gravel attached to the undersides of rocks in intertidal areas from Alaska to southern California.  In San Juan Island, Washington it broods continuously over a 6-mo period (July-December). A single brood consists of paired, elongated egg masses attached to the inside of the maternal tube.  Maximum oocyte diameter is 400┬Ám and fecundity averages about 30,000 eggs.  The eggs give rise to larvae that are brooded to the 1-setiger stage and then released. The larvae undergo a 1-d planktonic period before settling and becoming juveniles at the 8-setiger stage.  In laboratory culture at Friday Harbor Laboratories, Washington a 1-setiger stage is reached by about 3d at 14oC, a 3-setiger stage by 5d, and an 8-setiger stage by 26d.  At this time the juvenile is about 0.8mm in length and has a functional gut.  McHugh 1993 Biol Bull 185: 153.

NOTE  the study includes 3 other terebellid species that are not mentioned here.  Eupolymnia crescentis and Neoamphitrite robusta are both free-spawners, while Ramex californiensis is an internal-tube brooder like Thelepus

A spaghetti worm Thelpus crispus with tentacles spread out for
feeding. The worm's burrow is where the tentacles meet at the
bottom centre of the photo. If you look carefully, you can see
some of the tentacles transporting detrital food to the mouth 0.7X

 
Research study 3
 

drawing of spirorbid worm Bushiella abnormis showing opercular egg-sacdrawing of serpulid worm Protolaeospira eximia showing brood pouch that lies within the tube (tube not shown)drawing of spirorbid worm Paradexiospira vitrea showing eggs being incubated in a sac attached to the inner tube wall (tube not shown)In many marine-invertebrate taxa, small body size is associated with brooding.  A proposed explanation1 for this is that large individuals produce proportionately smaller broods because brood-space scales2 more slowly with increasing body size than does fecundity, thus making brooding a less effective strategy for larger species.  A study on several species of spirorbids in San Juan Islands, Washington, however, fails to show any scaling3 constraints on brood size.  Two of the species studied (e.g., Bushiella abnormis, see drawing upper Left) brood their young outside the tube in a pouch associated with the operculum. Three other species (e.g, Protolaeospira eximia, see drawing upper Right) brood inside the tube within a pouch, and Paradexiospira vitrea (see drawing lower Left) and Circeis armoricana (not illustrated) brood by attaching their eggs to the inner tube wall. Brood sizes for the 5 species studied range from 14-60. Hess 1993 Am Nat 141: 577.

NOTE1  see Strathmann & Strathmann 1982 Am Nat 119: 91

NOTE2  while fecundity is expected to scale as cube of body length in spirorbids, brood space (if a brood is held onto the body surface or attached to the inner wall of the tube) will scale only as square of body length.  If just the species that brood in external or internal sacs are considered, then fecundity should scale isometrically with brood size.  However, when body-wall or tube-wall brooders are thrown into the mix, then fecundity would likely scale in a negative allometric way with brood size.

NOTE3  the author uses a reduced major-axis regression method to determine scaling exponents rather than an ordinary least-squares regression method. The former apparently avoids under-estimation of scaling exponents and false indication of scaling restraints where none exists

 

 
Research study 4
 

photograph of a sabellid worm Terebrasabella heterouncinata courtesy Kuris & Culver 1999 Invert Biol 118: 391 Another species that broods its embryos inside its tube is the sabellid Terebrasabella heterouncinata, a pest species introduced from South Africa.  This worm is of concern to abalone culturists in California because it bores into the shells of their stock abalone Haliotis rufescens, seriously affecting growth and survival. Control measures include coating abalone shells with wax, manipulation of temperature, quarantine, and improved sanitation.  Up to the time of the this study, researchers are not sure whether the species is self-fertilising.  A study at the Bodega Marine Laboratoy, California in which F1 larvae are raised in isolation to fully reproductive adults (26-32wk at 18oC) that in turn produce fully functional F2 offspring, indicates that the species is a functional hermaphrodite.  Finlay et al. 2001 J Shellf Res 20: 883. Photo courtesy Kuris & Culver 1999 Invert Biol 118: 391.

NOTE  more on the biology of this species can be found in this section of tubeworms at HABITATS & ECOLOGY: PARASITIC TUBEWORMS

 

Abalone parasite
Terebrasabella heterouncinata

 
Research study 5
 

photograph of spionid worm Polydora cornuta courtest Rice et al. 2008 Invert Biol 127: 45A good description of reproduction in the internal-tube brooding Polydora cornuta is provided by a group of researchers based at the University of Tampa, Florida.  Sexes are separate and males produce spermatophores or sperm packets that they leave around the laboratory culture vessel.  Females locate and pick up these packets with their palps (see photograph) and pull them to the mouths of their tubes.  The packets break open and the sperm cloud is sucked into the tube along with the respiratory current.  One spermatophore provides enough sperm for 7wk or 8 spawning events, or around 2800 eggs in total.  Eggs are deposited within capsules (number per capsule not specified) inside the female’s tube, where they hatch to 3-setiger (3-segment) nectochaetes within 5d.  Interestingly, the female releases the larvae by breaking the capsules one by one with her mouth.  A bit of wiggling then carries the larvae from the tube. Metamorphosis occurs 6-9d after their release from the female’s tube (at 22-25oC) at a size of about 16 setigers. Adult worms have about 26 setigers.  The authors provide a wealth of information on various aspects of reproduction in P. cornuta not included here. Rice et al. 2008 Invert Biol 127: 45.

 
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