title used in learnabout sections of A SNAIL'S ODYSSEY
  Reproduction
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  Species that are poecilogonous
 

Tubeworm species that are poecilogonous are considered here, while SPECIES THAT BROOD INSIDE OF TUBE, SPECIES THAT BROOD OUTSIDE OF TUBE, and SPECIES THAT BROADCAST GAMETES are considered in other sections. There is also a general introductory section on tubeworm LARVAE.

This section on poecilogonous tubeworms is separated into genera for convenience

NOTE  lit. “varied offspring/reproduction” G. Poecilogony is usually defined as occurring in a single species, but has also been applied to an individual female in a species.  More information on peocilogony can be found in the ODYSSEY at LEARN ABOUT NUDIBRANCH: ALDERIA

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Boccardia

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Research study 1
 

drawing of 15-setiger larva of Boccardia proboscidea collected from planktonThe spionid worm Boccardia proboscidea is a common, tube-dwelling intertidal species in California that favours life amongst soft stones and within narrow rocky crevices.  An early report on its reproduction describes the enclosure of 20-80 eggs within soft, cocoon-like capsules attached by double stalks within the adult’s tube.  The author observes hatching of eggs and development to a 12-15 segmented stage that is active within the cocoon, with some cannibalism occurring (see drawing below), but these stages die before breaking out of the cocoon in this particular culture.  If freed, however, the larger stages forage about for a time, then settle and build tubes.  The author describes 3-15 segmented larvae as being common in the plankton in springtime.  Hartman 1941 Allan Hancock Pacific Expedition 7: 299.

NOTE the author describes only one feature of its reproduction. Later reports indicate a poecilogonous, or variable type of reproduction, as described for another spionid species Streblospio benedicti in Research Study 2 below. Note that this description could also have been included in the section "species that brood inside of tube"

 

 

 

Features of development:

 
drawing of freshly laid egg capsule of spionid worm Boccardia proboscidea drawing of young larval stage of spionid worm Boccardia proboscidea drawing of young larval spionid Boccardia proboscidea devouring undeveloped blastula within capsule drawing of 3-setiger larval stage of spionid worm Boccardia proboscidea
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Research study 2
 

photographs of different-aged planktotrophic larvae of the spionid worm Boccardia proboscideaA later study on Boccardia proboscidea by a researcher at the Scripps Institution of Oceanography, La Jolla confirms that this spionid actually has a poecilogonous1 type of development, that is, a female may produce planktotrophic larvae2, as described in Research Study 1 above, as well as bottom-crawling juveniles within a single egg capsule.  Poecilogony occurs among broods in different females, or within a brood from a single female.  Interestingly, the type of brood produced appears to be related to whether or not a female includes nurse eggs3 in the capsule.  Females with a low proportion of nurse eggs (about 15%) have larvae that become free-living at a 3-5 setiger (= segment) stage, are planktotrophic, and settle after about 20d.  However, females with a high proportion of nurses eggs (about 90%) have 2 types of offspring: ones that ingest nurse eggs, mature quickly, and hatch as crawling juveniles, and ones that are planktotrophic for about 30d before settling.  Finally, females with few or no nurse eggs have larvae that break free of the capsules at a 3-setiger stage and are planktotrophic for 30d before settling.  Adults of each type are morphologically similar, but consistently produce the same type of broods.  Thus, there appear to be 3 developmental patterns within a single population.  Reciprocal crosses of the last 2 developmental types described here yield interfertile offspring.  This observation relates to the question raised by the author as to whether the 3 reproductive morphs described belong to a single species.  Gibson 1997 Invert Biol 116 (3): 213.

NOTE1  poecilogony has apparently been reported before for this species, including in Research Study 1 above, but the present study is the first to describe 3 types of development in a single population

NOTE2  the author arbitrarily defines a larva as  having 16 setigers or less, while a juvenile has 17-45 setigers and a mature has more than 45 setigers.  Larvae feed on phytoplankton, while juveniles in the plankton feed on mixed algae  and small zooplankters including invertebrate larvae

NOTE3  nurse eggs are non-viable eggs that are eaten by the other  developing larvae, a behaviour termed adelphophagy

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Research study 3
 

photograph of tubeworm Boccardia proboscideaIn a later paper the author in association with research colleagues at Acadia University, Nova Scotia shows through molecular and morphological comparisons that Boccardia proboscidea is, indeed, a single species.  Most of the genetic variance found is attributable to individual differences and to variation between geographic regions.  There is no significant genetic difference between the developmental morphs.  Similarly, although there is some morphological differences between geographic areas, no major differences are disclosed between reproductive morphs.  The authors conclude that B. proboscidea is a single, poecilogonous species. Gibson et al. 1999 Mar Biol 134: 743.

NOTE  DNA amplification by RAPD-PCR technique.  Morphological comparisons are done using bright-field and scanning-electron microscopical techniques

 

 

Head end of female Boccardia proboscea with planktotrophic
development showing feeding palp and segments

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Research study 4
 

map showing collection sites for genetics study on Boccardia proboscideaA recent genetics study done by a consortium of researchers on Boccardia proboscidea throughout its west-coast distribution supports the view that it is a single species with 3 different reproductive strategies. Its genetic makeup, however, is interrupted by a putative break at Point Conception, California creating 2 groups, a larger, more genetically homogeneous northern group, and a smaller, more heterogeneous southern group.  The northernmost worms have just a single mode of reproduction, involving nurse eggs and hatching at an advanced larvae or crawl-away juveniles, while the southernmost ones have all 3 reproductive modes.  Other findings from the study are that higher latitude females supply their offspring with more extra-embryonic nutrition (nurse eggs) than do comparably reproducing southern females, and brood for a significantly longer time. Observations on worms in culture show, among other things, that the females play an active role in brooding of the offspring, a behaviour noted in previous studies.  Not only do they clean and ventilate the egg cases, but they liberate the embryos from their cases. Oyarzun et al. 2011 Evolution & Development 13 (6): 489.

NOTE  the researchers use 2 mitochondrial markers 16S rDNA  and Cyt b as well as some interbreeding experiments

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Streblospio

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Research study 1
 

map of North America showing distribution of the spionid worm Strblospio benedictiStreblospio benedicti is a small (10-20mm) tube-inhabiting spionid that lives close to the sediment-water interface.  It both suspension-feeds and deposit-feeds.  It is common on both sides of the North American  continent and, on the west coast1, inhabits beaches from southern California to at least southern British Columbia. Interestingly, on the Atlantic and Gulf of Mexico coasts its reproduction involves planktotrophic larvae, while on the Pacific coast the larvae are entirely lecithotrophic.  The presence of multiple reproductive modes in a species is known as poecilogony2.  In planktotrophic forms the females produce many small eggs (100-500 at 70-90µm), which they brood in paired dorso-lateral pouches and release as larvae at the 3- to 7-setiger3 stage of development.  The larvae bear long setae and feed for several weeks in the plankton before settling at the 9-setiger stage.  These planktotrophic larvae are strong swimmers and exhibit positive phototaxis.  In contrast, the lecithotrophic forms produce a few large eggs (10-50 at 100-200µm diameter) that are brooded in dorsal pouches for about a week (in culture at 20oC).  The larvae are released at the 9- to 12-setiger stage, with ample yolk supply, and are competent to settle immediately. However, in the absence of suitable substratum they graph showing lengths of planktonic larval life in the 2 contrasting life styles of the poecilogonous spionic Streblospio benedictimay remain planktonic for several days, subsisting on their yolk.  The author describes cross-breeding experiments that show that the planktotrophic and lecithotrophic forms of S. benedicti are not reproductively isolated and thus do not merit separate species status.  Levin 1984 Biol Bull 166: 494; see also Schulze et al. 2000 Evolution 54: 1247 for a discussion of the evolution of poecilogony and evolution of Streblospio spp. 

NOTE1  in a later paper researchers at the University of Georgia and NY University compare genetic structure of populations of Streblospio benedicti on both east and west coasts and, among other things, find that the recently introduced west-coast populations probably originated from a lecithotrophic population near Delaware about 100yr ago.  Zakas & Wares 2012 Mol Ecol 21: 5447.

NOTE2  mispelled "poicilogony" in the paper

NOTE3  “setiger” is a segment that bears setae. Similar terminology is a "chaetiger", referring to a segment that bears chaetae

 

 

 
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