title used in learnabout sections of A SNAIL'S ODYSSEY

Polychaetes as a group have a broad spectrum of reproductive modes. Within tubeworms are SPECIES THAT BROOD INSIDE OF TUBE, SPECIES THAT BROOD OUTSIDE OF TUBE, SPECIES THAT ARE POICILOGONOUS, and SPECIES THAT BROADCAST GAMETES. Some species have ASEXUAL REPRODUCTION.

This topic begins with a general introductory section on tubeworm larvae.

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Research study 1

drawing showing food-particle capture in a tubeworm larvaphotographs of developmental stages of tubeworm Schizobranchia insignisMany tubeworms produce free-swimming trochophore-type larvae, but whether they feed or not depends upon the taxon.  Larvae of marine invertebrates that have evolutionarily lost the requirement for feeding usually also lose the structures involved in capturing, ingesting, and digesting food.  Studies at Friday Harbor Laboratories, Washington, however, show that at least one species of tubeworm, Schizobranchia insignis, has a functional ciliary system for capturing and concentrating food particles, but that is the end of it. The larvae lack a functional gut and the food particles are rejected.  Note in the drawing on the Left that food particles are able to be captured by the cilia of the prototroch and directed to the food groove. Capture of particles is aided by the metatrochal cilia. Particles rejected at the mouth are moved posteriorly by cilia of the neurotroch. In other tubeworm larvae the edible particles are ingested and soon pack the gut. In S. insignis, however, the mouth has no connection to the midgut which, in any case, is compressed to small size by cells swollen with energy reserves. The author hypothesises that these sabellids may have lost the ability to feed relatively recently, or the ciliary bands may have an alternate function, perhaps swimming.  The prototrochal bands clearly function in swimming in other polychaete larvae, whether the larvae are feeding or nonfeeding.  Other functions considered by the author are tube formation and possible involvement in settlement.  Pernet 2003 Biol Bull 205: 295.

NOTE  in laboratory culture at 8-10oC the embryos become trochophores 2d after fertilisation, and are competent to settle and metamorphose 3-30d after fertilisation

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Research study 2

photograph of tubeworm Hydroides elegansphotograph of larval tubeworm Hydroides elegansThe small cosmopolitan tubeworm Hydroides elegans is an annoying and costly fouling species that inhabits tropical and subtropical areas around the world, and is now present in southern California. Extensive studies in laboratories in Hawai’i, Hong Kong, and elsewhere have provided much information on factors in bacterial films that induce settlement and metamorphosis of Hydroides larvae and, recently, a consortium of scientists working at the California Institute of Technology, Pasadena has added further details. The California researchers specifically investigate settlement-inducing features of a commonly occurring marine bacterial species Pseudoalteromonas luteoviolacea. The bacterium has an unusual spear-like projection, called a phage-tail, that it uses in defense of attack by other bacteria. When an Hydroides larva in search of a settlement site contacts a phage-tail it immediately ceases swimming, drops to the bottom, and begins shedding its locomotory cilia preparatory to metamorphosis. If the larva is shortly removed from contact with the bacterium, it reverses this behaviour and continues swimming. To determine what factors in the tail are involved, the researchers first delete the gene in the bacterium responsible for tail production and show that metamorphically competent Hydroides larvae will swim past these tail-less bacteria without slowing. In a follow-up series of genetic manipulations the researchers then identify a block of 6 genes that when deleted in otherwise normally tailed bacteria cause the larvae to settle but not shed their cilia or complete metamorphosis. It is not known what these genes do for the bacterium, but the Hydroides larvae are clearly recognising and responding to one or more of their products. Knowledge of the developmental cascade initiated in Hydroides by the bacterium has important implications for understanding bacteria-animal interactions related to biofouling. Shikuma et al. 2016 Proc Nat Acad Sci 113 (36): 10097-10102. Photographs of H. elegans courtesy Brian Nedved, Kewalo Marine Laboratory, University of Hawai'i.

NOTE such films are known cues for settlement of many marine invertebrates, including sponges, corals, crabs, sea urchins, and tunicates

NOTE this requires that the genome of Hydroides be sequenced and then gene expression upon exposure to bacterial phage-tail structures be assesssed