title used in learnabout sections of A SNAIL'S ODYSSEY
  Parasitic tubeworms
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Research study 0
 

drawing of spionid tubeworm Polydora sp. burrowing into a mollusc shellMany species of tubeworms infest the shells of abalones and other gastropods.  These include representatives from several families including Sabellidae, Serpulidae, Spirorbidae, and Spionidae.  Within Family Spionidae only genus Polydora and a few related genera are capable of boring.  How they do this is the subject of an early investigation by researchers at Pacific Marine Station, Dillon Beach, California.  The photograph of a scallop Patinopecten magellanicus infested with spionid tubeworms Spiona spp.worms commonly use commercially important species of oysters and scallops as hosts, but they may also inhabit gastropod shells, especially those inhabited by hermit crabs.  Burrows may not always penetrate the shell; if they do not, they are formed in a layer of mud on the shell.  Worms that do penetrate create a U-shaped tube (see drawings on Left), but whether they accomplish this mechanically with spines or chemically with acid/enzymes is a main subject of discussion in the paper.  However, the actual burrowing process is neither well described nor well figured. The authors include a review of world species of PolydoraBlake & Evans 1972 The Veliger 15 (3): 235.

X-ray photograph of scallop shell Placopecten
magellanicus
infested with spionid worms
Polydora conchatum
(larger multi-branched
tubes in central portion) and P. websteri
(smaller single-branched tubes near edges) 0.6X

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Research study 1
 

photograph of a shell of a dead abalone Haliotis rufescens heavily infested with boring organisms As we have seen from the above, spionid tubeworms may be most damaging because they settle on the outer surface and bore into the shell.  Spirorbid tubeworms are easily identified by their curled tubes and, because they usually inhabit the outer surface of the shell, are much less harmful to their hosts (see photo on Right).  Other shell-boring organisms, such as fungi, sponges, bryozoans, and barnacles are much less damaging to their host than some of the tubeworms because they settle mostly on old shell and not on apertural areas where shell-photograph of a spirorbid tubeworm Spirorbis sp. on the shell of an Astraea gibberosasecretory processes are ongoing. Information from Kuris & Culver 1999 Invert Biol 118: 391.

Tubeworm Spirorbis sp. on a
topshell Astraea gibberosa 1X

 

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Research study 2
 

photographs of juvenile abalone Haliotis rufescens infested and uninfested with the sabellid parasite Terebrasabella heterouncinataAn introduced sabellid Terebrasabella heterouncinata in California has become a serious pest in some abalone Haliotis rufescens mariculture facilities because it grossly deforms the abalone’s shell and causes temporary or permanent cessation of growth.   Respiratory holes may fuse shut, thus interfering with gas photograph of an abalone's respiratory hole infested with parasitic tubeworms Terebrasabella heterouncinataexchange, urinary release, and gamete dissemination. 

The worm lives inside apertures, such as shell holes in abalones, or main apertures of other gastropods, at the growing edge of the shell (see photo on Right). When the worm initially positions itself as a larva at the aperture edge of a potential host it covers itself in a mucous sheath.  The host recognises the worm as non-self and secretes nacreous shell material around it, in the same way that pearls or blister pearls are formed.  The host actually creates the tube of the parasite. 

Settlement of the worm at the shell aperture stops prismatic growth of the shell and only nacre is secreted. A typical sequence of events is shown here:

 
number 1 in a series of drawings showing how the presence of a parasitic sabellid worm Terebrasabella heterouncinata disrups normal shell formation in an abalone
Normal growth involves secretion of prism and nacre by the mantle edge
number 2 in a series of drawings showing how the presence of a parasitic sabellid worm Terebrasabella heterouncinata disrups normal shell formation in an abalone
Settlement of a worm disrupts prismatic secretion and protective nacre is secreted
number 3 in a series of drawings showing how the presence of a parasitic sabellid worm Terebrasabella heterouncinata disrups normal shell formation in an abalone
The abalone commences prismatic secretion only when the worms are covered
number 4 in a series of drawings showing how the presence of a parasitic sabellid worm Terebrasabella heterouncinata disrups normal shell formation in an abalone
The battle continues, resulting in aberrant growth of the shell

 

diagram of life cycle of the parasitic sabellid worm Terebrasabella heterouncincata in the shell of a a molluscphotograph of abalone parasite Terebrasabella heterouncinataEggs are laid within the tube and hatch to crawling larvae. The larvae are ready to adopt the adult mode of life within 12h of crawling.  Within 4-6wk an individual is capable of laying its own eggs.  The parasite is non-specific and has spread to many other gastropod groups, including limpets Lottia spp., trochids Calliostoma spp., and Tegula spp., and other genera, in which it may similarly cause growth deformities or a complete cessation of linear growth.  The authors have not found the parasite in bivalves.  Kuris & Culver 1999 Invert Biol 118: 391.

NOTE  the worm appears to be native to South Africa and may have entered California in the 1980s in shipments of South African abalones H. midae

 
Research study 3
 

drawing of the abalone parasite Terebrasabella heterouncinataX-ray photograph of an abalone shell infested with a sabellid parasite Terebrasabella heterouncinataA companion paper to Research Study 1 above on Terebrasabella heterouncinata describes its new genus and species status, and gives considerable detail on morphology, life history, and taxonomic status. Individuals are simultaneous hermaphrodites with eggs being incubated a few at a time in the burrow and being released as late-stage, non-feeding, larvae to infest nearby hosts.  Note in the photos of an infested abalone Haliotis rufescens from an aquaculture facility in southern California that the burrows are oriented perpendicular to the growing edge of the shell, and that densities within a shell may be quite high. The authors propose several useful research questions relating to its burrowing capability, particularly in relation to why some molluscs are susceptible to infestation and others not. Fitzhugh & Rouse 1999 Invert Biol 118: 357.

NOTE the worm has 11 setigers or segments and bears a prominant fecal groove running the length of the body to allow feces to be discharged

 
Research study 4
 

Following the 1996 discovery of an intertidal population of black turban snails Chlorostoma funebralis infested with the sabellid Terebrasabella heterouncinata at Cayucos, California near the site of an abalone culture1 facility, researchers devise a bold strategy to prevent further geographic spread of the pest. This involves the implementation of an eradication programme based on the theory of threshold of transmission2. It starts with the removal of 1.6 million black turban snails Chlorostoma funebralis, a highly susceptible and preferred host of the parasite, and additionally includes cleaning up shell debris and other potential gastropod hosts (Chlorostoma, Protomya, and other species), and even removal of hermit crabs inhabiting shells of these potential hosts.  As well, outfall screens3 are installed at the abalone culture facility to eliminate any further release of contaminated shell material.  The researchers collect uninfested C. funebralis from distant areas, mark them with coloured nail polish, and place them in the outfall area to act as “sentinal-hosts” that can be routinely re-collected and examined for infestation. Within 3yr no further evidence of spread of Terebrasabella is recorded.  The success of this containment-type of eradication is aided by the extremely short dispersal distance of the larvae, which become competent to settle within hours of release from the parent.  Thus, the potential for long-distance dissemination of larvae is minimal.  The authors remark that given the success of their programme, other exotic marine pests may be eradicated in similar proactive ways.  Culver & Kuris 2000 Biol Invasions 2: 245.photograph of a cluster of black turban snails Chlorostoma funebralis

NOTE1 the authors note that all abalone mariculture facilities in California have become infested with the sabellid parasite

NOTE2 the notion here is that a minimum threshold density of hosts is required to maintain a rate of transmission sufficient for a parasite population to persist.  Thus, simply remove the host and the parasite will disappear

NOTE3 mesh size of these screens is too large to retain Terebrasabella larvae, but at least the major outflow of potential shell habitat is restricted

 

Cluster of black turban snails
Chlorostoma funebralis 0.25X

 
Research study 5
 

schematic showing extent of infestation of 15 species of Californian gastropods by the sabellid parasite Terebrasabella heterouncinataA later study by the same authors on the sabellid tubeworm Terebrasabella heterouncinata reveals significant differences in levels of susceptibility to infestation among 15 species of California gastropods.  Note in the histogram that caenogastropods tend to be more resistant than vetigastropods and patellogastropods, thus suggesting a phylogenetic component of susceptibility to the parasite.  Although the exact causes of enhanced susceptibility are not known, the authors have lots of ideas.  For example, the comparatively larger, open apertures of patellogastropods and vetigastropods may provide larger, more accessible settlement sites for the settling larvae of the worm.  Also, water currents generated by mantle cilia in the susceptible taxa tend to circulate around the entire perimeter of the shells, rather than be restricted to the siphonal canals as in the caenogastropods.  Additionally, the most resistant species tend generally to have cleaner shells (free of epibionts), and therefore characteristics of fuzzy periostracum (Conus californicus) or highly polished surfaces (Cypraea spadicea, Olivella biplicata) may be important.  Species such as Calliostoma spp. and C. spadicea that exhibit active shell-cleaning behaviour with foot and/or mantle tend to be the least infested.  Slow growing species appear to be more susceptible than fast-growing ones, especially if the growth includes deposition of new calcite (prismatic material) around the shell aperture.  In support of this idea, older individuals that grow more slowly than younger ones also appear to be more susceptible to infestation, although the authors suggest that further research is needed to test this.  Chemical defenses are not ruled out by the authors.  Culver & Kuris 2004 Invert Biol 123: 316.

NOTE Asraea undosa as well as a few others used in the study are more commonly found in Baja California

NOTE a later publication on possible infestation by the same sabellid worm of turban snails Chlorostoma funebralis shows that while transmission of the worm can occur between individuals in experimental situations, it does so much more slowly than between individuals of red abalone Haliotis rufescens.  Moreover, collections made of turban snails and various limpets at sabellid-exposed sites near the Bodega Marine Laboratory, California reveal no presence of the worms.  The authors find that short-duration exposure (64sec) of T. heterouncinata larvae to fresh water will kill them.  Moore et al. 2007 J Shellf Res 26 (3): 869.

 
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