Guide to New Zealand Shell Polychaetes

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Polydora hoplura and eggmass within a blister This guide provides information on the marine polychaete worms which live on the scallops, abalone, oysters, and mussels, and other harvested shellfish of New Zealand. It especially deals with the worms which bore into live shells with resulting harm to their hosts.

Shell worms of commercial molluscs

The following boring or fouling species occur on New Zealand shellfish which are harvested commercially, and are dealt with here:

Family Spionidae, Polydora-group: Boccardia acus | Boccardia chilensis | Boccardia knoxi | Boccardia lamellata | Boccardia otakouica | Boccardia proboscidea | Carazziella quadricirrata | Dipolydora armata | Dipolydora dorsomaculata | Dipolydora giardi | Polydora cornuta | Polydora haswelli | Polydora hoplura | Polydora websteri |

Family Cirratulidae: Dodecaceria berkeleyi |
Family Onuphidae: Brevibrachium maculatum |

The species listed above most likely to be encountered as potential shell-penetrative shell-blistering pests are Polydora haswelli, P. websteri, and P. hoplura. Common species which may cause shell damage primarily through boring are Boccardia knoxi, B. acus, and Polydora armata. Common species more likely to be conspicuous as surface fouling or as secondary burrow occupants are Boccardia chilensis, B. proboscidea, and Polydora cornuta.


New Zealand shellfish often have a number of unwanted squatter organisms living on their shells, including small polychaete worms (marine bristleworms of Phylum Annelida). Some are harmless, and none actually eat their hosts, but those that specialise in boring deep into shells can do severe damage, especially if they penetrate right through to the shell interior. The problems for aquaculturists and shellfishers caused by worms settling on and boring into shells can include spoiled shell shape and quality, internal shell blisters, serious reduction in shell strength, damaged adductor muscles, energy wasted on shell repair, and increased vulnerability to serious pathogens. Worm populations may increase abnormally in intensive cultures of shellfish. The harvested end product can be unsaleable to consumers.

Polychaete shell borers mostly are Boccardia, Polydora, and Dipolydora species of the family Spionidae, belonging to the Polydora-group of species, often referred to as the polydorids. The commonest shell borer is the native Boccardia knoxi, which bores deep into shell, also protecting itself in a thin parchment-like tube lining. Paradoxically, it seems well adapted to New Zealand molluscs, would not normally penetrate the inner surface of shells, and is probably by itself harmless in thickened molluscs such as Haliotis and Ostrea species. Another very common shell borer is Boccardia acus which is the dominant species on the New Zealand intertidal clam (cockle) Austrovenus stutchburyi and can be common on shallow-water dredge oysters Ostrea chilensis. The best known pest worm of aquaculture is Polydora websteri, usually found in oysters, and often called the mudworm because of the mud-like fine particles it accumulates inside shell blisters. Polydora websteri occurs widely around the world. Most of the worms which cause problems belong in genus Polydora, and include three other species occurring in New Zealand - Polydora hoplura, Polydora cornuta, and Polydora haswelli. Polydora cornuta doesn't bore shells, but may settle in high densities on them as surface fouling, and cause problem due to mud build-up. Polydora haswelli has recently arrived in New Zealand. It is very similar to P. websteri, and may be a greater problem than P. websteri has been as it can attack green-shell mussels on longlines as well as oysters and scallops. [*etymology of Polydora]

In addition to the polydorids a number of non-spionid polychaetes occur on live shells as epifauna, but probably only two species will be found frequently inside borings. The cirratulid Dodecaceria berkeleyi probably mainly colonises and expands borings of other species. The onuphid Brevibrachium maculatum seems to colonise only Haliotis iris (abalone) shells. It is a large worm, much larger than any boring spionid or cirratulid.

Other borers: In New Zealand there are a number of other metazoan organisms that also bore into the shells of living commercial molluscs, creating hole-openings similar to those of worms. Pits of the yellow bioerosive sponge Cliona spp. may occur in a high densities in Haliotis iris and oyster shells. Colonies of the endemic boring barnacle Cryptophialus melampygos occur in Perna, Haliotis, and Mytilus species. The boring phoronid, Phoronis ovalis has been found on Perna canaliculus and Haliotis iris. A shell-burrowing bryozoan, Penetrantia irregularis has been found on Perna canaliculus.

terminology of keys explained on Boccardia proboscidea

Shell worm identification

All Polydora-group species have embedded spines on chaetiger 5. New Zealand shell boring worms which do not have chaetiger-5 spines might be Dodecaceria berkeleyi or Brevibrachium maculatum (q.v. or see species-key couplet 7).

Identification of the Polydora-group worms can sometimes be difficult because they are generally small, rather pale and semi-transparent, and some are quite similar-looking. Worms need to be undamaged and complete. If possible, it is best to look at live worms because features are much easier to see, emphasised by the colour present. Blood vessels will be showing in the gills, and sometimes there are coloured pigments on some segments and on the palps and caruncle (ridge between the palps), which can be very useful in quickly narrowing down the possibilities (see 'identification tips' on the species pages). Unfortunately the surface pigments of Polydora-group worms often fade badly when the worms are killed and preserved. Live worms can be anaethetised if necessary by a gradual mixing into their seawater of 7.5% Magnesium chloride (in tap water) . Note the shape of the pygidium (structure around anal-opening) - is it a fan, a plug, a pad, a group of cirri? Note the first appearance of gills (branchiae) - are there gills, small or large, before chaetiger 5? Note whether or not the prostomium tip (anterior end of the caruncle) is notched or T-shaped. The chaetae also need to be examined, especially the tips of the spines of chaetiger 5, either with the worm mounted whole on a slide, or with the relevant part excised with fine scissors or a dissecting needle. The first occurrence of hooded hooks in the neurochaetae (chaetiger 7 or 8) is useful in Boccardia. In Polydora there is a constriction (in-pinching) in the shaft of hooded hooks.

Key to Polydora-group genera in New Zealand






Key to New Zealand shell worm species







7(1) [Not Polydora-group].

New Zealand commercial shellfish and their companion worms

Shell-boring worms tend to prefer to live only on certain species of shellfish. Here are the common borer species for each shellfish, listed in approximate order of significance as potential pests.

Haliotis iris - Black Paua / Abalone

Principal species are Polydora hoplura, Boccardia knoxi, Dipolydora armata, Boccardia acus, Boccardia proboscidea, Dodecaceria berkeleyi, Boccardia chilensis, Brevibrachium maculatum. P. hoplura is a known problem species in aquaculture of juvenile paua. In addition to the borer species, spirorbine polychaetes (tiny, spiral-shelled serpulid polychaetes) may settle densely on shells as fouling, but are not a problem except in providing shelters inside or alongside which Polydora larvae can set up initial burrows. B. knoxi and Polydora hoplura have been reported as an aquaculture problem in Tasmania in Haliotis rubra. The borers of the other New Zealand abalone are unknown. There are no reports for Yellow-foot Paua, Haliotis australis, or Virgin Paua, Haliotis virginea.

Perna canaliculus - Green-lipped mussel

Principal species are Polydora haswelli, Boccardia knoxi, Boccardia acus, Dipolydora armata, Dipolydora giardi. P. haswelli is a recent serious problem species, penetrating shell interiors and causing major blistering.

Pecten novaezelandiae - Scallop

Principal species are Polydora haswelli and Polydora hoplura. Both species can cause carbuncular blistering inside the shell rim. Polydora websteri may also occur, though unconfirmed (possible misidentification of P. haswelli).

Ostrea chilensis - Dredge oyster [formerly Tiostrea, includes Ostrea lutaria]

Principal species are Polydora hoplura, Boccardia acus, Boccardia chilensis, Boccardia otakouica, Boccardia knoxi, Dipolydora dorsomaculata, Dipolydora giardi, Carazziella quadricirrata. No serious shell-borer problems have yet been reported for dredge oysters. This may change as the species becomes more widely cultivated in aquaculture. The shells are comparatively very thick which may be a factor lessening effects of borings.

Crassostrea gigas - Pacific / Japanese oyster

Principal species are Polydora haswelli, Polydora websteri, Polydora hoplura, Boccardia knoxi, Boccardia acus, B. otakouica, B. chilensis, Polydora cornuta. The three Polydora species may cause severe interior blistering. P. cornuta is a fouling species.

Saccostrea commercialis - Rock oyster [includes S. cucullata, Crassostrea glomerata]

Anecdotal reports exist of Polydora websteri damage. Rock oysters are likely to be vulnerable to damage by the three major Polydora pest species.

Austrovenus stutchburyi - Little neck clam / Cockle

Boccardia acus, B. chilensis, and an as-yet undescribed Boccardia occur, with B acus often boring extensively around the siphon area.

Paphies subtriangulatum, Paphies australis - Tuatua and Pipi

Occasional occurrences of Boccardia acus have been seen.

Atrina pectinata - Horse / Fan Mussel

Principal species are Polydora hoplura, Boccardia otakouica, Boccardia acus, Boccardia lamellata, Boccardia chilensis, Dipolydora giardi, Carazziella quadricirrata, Boccardia knoxi. Fan mussels often have severe, multi-layered, carbuncular blistering and deterioration of structure inside the shell rim due to presence of Polydora-group species, and a rich variety of epifauna, including polychaetes attach to the outside of the shell.

Etymology of Polydora:
In 1802 a scientist named Louis Augustin Guillaume Bosc wrote these words about a worm he had found: "Il l'a appelé Polydore, nom d'une nymphe de la mer de la suite de Nérée." In Greek mythology Polydora was actually one of the Oceanides nymphs, daughters of Tethys and Oceanus, and in Greece the name Polydora is still chosen today as a girls' name. Bosc was a French mycologist who visited America for a few years, and later published a little book about various worm-like organisms in which he included a rather sketchily described and illustrated Polydora cornuta, a worm he had seen was common in Charleston Harbour, South Carolina. Today we know that worldwide there are well over a hundred species related to Bosc's worm.


Compiled by Geoffrey B. Read (comment and error reports welcome). Original content copyright © 2004 NIWA.

The style and layout of shell-worm species pages were inspired by the species summaries in FishBase (2000 version) global information system on fishes.

This work was funded by the New Zealand Foundation for Research, Science and Technology (FRST) as part of FRST contract C01X0219, Biodiversity of New Zealand Aquatic Environments, to NIWA, National Institute of Water & Atmospheric Research, New Zealand.

The information found in this guide may be cited for publication or used for public education material.
Please fully acknowledge re-use or quote. The recommended citation for the shell-worm polychaete guide is as follows:

Read, G. B. (compiler) 2004. Guide to New Zealand Shell Polychaetes. Web publication. <> Date of access dd-mmm-yyyy