Histologia Informr 2 Unp

Burrow morphology, biometry, age and growth of piddocks

(Mollusca: Bivalvia: Pholadidae) on the south coast of England

Received: 11 August 2004 / Accepted: 27 January 2005 / Published online: 11 June 2005

Springer-Verlag 2005

Abstract Biometry and growth of three piddock species

Pholas dactylus, Barnea candida and B. parva, from

chalk and clay substrata were investigated between 1999

and 2000 at five low shore locations along the south

coast of England. Piddock burrow shape was significantly

different (P<0.01) amongst the locations. Burrows

at Lyme Regis showed the largest (height/

maximum diameter) ratio (4.86±2.00) whilst those at

Compton the lowest (3.73±1.62). Using the method of

Bhattacharya, the population structure of P. dactylus,

B. candida and B. parva was separated into eight, three

and five modal size classes, respectively. Age and shell

growth were determined from the number and spacing,

respectively, of annual growth lines present in acetate

peel replicas of shell sections. The von Bertalanffy

growth (VBG) equation fitted the size at age data

obtained for P. dactylus and B. candida (L¥ and K were

79.3±13.8 mm and 0.0011±0.22 and 29.6±1.5 mm

and 1.17±0.47, respectively) whilst the size at age data

for B. parva were linearly related and did not fit the

assumptions of the VBG equation. Male and female

gonads were mature and piddocks competent to spawn

between June and September 1999, with settlement of

juveniles observed between November 1999 and February

2000. A significant relationship between burrow

aperture diameter and age of the occupant piddock was

established for the three species. Burrow morphology

and spatial distribution of burrows were influenced by

substratum hardness and population density. Based on

estimates at Lyme Regis, piddocks are capable of

removing up to 41% of the shore substratum to a depth

of 85 mm over their lifespan (12 years), significantly

compromising the structural stability of the soft rock

shores they inhabit and contributing to bioerosion.

Introduction

Bivalve piddocks are one of the most characteristic

faunal groups inhabiting lower soft rock shores, such as

those comprised of limestone, chalk, sandstone, slate or

stiff clay (Harvey et al. 1980; Trudgill 1983, 1985;

George and Fincham 1989). Despite this, relatively little

is known about their general biology and ecology. Three

species of piddock commonly occur on the English south

coast: the common piddock Pholas dactylus L., the white

piddock Barnea candida (L.) and the little piddock

B. parva (Pennant) (Tittley et al. 1986; Wood and Wood

1986; George and Fincham 1989; Wood 1992). These

bivalves bore into the substratum and, consequently, it

may be this cryptic lifestyle that has resulted in the

general lack of knowledge about these species. This is

despite interest in them from early naturalists fascinated

by their luminescent properties and boring ability. For

example, Pliny the Elder (AD 77) documented the luminscence

of P.dactylus even whilst being eaten. Jefferies

(1865) refers to their use as food and fish bait and

Turner (1954) reported that the Romans ate them.

Pholas dactylus was once prevalent across the entire

Mediterranean and on the Atlantic coast of Europe, but

Communicated by O. Kinne, Oldendorf/Luhe

E. H. Pinn (&) Æ S. J. Hawkins

School of Biological Sciences, University of Southampton,

Southampton, SO16 7PX, UK

E-mail: eunice.pinn@jncc.gov.uk

Tel.: +44-1224-655718

Fax: +44-1224-621488

C. A. Richardson

School of Ocean Sciences, University of Wales Bangor,

Menai Bridge, Anglesey, LL59 5AB, UK

R. C. Thompson

School of Biological Sciences, University of Plymouth,

Drake Circus, Plymouth, PL4 8AA, UK

S. J. Hawkins

The Laboratory, Marine Biological Association, Citadel Hill,

Plymouth, PL1 2PB, UK

Present address: E. H. Pinn

Joint Nature Conservation Committee, Dunnet House,

7 Thistle Place, Aberdeen, AB10 1UZ, UK

Marine Biology (2005) 147: 943–953

DOI 10.1007/s00227-005-1582-0

they have disappeared from most sites due to human

collection for food and bait and as a result of pollution

(Michelson 1978). Various pholad species are still eaten

today in parts of Europe and Asia and there has been

recent interest in their mariculture (Bombace et al. 1995;

Marasigan and Laureta 2001).

Typical of most pholads, piddocks create conical

shaped burrows, which have a narrow entrance and a

larger rounded chamber. Piddock burrows increase

habitat complexity and provide a variety of microhabitats

for other species, thereby increasing local assemblage

diversity (Pinn et al., in preparation). Where

abundant, pholad borings, which are found in both

vertical and horizontal bedrock, can severely compromise

the structural stability of the shore, and can result

in increased rates of coastal erosion (Evans 1968a;

Trudgill 1983; Brookes and Stevens 1985; Trudgill and

Crabtree 1987).

Many studies on bio-erosion require an accurate

estimate of the demographic parameters (e.g. age

structure, growth and mortality rates) of the population

of organisms involved in erosion. The estimation of

these parameters can be a relatively straightforward

procedure when the age of individuals can be accurately

gauged (e.g. Anwar et al. 1990; Richardson 2001). Size

frequency distributions, surface shell rings or annuli and

internal growth lines are commonly used to estimate the

age of bivalve populations (e.g. Ramon and Richardson

1992). Distinct modal size classes in size frequency distributions

have been used as indicators of age in bivalves

(e.g. Thouzeau 1991; Chicharo and Chicharo 2001).

However, where there are extended periods of spawning

and/or recruitment or variable individual growth rates,

size classes may overlap (e.g. Gray et al. 1997) and it is

then often impossible to clearly identify the different size

(age) cohorts. The second method used to estimate the

age of bivalves is the use of surface shell rings and this

method has been used successfully to age many bivalve

species including the clam Ruditapes decussatus (Chicharo

and Chicharo 2001) and the wrinkled rock borer

Hiatella arctica (Trudgill and Crabtree 1987). However,

there are problems in distinguishing between annual

rings and those produced as a result of disturbance to

shell growth (see Richardson 2001). Evans and Le-

Messurier (1972) found that all the external rings on the

shells of pholads were not clear enough to be defined as

annual rings. A third and widely accepted method used

to estimate the age of bivalves, and one that has received

considerable attention, is the use of internal growth lines

and microbanding patterns present in sections of the

shell. The methodology and its application, reviewed in

Richardson (2001), has been successfully used to investigate

age and growth rates in a variety of mollusc species,

including horse mussels (Anwar et al. 1990),

mussels (Gray et al. 1997), razor clams Ensis siliqua and

E.ensis (Henderson and Richardson 1994) and the

pholad Pentilla penita (Evans and LeMessurier 1972).

In this paper we determine the burrow morphology,

spatial distribution, population structure, size distribution,

age and growth rate of three species of piddocks

P. dactylus, B. candida and B. parva from five locations

along the south coast of England. Using this information

we assess piddock bio-erosion of soft sedimentary

rock and attempt to predict their possible contribution

to coastal erosion.

Materials and methods

Piddocks were collected between March 1999 and September

2000 from five shores along the south coast of

England (Fig. 1), which exhibited different substratum

types: Lyme Regis (clay), Compton Bay (Isle of Wight,

clay), Bembridge (Isle ofWight, clay), Newhaven (chalk)

and Eastbourne (both in chalk and clay). At all sites, the

piddock beds were only accessible at spring low tide,

usually within 1 h of low water. The hardness of the

substratum at each location was assessed during August

2000 by measuring the depth of 10 replicate holes of

6 mm diameter drilled for 10 s (see Evans 1968b). Mean

depth drilled was converted to an index of hardness

using its reciprocal value.

On each shore, piddocks were removed from their

burrows by carefully breaking the surrounding rock.

The maximum shell length and shell width of each live

piddock removed from its burrow, was measured using

vernier callipers to the nearest 0.1 mm (see Fig. 2A), and

the shell width to shell length ratio calculated. Since

piddock collection is a destructive process involving the

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