Shell width and body live weight related to shell length of the endolithic bivalve Lithophaga lithophaga (date mussel) colonizing a specific habitat (vaults under boulders) formed by artificial and natural structures were examined. Artificial structures consisted of limestone boulders of a breakwater (Marina Rovinj, northern Adriatic Sea, Croatia) constructed 19 years before sampling of the date mussel. Date mussels’ density (around 80 individuals per 0.1 m2) did not differ between the two types of structure. However, the length frequency distribution in artificial structures (25th percentile = 3.20, median = 4.30 and 75th percentile = 5.10 cm) differed from that in natural structures (25th percentile = 3.66, median = 5.15 and 75th percentile = 6.20 cm) leading to a substantial difference in total biomass (0.3 and 0.8 kg per 0.1 m2 for artificial and natural structures, respectively). Parameter estimates of regression functions for width against length (linear function) and for live weight against length (allometric function) also significantly differed, indicating variations in date mussels’ morphometry between the two types of structure. Analyses of variance did not detect differences in width or weight for date mussels in the length range from 3 to 3.5 cm. However, width (average ± s.d., n = 18) of individuals in the range from 5.5 to 6 cm was significantly lower in artificial structures (1.46 ± 0.13 cm) than in natural structures (1.66 ± 0.10 cm). Consistent with this, live weight in artificial structures (8.36 ± 1.17 g) was significantly lower than that in natural structures (12.33 ± 1.48 g). It is suggested that these patterns reflect a growth rate of the date mussel that is higher in artificial than in natural structures. Information about date mussels’ biometric patterns in different habitats is important in planning studies assessing the resilience capability of natural populations after illegal destructive harvesting, particularly as,such studies are lacking.Istražene su širina ljušture i težina živog organizma u odnosu na dužinu ljušture endolitskogškoljkaša Lithophaga lithophaga (prstac) u specifičnom staništu (svodovi ispod blokova) sastavljenomod umjetnih i prirodnih struktura. Umjetne strukture su bile vapnenački blokovi lukobrana(Marina Rovinj, sjeverni Jadran, Hrvatska), izgrađenog 19 godina prije uzorkovanja prstaca.Brojnost prstaca (oko 80 jedinki po 0.1 m2) nije se razlikovala između tipova struktura. Međutim,raspodjela frekvencije dužina u umjetnim strukturama (25. percentil = 3,20, medijan = 4,30 i 75.percentil = 5,10 cm) razlikovala se od one u prirodnim strukturama (25. percentil = 3,66, medijan =5,15 i 75. percentil = 6,20 cm) što je dovelo do značajne razlike u ukupnoj biomasi (0,3 kg po 0.1m2 za umjetne i 0,8 kg po 0.1 m2 za prirodne strukture). Nadalje, procijenjeni parametri regresijskihfunkcija širine u odnosu na dužinu (linearna funkcija) i težine u odnosu na dužinu (alometrijskafunkcija) značajno su se razlikovali, što upućuje na morfometrijske razlike prstaca između tipovastruktura. Analizom varijance nije ustanovljena razlika u širini ili težini prstaca u rasponu dužineod 3 do 3,5 cm. Međutim, širina (aritmetička sredina ± s.d., n = 18) jedinki u rasponu od 5,5 do 6cm bila je značajno manja u umjetnim (1,46 ± 0,13 cm) nego u prirodnim strukturama (1,66 ± 0,10cm). U skladu s time, i težina jedinki u umjetnim strukturama (8,36 ± 1,17 g) je bila manja nego uprirodnim strukturama (12,33 ± 1,48 g). Pretpostavlja se da su navedene razlike bile posljedica većebrzine rasta prstaca u umjetnim nego u prirodnim strukturama. Informacije o biometrijskim karakteristikamaprstaca u različitim staništima su važne za planiranje studija o obnovi prirodne populacijenakon nezakonitog destruktivnog sakupljanja, te takve studije zasada nedostaju
