Aquatic botany

ELSEVIER Aquatic Botany 58 (1997) 151 - 164

Invertebrate borers in Posidonia oceanica scales: relationship between their distribution and

lepidochronological parameters

P a o l o Guidett i a S i m o n a Bussot t i u Maria Crist ina G a m b i b.*

Maur iz io Lorent i b

" lstituto di Anatomia Comparata. Uni~,ersidl di Genota, t'iale Benedetto XV, 5. 16132 Genoca. ltah- h Laboratorio di Eeologia del Benthos, Sta:ione Zoologiea ' A. Dohrn' di Napoli. Punta S. Pietro. 80077

lschia Porto Napoli, Italy

Accepted 27 February 1997

Abstract

The occurrence and pattern of distribution of invertebrate borers in the scales (remains of leaf bases still connected to the rhizome) of the seagrass Posidonia oceanica was studied in two meadows of the Mediterranean Sea. At Noli (Ligurian Sea), Posidonia shoots were sampled at 1, 5, 10 and 15 m depths, while at cape Coda Cavallo (Northern Sardinia) samples were collected only at 6 m depth. The lepidochronology technique (based on the annual cycle variation in the scale thickness) was employed in order to study the relationship between plant and borers and to characterize their microhabitat in the stem. Borers belong to Polychaeta and Crustacea Isopoda. Three species of polychaetes belonging to the family Eunicidae (Lysidice ninetta Audouin and M. Edwards, Lvsidice collaris (Grube) and Nematonereis unicornis (Grube)) and one species of isopod of the family Limnoriidae (Limnoria tuberculata (Sowinski)) were responsible for burrow- ing within Posidonia scales. Estimated density of borers reached in some instances values as high as 278 individuals m 2. Isopods occurred in younger scales, whereas polychaetes inhabited older scales. A positive trend was observed between polychaete width (3rd body segment) and scale thickness; however, both Spearman rank correlation and linear regression coefficients showed quite low significant values. In some instances polychaetes burrowed also into the living tissues of the shoots. Borers have been recorded also in several other Posidonia beds along the Italian

:~ Corresponding author. Tel.: + 39-81-991410; fax: + 39-81-984201 : e-mail: gambimc @alpha.szn.it

0304-3770/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0304-- 3770(97)00009-0

152 P. Guidetti et al./Aquatic Botany 58 (1997) 151-164

coasts. The role of scale borers for the decaying processes of the Posidonia system may be important. © 1997 Elsevier Science B.V.

Keywords: Seagrass; Borers; Polychaeta; Isopoda; Mediterranean Sea

1. Introduction

Meadows of the seagrass Posidonia oceanica are well known as highly complex biotopes within the coastal systems of the Mediterranean Sea and for supporting a variety of animal communities (Mazzella et al., 1992). The Posidonia rhizome layer harbours a diverse assemblage of benthic organisms which are relatively well studied as far as both sessile forms such as sponges, bryozoans and hydroids (Boero, 1981; Pansini and Pronzato, 1985), and vagile ones such as polychaetes and molluscs (Somaschini et al., 1994; Russo et al., 1986) are concerned. One major component of the rhizome layer is represented by scales, i.e. the remains of former leaf bases persisting along the rhizome. Overlapped scales build up a unique microhabitat characterized by enhanced surfaces, crevices and also accumulation of sediment (Pronzato and Belloni, 198 I). The spatial and functional microniche of single animal species has not yet been fully investigated at this level. Utilization of Posidonia tissues, both living and dead, as food resource and the role of spatial complexity are poorly known, particularly as far as stems (herein intended fundamentally as upright, orthotrope rhizomes) are concerned.

The analysis of cyclic variations in scale thickness in P. oceanica, known as lepidochronology (Boudouresque et al., 1983; Pergent et al., 1983), clearly show that each cyclic variation has a chronological significance, with each cycle corresponding to a one year period (Pergent and Pergent-Martini, 1990). This technique represents in P. oceanica a powerful tool for approaching rapidly problems already studied by classical methods as well as new research fields (Pergent et al., 1989). Examples are the estimation of growth rate and annual production of the rhizomes, the assessment of the number of leaves produced both in recent and past years, the modelling of leaf renewal cycles, the dating of previous flowerings, and the reconstruction of primary production accomplished by establishing correlations between scale thickness and blade length (Pergent and Pergent-Martini, 1990). Recent applications of lepidochronology are aimed at recording recent and past pollution events, based on the ability of scales to memorize physical and chemical events (Calmer et al., 1988; Romeo et al., 1995).

Recently, Brearley and Walker (1993, 1995, 1996) pointed out the importance of borers in seagrass systems of the western Australian coasts. Except for a few remarks by Brearley and Walker (1996), the presence of borers in the Mediterranean seagrass system was never investigated before. The present study is aimed at documenting the scale-boring behaviour of two groups of invertebrates, polychaetes and crustacean isopods and their relationships with the lepidochronological parameters and scale annual cycle. Such relationships may be of two kinds: one based on the spatial distribution of scale features (fundamentally thickness), and the other based on the scale age as reflected in its state of decay. The hypothesis is that borers respond to variations in scale characteristics based on their morpho-functional, ecological and behavioural require-

P. Guidetti et al. / Aquatic Botany 58 (1997) 151-164 153

ments. Polychaetes and isopods are major components of P. oceanica faunal stock (Gambie t al., 1992). A preliminary survey conducted in several Posidonia stands along the western coasts of Italy revealed the extent of the occurrence of boring species (Gambi et al., 1997).

2. Material and methods

Orthotrope rhizomes were collected by SCUBA diving at two sites located at Noli (Ligurian Sea: 44°13 ' lat. N, 08025 ' long. E) and at Coda Cavallo cape (Eastern Sardinia: 40°50 ' lat. N, 09o45 ' long. E), respectively. The former is a continuous meadow extending from 1 to 15 m depth, growing on sandy bottoms interspersed with some rocky formations and where the plant does not build the typical terrace-like formation (interlacing of roots, rhizomes and sediment), defined in Posidonia as "matte ~. Rhizomes were collected at four depths (1, 5, 10 and 15 m) in May, June and October 1995. At Coda Cavallo cape, rhizomes were collected in May and August 1995 at 6 m depth in a patchy bed established both on sandy bottom and 'matte" formation with rocky cliffs about 30 m apart. At all sites and depths considered, stems and shoots were collected randomly.

In the laboratory, lepidochronological analyses were carried out on a number of rhizomes ranging from 36 to 47 (Table 1). Scales were carefully detached from the rhizome according to their rank insertion (Pergent et al., 1983), from the oldest (toward the stem base) to the most recent (close to the outer living leaves). Scale thickness was measured, by means of a manual micrometer, on a transverse section cut at 10-12 mm above the insertion point on the rhizome. The presence of either boring organisms or their empty burrows were noted for each scale. Scales having traces of burrowing and animals, when present, were preserved in 4% formaldehyde. The occurrence of more

Table 1

Lepidochronological parameters and abundance of borers in the studied PosMonia beds at various depths (m)

Parameter measured Null ( I ) Noli (5) Noli (10) Noli (15) Coda Cavallo (6)

Mean no. of shoots m- 2 644 347 331 21)9 585

Bed covering (~;4) 61) 100 95 11)0 70

Meanno . ol scales per annual cycle 7 .0(0 .6) 6 .9(0 .8) 7.1 (0.1) 7 . 4 ( I . I ) 6.3(0.6}

Mean scale thickness ( /xm) 486.6 (181) 530.4 (221)) 540.8 1'234) 577.8 (236) 592.5 (292) No. of shoots observed 43 46 36 47 41'1

No. of shoots with borer tracks 18 20 10 23 12

No. of shoots with l iving borers 10 9 I I 15 13

Actual no. of borers 14 11 11 17 19 No. of borers m • 209 83 101 75 278 No. of polychaete borers 5 9 I 1 17 I 1

No. of polychaete borers m -~ 75 68 101 75 161 No. of isopod borers 9 2 1) () 8 No. of isopod borers m ~' 135 15 0 0 [ 17

Number in parenlheses represent standard deviations.

154 P. Guidetti et al. / Aquatic Botany 58 (1997) 151-164

than one individual per scale was noted, and average number of individuals per scale was calculated. In addition, for polychaetes, the specimens collected at all sites and depths were measured to estimate size structure of the population. As a size parameter we selected the width of the body at the level of the 3rd segment. The measure of this parameter allowed us also to test the statistical correlation (Spearman rank pairwise test

and linear regression) between scale thickness and worm width. For isopods, the pairing of sexes and the occurrence of gravid females were recorded. In order to study the microdistribution of borers and its relationship with the

lepidochronological annual cycle, a subdivision of scales into discrete thickness intervals was attempted. Scale thickness values were pooled into two groups above and below the mean thickness (Table 1) based on the fact that maximum scale thickness corresponds

generally to leaves produced in September-October, while a minimum is recorded approximately in February-March (Crouzet et al., 1983; Pergent, 1990).

3. Results

3.1. Lepidochronological analysis

The main results from the lepidochronological analysis are reported in Table 1. Scale

thickness increased with depth at the Noli stands, while mean number of scales per annual cycle remained quite constant, in contrast to the Coda Cavallo site where a

slightly lower value was recorded. Except for the shallowest stand at Noli (1 m), values of scale thickness from all depths lay in the same range. The number of detectable lepidochronological years was found to decrease with increasing depth (Table 2).

Both at Noli and at Coda Cavallo two groups of invertebrate borers were identified,

belonging to Polychaeta and Crustacea Isopoda, respectively.

Table 2 Number of bored scales for each annual cycle, and total number of scales observed (in parenthesis) in the studied Posidonia beds at various depths (m)

Annual cycles Noli (1) Noli (5) Noli (10) Noli (15) Coda Cavallo (6)

1994 10 (221) 7 (255) 9 (186) 6 (320) 9 (199) 1993 11(275) 21(339) 19 (262) 26 (358) 18 (267) 1992 11(285) 20 (330) 17 (282) 46 (344) 15 (267) 1991 8 (291) 14 (325) 15 (269) 32 (273) 5 (244) 1990 9 (296) 13 (310) 2 (221) 9 (212) 9 (215) 1989 10 (285) 10 (241) 3 (199) 11 (143) 1 (77) 1988 8 (259) 4 (154) 3 (135) 2 (79) 3 (50) 1987 3 (226) 6 (147) 3 (73) 1 (6) 2 (38) 1986 3 (164) 1 (78) 0 (10) 2 (6) 2 (33) 1985 2 (113) 3 (68) 0 (7) 0 (4) 5 (22) 1984 0(71) 1 (35) - - 5 (18) 1983 0 (37) 0 (16) - - 0 (16) 1982 0 (36) 0 (6) - - 2 (15) 1981 0 (22) 0 (4) - - - 1980 0 ( 1 6 ) . . . . 1979 0 (3) . . . .

P. Guidetti et al. /Aquatic Botany 58 (1997) 151-164 155

3.2. Polvchaetes

Three species of polychaetes were identified as borers of Posidonia scales: Lvsidice ninetta Audouin and M. Edwards, Lysidice collaris (Grube) and Nematonereis unic~Jr- his (Grube). All three species belong to the family Eunicidae, which is characterized by the presence of a unique and complex maxillary apparatus composed by two pairs of mandibles and a set of maxillae inserted in a strong muscular, eversible pharynx (Fauchald and Jumars, 1979). A detailed morphological description of the species can be found in George and Hartmann-Schri~der (1985) (L. ninetta and N. unicon~is), and in Martin (1987) ( L. collaris).

Among the three taxa, L. ninetta was the most frequent species at Noli stands, while N. unicornis was more abundant at Coda Cavallo cape. L. collaris was found only in the Noli stands. Polychaetes formed sinuous burrows, whose width generally fitted the animal's dimensions (Fig. l a-c). Burrows were produced by removing the plant tissues.

A

B

C t I

1Omm

Fig. 1. (A) Posidonia scales with empty burrows of polychaetes; (B) scales with piercing groves and holes made by polychaete borers: (C) scales with worms inside their burrows (indicated by arrows).

156 P. Guidetti et al. /Aquatic Botany 58 (1997) 151-164

Noli s tands (1-15 m)

ca

"o 7,

lOi J

4~

0 - - - 0 .3 0.5

L. nlnetta

L. oollarl8

0.7 0.9 1.1 1.3 1.5 worm width (ram)

Fig. 2. Size-frequency distribution of Lysidice ninetta and L. collaris populations from Noli Posidonia stands (specimens from all depths).

which are likely to be utilized as food by the worms. Burrows deserted by polychaete borers were filled with sediment and often occupied by other worms, such as representa- tives of the polychaetous families Syllidae, Phyllodocidae, Terebellidae, Sabellidae, Cirratulidae, Spionidae, Nereididae and sipunculids (Aspidosiphon muelleri, Golfingia

sp.). Some specimens belonging to the genus Lysidice, whose body width exceeded 1.2 mm, were often observed to pierce through the entire scale (Fig. lb), sometimes affecting also the living meristem, and in some cases causing the detachment of the whole shoot. In one instance the hole produced at the leaf base persisted along the growing leaf blades. In general, one individual per scale and per stem was observed (Fig. lc); in a few cases two individuals were found to co-occur in the same scale or in the same stem. Boring behaviour was observed to occur in all three species also in the laboratory, when animals were offered intact scales (M.C. Gambi, pers. obs.).

A few reproductive females of L. coUaris were observed, as well as one eunicid juvenile, probably belonging to one of the above species. Length of specimens of the three taxa varied from about 5 mm up to 60 ram. Due to the paucity of specimens of N. unicornis, the analysis of size-frequency distribution was possible only for L. ninetta

and L. collaris populations, and for these latter species only for Noli stands (Fig. 2). Results show that only L. ninetta occurred with a wide range of sizes, following a normal distribution with a modal class at 0.9 mm, while L. collaris showed a more limited range mainly represented by large individuals. However, due to the relatively limited number of specimens analyzed, differences between the two distributions cannot be statistically tested.

3.3. Isopods

The bulk of isopods found boring in Posidonia scales at all sites investigated seem to belong to the species Limnoria tuberculata (Sowinski) as defined by Cookson (1991)

P. Guidetti et al. / Aquatic Botany 58 (1997) 151 . 164 157

(Fig. 3a). Identification needs to be confirmed through more taxonomic work, as L. tuberculam is a poorly-known species, like all Mediterranean limnoriids boring in materials other than wood. It is noteworthy that L. tuberculata is considered a

A

1ram

B

Fig. 3. (A) SEM micrograph of the borer isopod Limnoria tuberculala from Posidonia scale: (B) Posidonia scales showing burrows made by isopods.

158 P. Guidetti et al. /Aquat ic Botany 58 (1997) 151-164

wood-borer (Geldiay and Kocatas, 1972; Cookson, 1991); wood- and seagrass-borers within the genus share some morphological features (presence of 'rasp and file' on mandibles). In our samples, Limnoria typically formed branching burrows starting from the upper portion of scales still holding a ligula (Fig. 3b). They bored inside the middle tissues, with epidermal outer layers generally remaining as a sort of transparent screen. Burrowing appears to progress downward and to extend not further than about one-third of scale length. Scales occupied by active limnoriids were generally the younger, closer to the living shoot in comparison to those inhabited by polychaetes, as will be further shown. In a few cases, older scales had the marks of empty burrows, often full of fecal pellets and sometimes occupied by tanaids or harpacticoid copepods. Living leaf bases were never found to be affected by isopods. In the scales where living animals were found, the number of limnoriids per scale averaged 2 at Coda Cavallo and 1.2 at the Noli stands. Occurrence of pairs (one at Noli in May, and two in October; two at Coda Cavallo in August) and gravid females (one at Noli in October and at Coda Cavallo in August, respectively) was recorded.

A qualitative survey of Posidonia stems from other beds located along the coasts of the Ligurian and Tyrrhenian sea (comprised between 44020 ' lat. N and 09014 ' long. E at Prelo in the Ligurian sea, to 35052 ' lat. N and 02052 ' long. E at Linosa island in the Sicily channel), along a depth interval ranging from I m-28 m, revealed the extent of the phenomenon and the occurrence of the same taxa of polychaetes and isopod borers at several sites (Gambi et al., 1997). While polychaetes were evenly distributed in almost all the beds investigated and along a wide depth range, isopods occurred at fewer sites (Gambi et al., 1997).

3.4. Relationships of borers' microdistribution with meadow features and lepidochrono- logical parameters

The proportion of Posidonia stems with scales having unoccupied burrows ranged from 28% (Noli, 10 m) to 49% (Noli, 15 m), while the proportion of stems with living borers ranged from 23% (Noli, 1 m) to 32.5% (Coda Cavallo, 6 m) (Table 1). On the

3O T

~ 2o

~ 10

5

lepidochronological annual cycles

[ ] Noli (5 m)

[] Coda Cavallo

Fig. 4. Distribution of bored scales among lepidochronological annual cycles at the shallow stands (5-6 rn depth) of Coda Cavallo and Noli Posidonia beds (for data see also Table 2).

P. Guidetti et al. / Aquatic" Botany 58 (1997) 151-164 159

6 0

5O !

40

30

o 2o

10

0

Noli (-5 m)

i Coda Cavallo

!!;::)i:i

0,3 0.5 0.7 0.9 1.1 1.3 s c a l e t h i c k n e s s (mm)

Fig. 5. Thickness-frequency (as percentage) distribution of scales showing traces of burr,owing.

whole, the proportion of stems affected by burrowing ranged from 58% (Noli 10 m) to 81% (Noli 15 m). When the average number of borers per stem was converted to a density per square metre of meadow, values as high as 278 individuals were obtained at Coda Cavallo, while at Noli the highest density was recorded at I m with 209 individuals (Table 1). At shallow depth isopods dominated, while at deeper stands only polychaetes occurred. The frequency of burrowed scales in relation to scale thickness is shown in Fig. 4, in which the trend at Coda Cavallo is compared with that found at Noli at a similar depth (5 m). A pattern can be observed in which a higher number of bored

E g

61 ._o

• T V O

• • • A ~ U

T I • L. ninetta

• L. s I ¢ollaris • N, unicornis

1 2

worm width (mm)

Fig. 6. T r e n d o f worm width (mm, 3rd body segment) and scale thickness (mm). Spearman correlation coefficient tor cumulative data of the three polychaete species is 0.25 ( d r = 98, P < 0.01)" pa rame te r s o f the

regression analysis are: a = 0.52; b = 0.38; r = 0.41 ( d . f = 98, P < 0.01 ).

160 P. Guidetti et al. / Aquatic Botany 58 (1997) 151-164

25 T . oi ! I N. unicornis

L. collaris

_~ L. ninetta

[ ] Isopods

lepidochronological annual cycles

Fig. 7. Microdistribution and abundance of borers as related to lepidochronological annual cycles.

scales occur in the portion of thickness range above the mean values (Table 1), that include the maxima in lepidochronological annual cycles.

The thickness-frequency (%) distribution of bored scales (Fig. 5) showed, in fact, a modal class of 0.7 mm for both studied sites. A positive trend between scale thickness and size (body width) seemed to occur for polychaete species (Fig. 6). The relationship showed a higher scattering for smaller worms, while variability decreased towards the larger worms, which are more constrained by scale thickness. However, both the Spearman rank pairwise correlation and the linear regression for pooled data of the three species showed quite low significant values (Spearman correlation = 0.25, d.f = 98; P < 0.01; r = 0.41, d.f. = 98, P < 0.01); the correlation slightly increased when data of the most abundant species L. ninetta were analyzed separately (Spearman correlation = 0.45, d.f = 51, P < 0.01; r = 0.48; d.f = 51; P < 0.01). These results suggest that not a clear positive correlation exists between the two considered variables.

Finally, the pattern of microdistribution of different borer species, obtained by pooling data from both studied sites, is shown in Fig. 7. It is clear that isopods are the only borers settled in the first scale and are more frequent in the less-aged scales of the first lepidochronological annual cycle, while polychaetes occurred with high frequencies in scales of the second and third lepidochronological annual cycles and burrowed also into very old scales, although with comparatively fewer individuals.

4. Discussion

Our observations on scale-borers organisms in P. oceanica document the extent of the phenomenon which appears to be a complex one, involving several forms taxonomi- cally and functionally distinct. The records of burrowing into Posidonia scales in several localities along the Italian coasts (Gambi et al., 1997), demonstrate that borer organisms are widespread and not limited to the local conditions of the studied sites.

Among polychaetes, L. ninetta is a vagile but cryptic form, known to inhabit Posidonia meadows, especially at the level of stems (Somaschini et al., 1994; Gambi et

P. Guidetti et al . /Aquatic Botany 58 (1997) 151- 164 I~1

al., 1995). Kernels (1960), on a short anecdotal sentence in a more general paper on the fauna of Posidonia meadows, already noticed the scale-boring habit of this species, which suggests that such behaviour is not a recent phenomenon. L. ninetta is also common in the Mediterranean Sea in shallow and deep hard bottoms covered mainly by algal calcareous forms (Giangrande, 1988; Sard'~, 1991), which in deep bottoms are known as 'coralligenous' formations. L. collaris is a species very close to the former and easily recognizable on the ground of some obvious morphological characters. However, as stated by Martin (1987), who first reported the species from the Mediter- ranean Sea, it has possibly been often confused a, ith L. ninetm. The species has been found associated with the calcareous algal mat on shallow hard bottoms of the Catalonia coasts (Martin, 1987). Both species of Lvsidice are known to burrow into calcareous algae, while other species of the genus are reported to bore also inside dead corals (Fauchald and Jumars, 1979). Therefore, they have the morphological tools for piercing the Posidonia scales. N. unicornis is a smaller species, and shows a wider ecological distribution in comparison with the other two taxa, being common both on hard and soft bottoms (Bellan, 1964), and also in Posidonia beds (Cantone, 1993). Scale boring behaviour of both L. collaris and N. unicornis was never reported before.

To our knowledge, also isopod borers have never been recorded from Mediterranean seagrasses. In other geographical areas, limnoriids have been found in stands of Thalassia testudinum (Florida, Caribbean Sea), Phyllospadix iwatensis (Northern Pa- cific) (Cookson, 1991), Posidonia of the australis and ostet!l'eldii groups. Hetero- zostera tasmanica and Amphibolis griffithii (Australia) (Brearley and Walker, 1993, 1996).

At the sites in which meadow features and lepidochronological parameters were analyzed, a clear difference in the pattern of occurrence was found between polychaetes and isopods. Isopods are present in shallower stands (1-6 m), while polychaetes are more evenly distributed. Whether this pattern is due to stand morphology and/or some abiotic (e.g., environmental energy) and biotic (e.g., recruitment pattern, migration) factor is at present unknown. Recruitment patterns differ greatly between the two faunal groups. In fact, while isopods brood their offspring and probably migrate as adults, the three eunicids are likely to have pelagic larvae (Fage and Legendre, 1927), so that their capability for dispersal is potentially greater. The occurrence of populations character- ized by a wide size range (with a unimodal distribution), comprising many small specimens as shown at least for L. ninetta by the size-frequency distribution, points to early colonization events by juvenile eunicids in the stem habitat. However, except for a few details (Fage and Legendre, 1927), the life cycle and type and period of reproduc- tion of the three polychaete species are very poorly known.

The lepidochronological analysis allowed us to distinguish different patterns of microdistribution between the two faunal groups, according to scale age and scale thickness. While scale thickness holds a more direct relationship to animal functional requirements (space and food quantity), scale age reflects on the degree of tissue decay and does not have to be considered as related to the dating of the colonization event. Preference of isopods for younger and comparatively intact scales is probably due to an adaptation of their mouth parts to deal with tougher tissues. On the other hand, eunicid polychaetes require softer ageing tissues at a comparatively more advanced stage of

162 P. Guidetti et aL / Aquatic Botany 58 (1997) 151-164

decay, which they can easily attack with their maxillary apparatus. As a result, spatial partitioning along the stem between isopods and polychaetes may tend to reduce competition. Polychaetes showed also a relationship between their body width and scale thickness, which can be fundamentally due to dimensional constraints, affecting mainly larger animals. Contrary to isopods and smaller individuals, larger eunicid worms (width > 1.2 ram) exploit more than one scale at a time. In some instances they can affect the whole base of the shoots, including meristems. This may have far-reaching implication for the health of individual shoots and of the meadow altogether. However, the major consequence for the meadow dynamics may be the enhancement of decaying processes as far as the stem layer is concerned. From this point of view, borers act in various ways, by mechanically disrupting the scale tissues, by enhancing the microbial activity (e.g., through deposition of fecal pellets inside the scales), and by favouring the settlement of other invertebrate species which exploit empty burrows. All the above issues require specific approaches, also considering that time-scale of study must be expanded to include the colonization dynamics and life history traits of the borer species which are relevant to the phenomenon.

Acknowledgements

We thank Giovanni Gragnaniello (Stazione Zoologica, Naples) for technical assis- tance in SEM examination of Limnoria. Anne Brearley (The University of Western Australia) provided information on the habitat and distribution of limnoriids in seagrass systems. We are also grateful to Maria Cristina Buia (Stazione Zoologica, Naples), Randall S. Alberte (UCLA, Los Angeles, U.S.A.) and Peter A. Jumars (UW, Seattle, U.S.A.) for critical reading of the manuscript and useful suggestions.

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