JOURNAL OF CRUSTACEAN BIOLOGY, 34(1), 47-53, 2014

CURRENT DISTRIBUTION OF TWO SPECIES OF TYLOS (ISOPODA: ONISCIDEA) INTHE CENTRAL MEDITERRANEAN AND THE INFLUENCE OF BEACH SAND

GRAIN-SIZE PARAMETERS

Giuseppe Montesanto 1,∗, Alan Deidun 2, Arnold Sciberras 2,Jeffrey Sciberras 2, and Bianca Maria Lombardo 1

1 Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Sezione di Biologia Animale “Marcello La Greca,”Università di Catania, Catania, Italy

2 IOI-MOC, University of Malta, Msida, Malta MSD 2080

A B S T R A C T

The psammophilic genus Tylos Audouin, 1826 is represented within the Mediterranean basin as two species: T. ponticus Grebnicki, 1874,and T. europaeus Arcangeli, 1938, according to Taiti and Ferrara (1996). It is impossible to define precisely the current distribution of thetwo species, mainly because, in literature, both taxa have been described as subspecies of T. latreillii or even, in some cases, as T. latreilliisensu lato. The current study surveyed populations of Tylos spp. on sandy beaches along the coastline of Sicily, circum-Sicilian islands andthe Maltese archipelago. The aims of this study are to fully and correctly identify all the sampled populations, pursuant to characterizing thecurrent distribution of the two species, which occupy similar ecological niches, and to investigate any possible granulometric preferencesby the two species. Data acquisition proceeded along three tiers: in situ sampling, manually and through the deployment of pitfall trapconstellations, literature review and by analyzing preserved specimens of Tylos held within the ‘Caruso’ collection (University of Catania,Catania, Italy). Generally, T. europaeus showed a preference for fine sand, whilst T. ponticus showed a preference for fine-medium sand,although, statistically, differences in the distribution of the two species could not be explained in terms of median grain size. The twospecies were never recorded on the same beach. The degree of inter-specific morphological differentiation was also evaluated through adetailed morphological study of sampled Tylos spp.

KEY WORDS: distribution, Isopoda, morphology, Oniscidea, sediment grain size, Tylos

DOI: 10.1163/1937240X-00002206

INTRODUCTION

Tylos Audouin, 1826 represents a cosmopolitan, semi-ter-restrial oniscid isopod genus, typical of the upper zonesof sandy shores where back-beaches are adequate, espe-cially along the Atlantic coasts of Europe and the Mediter-ranean, where oniscid isopods and talitrid amphipods con-stitute an important component of the eulittoral and supralit-toral (McLachlan and Brown, 2006). Some species of Tylos,e.g., T. europaeus Arcangeli, 1938, are considered by someauthors (Goncalves et al., 2005) as a useful bioindicator ofenvironmental quality in sandy shores, since oscillations inpopulation dynamics, reproductive strategies, and productiv-ity of these species might reflect environmental changes.

Several studies have been conducted on the populationdynamics (Hammer et al., 1969; Hayes, 1974; Goncalveset al., 2005), zonation (Kensley, 1974; Hayes, 1977; Fal-laci et al., 1996), burrowing behaviour (Brown and Trueman,1996), spatial distribution (Odendaal et al., 1999), bioecol-ogy (Kensley, 1974; Fallaci et al., 1996), field orientation(Ugolini et al., 1995), surface activity (Colombini et al.,1996; Fallaci et al., 1996) and reproduction (Suzuki et al.,2013) of different species of Tylos.

∗ Corresponding author; e-mail: g.montesanto@unict.it

According to Schmalfuss (2003), within the Mediter-ranean basin, there are currently two species belonging to thepsammophilic genus Tylos: T. ponticus Grebnicki, 1874; andT. europaeus. In the literature, however, many different syn-onyms for Tylos in the Mediterranean have been proposed inthe past.

Within Sicily and the circum-Sicilian islands, the existingreports of Tylos (Caruso, 1973a, b, 1974; Caruso andLombardo, 1982; Caruso et al., 1987; Caruso and Lombardo,1995) list all such samples under “Tylos latreillei.” Deidunet al. (2003) and Deidun and Schembri (2008) both list T.europaeus as one of the most abundant macrofaunal specieson the beach of Ramla l-Hamra in Gozo, whilst Deidunet al. (2011) elaborate on the distribution of Tylos spp.in the Maltese Islands and on the population dynamics ofT. europaeus. The latter authors list the occurrence of T.sardous, besides T. europaeus, in the Maltese Islands, butthe former species is currently recognized as T. ponticus(Schmalfuss and Vergara, 2000). It is now generally assumedthat the synonym T. latreilii = T. ponticus, whilst whetherthe synonym T. sardous = T. europaeus is still debatable (seealso the remarks reported in: Taiti and Ferrara, 1996).

It is impossible from the literature to define precisely thecurrent distribution of T. europaeus and T. ponticus; both

© The Crustacean Society, 2014. Published by Brill NV, Leiden DOI:10.1163/1937240X-00002206

48 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 34, NO. 1, 2014

taxa have been described as subspecies of T. latreillii oreven, in some cases, as T. latreillii sensu lato (Vandel, 1962).The two congeners have never been reported from the samebeach.

The two species of Tylos can be distinguished from eachother by the following morphological characters: shape ofapex of the endopod of the second pleopod; lateral marginof pereionite 1 and epimera of pereionites 2-7; apical marginof the ventral plate of pleonites 4 and 5; and the shape of theprominent process on the basis of pereiopod 1 on the tergalmargin (Giordani Soika, 1954; Vandel, 1962; Kussakin,1982; Taiti and Ferrara, 1996). Moreover, up to the present,it has always been considered that the distribution of thesetwo species was strictly related to the grain-size of the beachwhere the populations live: T. europaeus (and its synonyms)was considered as being exclusive to fine-sand beaches,while T. ponticus (and its synonyms), as being exclusive tocoarse-sand beaches (Giordani Soika, 1954, 1956; Vandel,1962; Kussakin, 1982; Caruso et al., 1987; Taiti and Ferrara,1996).

Among past works on Tylos, only Giordani Soika (1954)and Goncalves et al. (2005) reported data on grain-sizemeasurement. A lack of granulometric information from thebeaches sampled for Tylos is still very real and, generally,this type of data is never precisely reported in Tylos-relatedliterature; in fact, there is only qualitative information inliterature regarding such an aspect. Otherwise, since thework of Pardi (1955), the burrowing/digging behavior of thisgenus have been well studied. The isopods show a strongsurface activity during the night, while they occur, mainlyinactive, deep down in the sand during the day (as reviewedin Fallaci et al., 1996).

The aims of this study are: to fully and correctly identifyall the Tylos spp. populations within the Central Mediter-ranean region from sampled populations in Sicily, islandsaround Sicily, and from the Maltese archipelago pursuant tocharacterizing the current distribution of these species thatoccupy a similar coastal habitat; and to investigate any possi-ble granulometric preferences of the two species, with quan-titative measures. The study also aims to assess the degreeof interspecific morphological variation within the variouspopulations sampled, in order to confirm the validity of thesynonyms currently accepted by taxonomists, thereby con-firming the presence of the two Mediterranean species, T.ponticus and T. europaeus, for the surveyed geographic area.

MATERIAL AND METHODS

A total of 32 beaches in Sicily, 3 beaches on the Aeolian Islands, individualbeaches on the islands of Ustica, Favignana, Pantelleria, Linosa andLampedusa and 3 beaches on the Maltese Islands (Table 1 and Fig. 2),were sampled by means of pitfall trap constellations or, where this was notpossible (e.g., due to the restricted dimensions of the beach), by means ofindividual traps and through direct collections with entomological forceps.Pitfall trap constellations deployed were similar to those described inDeidun and Schembri (2008). A total of 346 individuals were collected fromjust 22 out of the 43 sampled beaches.

In addition, more than 200 individuals from the collection of Prof.Domenico Caruso, housed at the Museum of Zoology (University ofCatania, Catania, Italy), were also re-examined in order to assign eachpopulation to the two currently-accepted species of Tylos and thus toretrieve data about their distribution. Such a morphological study was alsouseful to further define the degree of interspecific variation within the

various populations of Tylos by examining a relatively large number ofCentral Mediterranean populations of the two species.

We also evaluated previous records in the literature for Tylos recorded forthe same geographical area (Giordani Soika, 1954). A list of all the beachesyielding records of Tylos (either directly, through sampling, or indirectlythrough literature records) is reported in Table 1. The distribution of thesetwo species of Tylos was thus mapped by integrating results from thesampling survey, from individuals housed within the ‘Caruso’ collection,and from the literature.

Nineteen sand samples destined for granulometric analyses were col-lected from the same sub-surface locations on the beach from which bur-rowed Tylos were collected, i.e., sand replicates were collected from theimmediate vicinity of the burrowed isopods. This was done both on beachesthat were characterized by a uniform granulometry and on beaches wheregrain-size parameters varied considerably, spatially.

Sediment particle diameter was determined through the deployment ofthe stereoscope Leica M205C and its complementary software LAS (LeicaApplication Suite ver. 3.3.0). Given the high resolution of this analysis andthe operator’s potential bias when choosing which parts of the sample touse for the image analysis, the sample was subdivided using the quarteringtechnique, described by Dyer (1979). Such analysis allowed the plotting ofbox-plots for each of the sampled beaches, using the Java-based applicationBox Plot Simulation (available online at http://cnx.org).

The influence of differing grain-size values on the distribution of the twospecies was processed through a two-sample t-test for independent samplesand a single-sample t-test. The two-sample t-test for independent samplesperforms the Student’s t-test, which assumes that the two samples haveequal variances, while the alternative t-test assumes that the two sampleshave unequal variances. The single sample t-test considers the significanceof the difference between the observed mean of a sample and a hypotheticalmean of the population from which the sample is randomly drawn. For boththese Student’s tests, the median value of the grain-size values was used,and the value of α was set to 0.05. Any correlations between this physicalparameter and the relative distribution of the two species were investigated.

The morphology of the first pair of pereiopods, of the apical margin ofthe ventral plates of pleonites 4 and 5, of the tergite margins of pereionite 1and of the endopod apexes of the first pair of pleopods, were used to assessthe degree of inter-specific differentiation.

RESULTS

No significant deviation from the 1:1 sex ratio has beenfound for the studied populations. All T. europaeus exhibita more developed groove on the lateral margin of pereionite1 and the epimera of pereionites 2-7 are considerably widerthan in T. ponticus (Fig. 1A-D). With respect to T. ponticusindividuals, the male pleopod 2 endopod has a narrow andmore curved apex (Fig. 1E-F); the apical margin of theventral plates of pleonites 4 is shorter and more curvedwhile pleonite 5 have a more rounded margin, rather thantruncate, as in T. ponticus (Fig. 1G-H); the shape of theprominent process on the basis of pereiopod 1, on thetergal margin (Fig. 1I-J), is triangular and more pronounced,where the same process in T. ponticus is shorter and morepointed.

The distributional data on the two studied species wasgeo-referenced and plotted on maps for Sicily, circum-Sicilian, and Maltese Islands, using data derived either fromthe sampled populations, from literature (Giordani Soika,1954), or from the Domenico Caruso collection. The dis-tribution of the two species, as recorded within the surveyedgeographical area, is reported in Fig. 2 (the correspondingnumbers of the cited beaches are also reported in Table 1).In Sicily, although T. ponticus and T. europaeus dominatebeaches in the north-west and north-east of the island, re-spectively, there is no clear spatial separation between thetwo species, with an overlap between the geographical dis-tribution of the two being observed all over the island, es-

MONTESANTO ET AL.: DISTRIBUTION OF TYLOS IN THE MEDITERRANEAN 49

Table 1. List of the beaches sampled in the present study. WGS84 coordinates are reported. CCS, “Caruso Collection” samples; LIT, Data from Giordani-Soika (1954); PSS, Present study samples; TE, Tylos europaeus; TP, Tylos ponticus.

No. Beach Area Latitude N Longitude E Data source Species

1 Alicudi Aeolian Is. 38°33′01′′ 14°21′57′′ CCS TP2 Brucoli Sicily 37°17′28′′ 15°10′34′′ PSS TP3 Calcara Sicily 37°59′25′′ 12°30′09′′ PSS TP4 Castellammare del Golfo Sicily 38°01′43′′ 12°53′02′′ PSS TP5 Cornino Sicily 38°05′38′′ 12°39′46′′ PSS TP6 Donnalucata Sicily 36°44′37′′ 14°40′05′′ CCS TP7 Favignana Egadi Is. 37°55′47′′ 12°19′34′′ CCS TP8 Filicudi Aeolian Is. 38°33′32′′ 14°35′08′′ PPS TP9 Foce Fiume Belice Sicily 37°34′58′′ 12°51′38′′ CCS TE

10 Foce Fiume Naro Sicily 37°14′11′′ 13°37′10′′ CCS TE11 Fontane Bianche Sicily 36°57′42′′ 15°12′23′′ PSS TE12 Giampilieri Sicily 38°04′03′′ 15°29′11′′ PSS TP13 Isola delle Correnti Sicily 36°39′02′′ 15°04′43′′ PSS TP14 Lampedusa Pelagie Is. 35°30′46′′ 12°33′30′′ CCS TE15 Linosa Pelagie Is. 35°51′53′′ 12°52′02′′ CCS TP16 Lipari Aeolian Is. 38°26′30′′ 14°57′02′′ CCS, PSS TP17 Marina di Modica Sicily 36°42′20′′ 14°47′25′′ PSS TP18 Marina di Ragusa Sicily 36°46′47′′ 14°34′14′′ CCS TP19 Marzamemi Sicily 36°43′14′′ 15°07′10′′ CCS, PSS TE20 Mazara del Vallo Sicily 37°38′03′′ 12°36′50′′ LIT TP21 Mondello Sicily 38°12′00′′ 13°19′42′′ LIT TE22 Morghella Sicily 36°42′11′′ 15°07′23′′ PPS TE23 Oliveri Sicily 38°07′19′′ 15°06′03′′ PSS TE24 Pantelleria Pantelleria Is. 36°50′12′′ 11°56′50′′ CCS TP25 Paradise Bay Maltese Arc. 35°58′55′′ 14°19′59′′ PSS TP26 Playa di Catania Sicily 37°27′10′′ 15°05′11′′ CCS TE27 Ponte Schiavo Sicily 38°05′17′′ 15°30′03′′ LIT TP28 Portopalo, Menfi Sicily 37°34′39′′ 12°54′50′′ CCS TE29 Punta delle Formiche Sicily 36°39′44′′ 15°03′40′′ PSS TE30 Punta Faro Sicily 38°15′56′′ 15°38′56′′ LIT TE31 Ramla I-Hamra Maltese Arc. 36°03′42′′ 14°17′09′′ PSS TE32 Roccalumera Sicily 37°58′21′′ 15°23′37′′ CCS TP33 Ronciglio Sicily 38°00′04′′ 12°30′32′′ CCS, PSS TP34 San Blas Maltese Arc. 36°03′25′′ 14°18′04′′ PSS TP35 San Vito Lo Capo Sicily 38°10′33′′ 12°44′40′′ PSS TP36 Sciacca Sicily 37°30′10′′ 13°02′56′′ CCS TP37 Selinunte Sicily 37°34′57′′ 12°49′08′′ LIT, CCS TE38 Sindaro Marino Sicily 38°16′01′′ 15°28′21′′ PPS TE39 Siracusa Sicily 37°03′15′′ 15°16′13′′ LIT, CCS TP40 Spadafora Sicily 38°13′46′′ 15°23′38′′ LIT TE41 Torretta Granitola Sicily 37°35′07′′ 12°38′42′′ LIT, PSS TP42 Ustica Ustica Is. 38°42′29′′ 13°09′41′′ PSS TP43 Vaccarizzo Sicily 37°26′16′′ 15°05′17′′ CCS TE

pecially in the south-eastern sector. The circum-Sicilian is-lands, with the exception of the island of Lampedusa, featureexclusively T. ponticus, whilst only T. europaeus and only T.ponticus was recorded from the islands of Gozo and Malta(Maltese archipelago), respectively.

A list with the median particle diameter for a number ofSicilian beaches from where Tylos were recorded is shownin Table 2. An analyses of such parameters reveals that thelowest median particle diameter value was reported for thebeach at Playa di Catania (0.344), whilst the highest suchvalue was reported for the beach at Giampilieri (1.771),where the highest standard deviation value was also recorded(1.201), whilst the lowest such value was recorded for thebeach at Ramla (0.099). A similar pattern for standard errorwas reported, with the highest (0.118) and lowest (0.006)

values being reported for the beaches at Playa di Cataniaand Giampilieri, respectively. The grain-size results allowedthe plotting of the box-plot shown in Fig. 3.

With respect to beach sediment particle mean diameter,the two species appear to exhibit partial niche partitioning.In fact, T. europaeus in general demonstrated a preferencefor finer particle sizes, being restricted to a range of particlediameters extending from fine to coarse sand (according tothe Wentworth scale), whilst T. ponticus could tolerate abroader range of particle sizes, being recorded from beachescharacterized by fine to very coarse sand.

The Student’s t-test for the difference between the meandistribution of the two targeted species (independent sam-ples), using the median values of grain-size for each beach,revealed an insignificant difference (t = 1.8431; df = 17;

50 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 34, NO. 1, 2014

Fig. 1. Diagnostic morphological characters of Tylos ponticus (from Filicudi beach, Aeolian islands) and Tylos europaeus (from Fontane Bianche beach,Sicily). A, C, T. ponticus; B, D, T. europaeus: lateral margin of pereionite 1 (white arrows). A, T. ponticus; B, T. europaeus: epimera of pereionites 2-7. E,T. ponticus; F, T. europaeus: apex of pleopod 2 endopod. G, T. ponticus; H, T. europaeus: pleon and uropods, ventral vision. I, T. ponticus; J, T. europaeus:process in basis of pereiopod 1 (white arrows). Scale bars: A, 10 mm; B, 10 mm; E, 100 μm; F, 100 μm.

P = 0.0828) between these, indicating that the distribu-tion of the two species cannot be attributed solely to differ-ences in sediment grain-size. Despite this, it is worth ob-serving that, through the testing of the median values ofgrain-size for each species, by choosing different thresholds

of expected median grain sizes (hypothetical means), non-significant differences under 0.500 mm for T. ponticus andunder 0.300 mm for T. europaeus emerged. The differencesare significant above 0.600 mm for T. ponticus and above0.350 mm for T. europaeus, possibly indicating an upper

MONTESANTO ET AL.: DISTRIBUTION OF TYLOS IN THE MEDITERRANEAN 51

Fig. 2. Map of the geographical distribution within the Central Mediterranean surveyed within the current study of Tylos ponticus and Tylos europaeus;numbers are given in Table 1.

Table 2. Granulometric data for a number of sandy beaches in Sicily, where Tylos spp. individuals was recorded in the present study. SD, standard deviation;SE, standard error.

Beach Mean Max. Min. Mode Median SD SE Confidence interval (95%) No. of samples

Brucoli 1.108 7.247 0.525 1.008 0.976 0.738 0.061 0.989-1.200 150Cornino 0.731 2.221 0.122 0.493 0.675 0.272 0.015 0.701-0.761 318Filicudi 0.428 1.244 0.145 0.206 0.403 0.164 0.009 0.410-0.446 309Fontane Bianche 0.484 1.315 0.025 0.333 0.471 0.163 0.012 0.462-0.500 200Giampilieri 1.771 8.444 0.515 0.515 1.474 1.201 0.118 1.540-2.000 104Isola delle Correnti 0.642 1.956 0.146 0.346 0.346 0.247 0.014 0.615-0.669 324Marina di Modica 0.703 1.592 0.331 0.563 0.685 0.185 0.015 0.672-0.733 143Marzamemi 0.908 2.219 0.453 0.744 0.862 0.264 0.022 0.866-0.900 150Morghella 0.420 0.901 0.005 0.354 0.406 0.139 0.008 0.405-0.435 323Paradise Bay 0.673 1.697 0.278 0.442 0.634 0.201 0.013 0.648-0.698 254Playa di Catania 0.344 0.876 0.008 0.297 0.338 0.111 0.006 0.332-0.300 306Punta delle Formiche 0.622 1.609 0.272 0.409 0.589 0.199 0.011 0.600-0.644 311Ramla 0.384 0.754 0.214 0.2515 0.3645 0.010 0.009 0.367-0.400 273Ronciglio 1.047 2.643 0.335 0.846 0.983 0.490 0.036 0.977-1.118 184San Blas 0.422 0.8505 0.212 0.317 0.403 0.111 0.008 0.406-0.438 380San Vito Lo Capo 0.476 1.258 0.034 0.274 0.454 0.16 0.009 0.459-0.492 351Sindaro Marino 0.361 0.742 0.129 0.326 0.348 0.12 0.007 0.347-0.374 303Torretta Granitola 1.007 2.652 0.417 1.277 0.921 0.365 0.024 0.961-1.054 237Ustica 0.887 1.94 0.189 0.592 0.851 0.322 0.018 0.851-0.923 307

52 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 34, NO. 1, 2014

Fig. 3. Grain-size preferences for Tylos ponticus and Tylos europaeus in the geographical area surveyed within the current study.

limit of median grain size values for each species, abovewhich each species does not occur. The results of this Stu-dent’s t-test (one sample) are reported in Table 3.

DISCUSSION

The rigorous morphological analyses conducted within thisstudy have enabled the correct identification of populationswithin the surveyed geographical area previously recordedas “T. latreillei” or as T. latreillii sensu lato. According toTaiti and Ferrara (1996), our opinion is to follow Kussakin(1982) viewpoint, considering two valid species in thepresent study area: T. ponticus and T. europaeus.

The populations sampled in this study, the re-examinationof the material from the studied collection, and the previ-ous records (Giordani Soika, 1954) allowed us to determineconclusively to which of the two species all the examinedindividuals belong. All the morphological characters studiedin the observed specimens seem to confirm the synonymscurrently accepted, and therefore the validity of the twospecies in the study area. In terms of inter-specific diagnos-tic morphological traits, the characters with the highest valuewere the shape and length of the ventral plate of pleonites 4and 5, and the shape of the prominent process on the ba-sis of pereiopod 1 on the tergal margin. The most prominent

Table 3. Results of single-sample t-test for the species of Tylos consideredin this study. ∗Significant P values (P < α).

Species Hypotheticalmean

t df P

One-tailed Two-tailed

Tylos ponticus 0.500 2.4715 11 0.0155∗ 0.0310∗0.550 1.9429 11 0.0390∗ 0.07810.600 1.4144 11 0.0925∗ 0.1849

Tylos europaeus 0.300 2.5574 6 0.0215∗ 0.0431∗0.325 2.2073 6 0.0347∗ 0.06940.350 1.8571 6 0.0563 0.1127

intra-specific morphological differences were recorded forthe populations of T. ponticus; T. europaeus exhibited a highdegree of homogeneity between the different sampled popu-lations. The differences reported for the T. ponticus seem tobe fall within the normal range of intra-specific variation, al-though future morphometric and genetic studies on the sam-pled populations will better assess such a degree of intra-specific differentiation, besides qualitative characters (Rohlfand Marcus, 1993).

This study is the first to investigate in detail the influenceon the distribution of different populations of Tylos ofvarying sediment particle diameters. In view of the snapshotnature of the sampling conducted in the present study, thedistributional data for the two taxa of Tylos in questionare not absolute. To further substantiate the distributionaldata, the conduction of a multi-seasonal, and possibly evena multi-year, sampling should take place on the beacheswe studied. The data emerging from our current studycontradicts previous reports on the grain-size preferences ofT. ponticus, which was thought to be partial to coarse andvery coarse-sand conditions (Giordani Soika, 1954, 1956;Vandel, 1960; Kussakin, 1982; Caruso et al., 1987). Ourdata also suggests that T. europaeus shows a lower degreeof plasticity in adapting to beaches with variable sedimentgrain sizes, whilst T. ponticus is more euryecious in that itcan tolerate a broader range of sediment grain sizes. In fact,T. europaeus appears to be competitively excluded from verycoarse-grained beaches.

In 40 years of research on isopod taxonomy and ecologyconducted by the research group headed by Prof. Caruso, thetwo species of Tylos in question have never been reported insympatry from the same beach. This can be attributed to anumber of factors including the urbanization of the coast-line, increasingly colonized by man-made artefacts, such asconcrete walls, e.g., on the beaches of Brucoli and Castella-mare del Golfo, which could potentially thwart the longitu-dinal inter-beach dispersion of Tylos. This hypothesis will beassessed through a forthcoming genetic study that will eval-

MONTESANTO ET AL.: DISTRIBUTION OF TYLOS IN THE MEDITERRANEAN 53

uate the degree of divergence between the various popula-tions of Tylos in Sicily and the circum-Sicilian islands. Suchstudies will also highlight any incidence of genetic introgres-sion within boundary areas separating current ranges of T.europaeus and T. ponticus. This will make necessary a sec-ond sampling campaign that would focus on the coastal ar-eas separating the two sets of populations – such zones arepresent in Sicily but are completely absent on the circum-Sicilian and the Maltese Islands.

Biological interactions between the two species, namelyin the form of competitive exclusion, could also be operat-ing, especially since the niches of the two species partiallyoverlap. Because the current study concluded that mediangrain size cannot explain the allopatric separation of thetwo species, niche separation between the two targeted Ty-los could be attributed to different variables, which mightinclude the water and organic content of the sand, the degreeof porosity and penetrability of the same beach sand, and theexposure to wave action, and the slope of the beaches.

ACKNOWLEDGEMENTS

The authors thank Prof. Paula B. Araujo, Universidade Federal do RioGrande do Sul, Porto Alegre (Brasil), for critical comments on the firstversions of this paper. We wish to thank Prof. Domenico Caruso forproviding us the opportunity to access the “Caruso Collection” of terrestrialisopods, housed at the Museum of Zoology (University of Catania,Italy), and Prof. Richard Muscat from the University of Malta ResearchCommittee for funding such a study. The authors would also like to thanktwo anonymous referees whose constructive comments greatly improvedthe value and rigour of the manuscript.

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RECEIVED: 15 May 2013.ACCEPTED: 30 September 2013.AVAILABLE ONLINE: 13 December 2013.