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48 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 34, NO. 1, 2014
taxa have been described as subspecies of T. latreillii or
even, in some cases, as T. latreillii sensu lato (Vandel, 1962).
The two congeners have never been reported from the same
beach.
The two species of Tylos can be distinguished from each
other by the following morphological characters: shape of
apex of the endopod of the second pleopod; lateral margin
of pereionite 1 and epimera of pereionites 2-7; apical margin
of the ventral plate of pleonites 4 and 5; and the shape of the
prominent process on the basis of pereiopod 1 on the tergal
margin (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 these
two species was strictly related to the grain-size of the beach
where 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 to
coarse-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-size
measurement. A lack of granulometric information from the
beaches sampled for Tylos is still very real and, generally,
this type of data is never precisely reported in Tylos-related
literature; in fact, there is only qualitative information in
literature regarding such an aspect. Otherwise, since the
work of Pardi (1955), the burrowing/digging behavior of this
genus have been well studied. The isopods show a strong
surface activity during the night, while they occur, mainly
inactive, deep down in the sand during the day (as reviewed
in Fallaci et al., 1996).
The aims of this study are: to fully and correctly identify
all the Tylos spp. populations within the Central Mediter-
ranean region from sampled populations in Sicily, islands
around Sicily, and from the Maltese archipelago pursuant to
characterizing the current distribution of these species that
occupy 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 degree
of interspecific morphological variation within the various
populations sampled, in order to confirm the validity of the
synonyms 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, individual individuals, the male pleopod 2 endopod has a narrow and
beaches on the islands of Ustica, Favignana, Pantelleria, Linosa and
Lampedusa and 3 beaches on the Maltese Islands (Table 1 and Fig. 2),
were sampled by means of pitfall trap constellations or, where this was not
possible (e.g., due to the restricted dimensions of the beach), by means of
individual traps and through direct collections with entomological forceps.
Pitfall trap constellations deployed were similar to those described in
Deidun and Schembri (2008). A total of 346 individuals were collected from
just 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 of
Catania, Catania, Italy), were also re-examined in order to assign each
population to the two currently-accepted species of Tylos and thus to
retrieve data about their distribution. Such a morphological study was also
useful to further define the degree of interspecific variation within the
various populations of Tylos by examining a relatively large number of
Central Mediterranean populations of the two species.
We also evaluated previous records in the literature for Tylos recorded for
the same geographical area (Giordani Soika, 1954). A list of all the beaches
yielding records of Tylos (either directly, through sampling, or indirectly
through literature records) is reported in Table 1. The distribution of these
two species of Tylos was thus mapped by integrating results from the
sampling 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 the
immediate vicinity of the burrowed isopods. This was done both on beaches
that were characterized by a uniform granulometry and on beaches where
grain-size parameters varied considerably, spatially.
Sediment particle diameter was determined through the deployment of
the stereoscope Leica M205C and its complementary software LAS (Leica
Application Suite ver. 3.3.0). Given the high resolution of this analysis and
the operator’s potential bias when choosing which parts of the sample to
use for the image analysis, the sample was subdivided using the quartering
technique, described by Dyer (1979). Such analysis allowed the plotting of
box-plots for each of the sampled beaches, using the Java-based application
Box Plot Simulation (available online at http://cnx.org).
The influence of differing grain-size values on the distribution of the two
species was processed through a two-sample t-test for independent samples
and a single-sample t-test. The two-sample t-test for independent samples
performs the Student’s t-test, which assumes that the two samples have
equal variances, while the alternative t-test assumes that the two samples
have unequal variances. The single sample t-test considers the significance
of the difference between the observed mean of a sample and a hypothetical
mean of the population from which the sample is randomly drawn. For both
these 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 physical
parameter and the relative distribution of the two species were investigated.
The morphology of the first pair of pereiopods, of the apical margin of
the ventral plates of pleonites 4 and 5, of the tergite margins of pereionite 1
and of the endopod apexes of the first pair of pleopods, were used to assess
the degree of inter-specific differentiation.
RESULTS
No significant deviation from the 1:1 sex ratio has been
found for the studied populations. All T. europaeus exhibit
a more developed groove on the lateral margin of pereionite
1 and the epimera of pereionites 2-7 are considerably wider
than in T. ponticus (Fig. 1A-D). With respect to T. ponticus
more curved apex (Fig. 1E-F); the apical margin of the
ventral plates of pleonites 4 is shorter and more curved
while pleonite 5 have a more rounded margin, rather than
truncate, as in T. ponticus (Fig. 1G-H); the shape of the
prominent process on the basis of pereiopod 1, on the
tergal margin (Fig. 1I-J), is triangular and more pronounced,
where the same process in T. ponticus is shorter and more
pointed.
The distributional data on the two studied species was
geo-referenced and plotted on maps for Sicily, circum-
Sicilian, and Maltese Islands, using data derived either from
the sampled populations, from literature (Giordani Soika,
1954), or from the Domenico Caruso collection. The dis-
tribution of the two species, as recorded within the surveyed
geographical area, is reported in Fig. 2 (the corresponding
numbers of the cited beaches are also reported in Table 1).
In Sicily, although T. ponticus and T. europaeus dominate
beaches in the north-west and north-east of the island, re-
spectively, there is no clear spatial separation between the
two species, with an overlap between the geographical dis-
tribution of the two being observed all over the island, es-