INTRODUCTION

The lagoon of Tunis is located in the northeast of Tuni-

sia. Cutting across the lagoon and connecting the Medi-

terranean with the old port of Tunis, the Canal of Tunis

(NC) divides the lagoon into two basins, commonly

known as the southern and the northern lagoon of Tunis.

In the last decades, Tunis southern lagoon was polluted

by local anthropic activities (domestic and industrial dis-

charge), which induced dystrophic crises together with

destruction of benthic communities (Ben Souissi 2002).

In order to resolve the pollution problem and to improve

the water quality of the Tunis southern lagoon for ecolog-

ical and economical purposes, an environmental restora-

tion project was conducted from April 1998 to July 2001.

The overall aim of the project was to achieve a good eco-

logical status in the lagoon and to realize substantial land

reclamation all around.

The Tunis southern lagoon, azoic during the restora-

tion, was progressively invaded by several species, there-

fore, new activities have been developed such as the col-

lection of mussel on artificial rocky banks of the lagoon

and fishery of cephalopod species such as common cuttle-

fish Sepia officinalis Linnaeus, 1758 and musky octopus

Eledone moschata (Lamarck, 1758) (Ounifi Ben Amor et

al. 2019a).

Several Indo-Pacific immigrants were established in

the area since autumn 2001 (Ben Souissi et al. 2005).

For crustaceans, thirteen alien species were recorded in

the area, among them 5 isopods, of which two spheroma;

Sphaeroma walkeri and Sphaeroma venustissimum are

totally established (Ounifi Ben Amor et al. 2015, 2018).

The alien isopod Paracerceis sculpta (Holmes, 1904)

was firstly recorded in the Tunis lagoon, in the northern

part (TNL) (Rezig 1978) and in its southern part (TSL)

(Ounifi Ben Amor et al. 2006).

Invasive Alien Species (IAS) have negative ecologi-

cal and socio-economic impacts and represent a major

threat to marine biodiversity by displacing native species,

changing community structure and food webs (Streftaris

& Zenetos 2006). While in the Tunis southern lagoon, the

negative impact of the introduced species is not highlight-

ed, alien and native species survive and coexist together

(Ounifi Ben Amor et al. 2017).

Paracerceis sculpta, subject of the present study, native

to the Northeast Pacific from southern California to Mex-

ico (Menzies 1962, Miller 1968), has been recognized

as alien species in several marine environments (Brusca

1980). Several authors pointed that isopods possess typi-

cal traits of non-indigenous species (NIS), such as high

dispersal ability, tolerance of wide range of environmen-

tal conditions, phenotype plasticity… employing ships

as vectors and colonizing ports and nearby marine envi-

ronments worldwide (Carlton & Iverson 1981, Hewitt &

Campbell 2001). Paracerceis sculpta essentially associ-

ated with algae and calcareous sponges are considered as

NIS in the Atlantic coast of Europe, also in Southwestern

Atlantic (Rodriguez et al. 1992), the Mediterranean Sea

(Tunisia: Rezig 1978; Italy: Forniz & Maggiore 1985),

Hong Kong (Bruce 1990), and Australia (Harrison &

VIE ET MILIEU - LIFE AND ENVIRONMENT, 2021, 71: 55-61

ON THE SUCCESSFUL ESTABLISHMENT OF THE ALIEN ISOPOD

PARACERCEIS SCULPTA IN TUNISIAN WATERS (CENTRAL

MEDITERRANEAN SEA)

K. O. BEN AMOR

1,2

, M. M. BEN AMOR

, J. BEN SOUISSI

Laboratoire de Biodiversité, Biotechnologie et Changements Climatiques, Faculté des Sciences de Tunis,

Université Tunis El Manar, Tunis Tunisie

Institut National des Sciences et Technologies de la Mer, port de pêche, 2025 La Goulette, Tunisia

Département des Ressources Animales, Halieutiques et des Technologies agroalimentaires, Institut National Agronomique de

Tunisie, Université de Tunis Carthage, Tunis Tunisie

* Corresponding author: benamor7@yahoo.fr

ABSTRACT. – A total of 2103 specimens of an alien species Paracerceis sculpta (Holmes,

1904) was collected between November 2016 and July 2017 in Tunis Southern Lagoon, brack-

ish area located in northern eastern Tunisia. The species is a native from southern California and

invaded the Mediterranean Sea during the late seventies. P. sculpta was found below stones cov-

ered by biofouling and empty tests of barnacles. From the three distinct sexually mature male

morphs of P. sculpta, only the larger (alpha) was found, confirming the absence of the two

smaller male morphs (beta and gamma) in an introduced population. Females reached size at

first sexual maturity from 4 mm in length, the fecundity ranged from 5 to 14 eggs. The high den-

sities and occurrence of all stages of development of the intertidal isopod Paracerceis sculpta in

the Tunis southern lagoon, suggests that the species is established in its new environment.

ECOLOGICAL ASPECTS

POPULATION DYNAMICS

REPRODUCTIVE TRAITS

SETTLEMENT

56 K. O. BEN AMOR, M. M. BEN AMOR, J. BEN SOUISSI

Vie Milieu, 2021, 71

Holdich 1982, Hass & Knott 2000). Paracerceis sculpta

formed the subject of several studies concerning physiol-

ogy of moult and growth (Shuster 1989; Shuster & Guth-

rie 1999), population biology (Munguia & Shuster 2013,

Rumbold et al. 2018) and reproductive behavior (Shuster

1989, Shuster & Wade 1991, Shuster 1992).

The intertidal isopod P. sculpta is a model organism

for understanding alternative mating strategies relat-

ed to its three different male morphs: alpha, beta and

gamma males (Shuster 1987, Shuster 1990, Shuster &

Wade 1991). Alpha males are the largest specimens with

enlarged pleotelsons and elongated uropods, which guard

cavities of their common habitat, the calcareous sponge

Leucandra losangelensis (Laubenfels, 1930). Within

these cavities, alpha males maintain a harem composed

of one or more females. Beta males are morphological-

ly similar to females and live as females, do to infiltrate

harems and mate (Shuster 1987). Finally, gamma males

are very small relative to the other two morphs and are

able to sneak undetected into harems (Shuster 1987).

Paracerceis sculpta is well known as a component of

the biodiversity studies of Tunis Southern Lagoon (Ounifi

Ben Amor et al. 2017). However, some aspects of their

population dynamics, reproductive traits and the distribu-

tion / absence of the two male morphs are to date poorly

known. The aim of the present study is to analyze its life

history strategies, population biology and reproductive

traits in order to state if a viable population is at present

successfully established in Tunis southern Lagoon.

MATERIALS AND METHODS

Study area: Tunis Southern Lagoon adjoins the city of Tunis

and is located in the southwestern region of the Gulf of Tunis

(36°47’N and 10°17’E). It constitutes the southern part of Tunis

Lagoon divided in two areas by a man made navigation chan-

nel (NC) (Fig. 1). Tunis Southern Lagoon extends over an area

of 7.2 km² with a regular depth of about 2.1 m, the maximum

depth being 4 m. It appears as an ellipse stretching in a SW-NE

direction, between 36°46’47” and 36°48’00”N and 10°12’22”

and 10°16’41”E. Its shores have been excavated and artificial-

ly protected by large rocky stones. Tunis southern lagoon is a

coastal lagoon, which maintains a connection to the sea through

the navigation Channel.

The lagoon is characterized by a semiarid Mediterranean

climate. The annual temperature ranges between 7 and 30° C,

the mean annual rainfall is 528 mm with interannual variations.

The luminosity is about 6 h per day, with 155 h in December

and 359 h in July, evaporation ranges between 62 and 201 mm,

respectively, in January and July (Abidi et al. 2018).

Sampling & laboratory procedures: Paracerceis sculpta

were manually collected from November 2016 to July 2017 in

the intertidal zone of the lagoon, not exceeding 0.5 m depth,

among rocky shores covered by algae. The empty barnacle tests

were removed gently at the base of the barnacle using a mallet.

Each test and its occupants were carefully placed inside a poly-

ethylene zip-lock bag. Small stones covered by algae containing

isopods were also transported to the laboratory for identification

and counting. The densities were assessed using a one 0.1 m

quadrate sampler with 10 replicates per month totalizing an area

of 1 m². Simultaneously with the biological sampling, the water

temperature were measured using a thermometer with accuracy

of 1/10° C and a multi-type Lab (WTW) for other parameters

such as pH, salinity and dissolved oxygen.

In the laboratory, specimens were removed from algae by

washing and sieving through a 0.47 mm sieve, all occupants of

each barnacle test were removed. Samples were preserved in a

70 % ethanol solution and subsequently identified using taxo-

nomic description (Loyola e Silva et al. 1999). The total length

of specimens was measured from the frontal edge of the cepha-

lon to the posterior end of the pleotelson under a stereoscopic

microscope with a micrometric ocular (± 0.01 mm accuracy).

Animals were classified into sex and age groups: juveniles (indi-

viduals lacking any observable trait of sexual differentiation),

alpha males (possessing rugose pleotelsons and elongated uro-

pods), ovigerous females (i.e., with eggs in the marsupium) and

non-ovigerous females (i.e., without marsupium). The eggs in

the marsupia of ovigerous females were removed and counted.

Data analysis: Densities and size-frequency distributions

were plotted. Sex ratio (Sr) was calculated each month, the het-

erogeneity hypothesis and the 1: 1 sex ratio hypothesis are tested

by the Chi-square χ² test (Scherrer 1984). The monthly average

data were submitted to the analysis of variance (ANOVA) and,

if necessary, to posthoc Tukey test, with a significance level at

P < 0.05 (Zar 2010).

Fig. 1. – A: Map of Tunisia pointing out the site of Tunis South-

ern Lagoon (TSL) located in the north. B: Tunis Northern

Lagoon (TNL) separated from Tunis Southern Lagoon by a nav-

igation channel (NC).

PARACERCEIS SCULPTA FROM TUNIS SOUTHERN LAGOON 57

Vie Milieu, 2021, 71

Linear regression and the Pearson’s correlation coefficient

(Zar 1999) were calculated to assess the relationship between

female size and the number of eggs in the marsupium and

between the number of ovigerous females and the lagoon water

temperature. Monthly reproductive activity was determined

as the proportion (%) of ovigerous females (Guarino et al.

1993). Size at first sexual maturity was considered as the size

of the smallest ovigerous female (Garcia-Guererro & Hendrickx

2005).

RESULTS

Environmental parameters

The water temperature range between 12° C in Janu-

ary 2017 and 27.1° C in July 2017 with an average of

18.77° C. During the study period, the salinity oscillated

between 36.1 and 38 with an average value of 37.05. The

average pH range between 7.70 in December 2016 and

8.29 in June 2017 with a mean value of 8.02.

Some ecological aspects

Paracerceis sculpta is an intertidal lucifugous spe-

cies, often found inside empty tests of barnacles of the

two species Amphibalanus eburneus (Gould, 1841) and

A. amphitrite (Darwin, 1854) and below stones covered

by biofouling. The species is rather found below rough

stones with crevasses colonized by algae and rare or

absent in the smooth stones without fouling. Males and

ovigerous females are more abundant in the empty tests

of barnacles, while non-ovigerous females and juveniles

were mainly observed among algae (Table I). The species

was observed associated, both in algae and in empty bar-

nacle tests, with other crustaceans species essentially its

congener Sphaeroma venustissimum and the native iso-

pods Dynamene edwardsi (Lucas, 1849) and Cymodoce

truncata Leach, 1814.

Description and population biology

From November 2016 to July 2017, a total of 2103

of Paracerceis sculpta were sampled and sorted into

three different classes; alpha males (824), females

(1136) and juveniles (134). The mean size of males

was 6.80 ± 1.21 mm, females (5.3 ± 1.3 mm). Juveniles

showed a mean size of 2.8 ± 0.5 mm. Alpha-males were

larger than females, and exhibited robust ornamented tel-

sons and elongated uropods (Fig. 2).

Alpha male, 7 mm (Fig. 2A): body relatively slender,

about 2.1 times as long as wide. Anterior margin of head

with median projection. Dorsal surfaces of head and pere-

on almost smooth, each bearing 3 or 5 bundles of setae

posteriorly. Pleon and anterior part of pleotelson granu-

lose. Posterior margin of pleon and middle part of pleotel-

son each with 3 setose tubercles. Apex of pleotelson cleft,

with 3 pairs of notches, anterior and middle notches deep

and posterior shallow. Uropod with endopod reduced;

exopod greatly elongate, gradually curved, bearing many

bundles of setae, apex acute.

Adult female (non-ovigerous), 5.5 mm (Fig. 2B): body

ovate, about 1.5 times as long as wide. Anterior margin

of head rounded. Dorsal surfaces of head and pereon

smooth, each bearing a few posterior setae. Pleon also

smooth, posterior margin with three bundles of setae.

Middle part of pleotelson with 3 setose tubercles. Apex

of pleotelson shallowly concave. Endopod and exopod of

uropod flattened with marginal setae; exopod lanceolate,

apex pointed. Males are light brown and pink. Females

are dark red with a dorsal dark or light brown band. Juve-

niles do not show the reproductive structures that adult

isopods have, e.g. enlarged penes or oöstegites, and there-

fore cannot be sexed. Our data showed the coexistence of

all cohorts throughout the study period, however during

the warm period (April to June) different cohorts coexist

in almost similar proportions (Fig. 3).

The highest densities were observed for the largest

cohorts (from 5 to 7 mm) during the sampling period; 5.1

to 6 mm for females and 6.1 to 7 mm for males (Fig. 3).

No significant difference was found between the mean

sizes of males and females (P = 0.2422). Juveniles rep-

resent 7 % of the entire population sampled. The densi-

ties of juveniles decreased from March to July 2017 when

the adults appeared in large numbers. Females outnum-

bered males with significant differences from November

to February, while males predominated over females from

March to May (Table II).

Table I. – Different associations and total number of Paracer-

ceis sculpta individuals recorded in Tunis Southern Lagoon. ov:

ovigerous females.

Association Males Females Juveniles

Empty tests of barnacles 530 275 (ov: 95) 51

Algae 294 861 (ov: 50) 92

Fig. 2. – Paracerceis sculpta. A: Adult male in dorsal view; B:

Adult female in dorsal view. Scale bar: 3 mm.

58 K. O. BEN AMOR, M. M. BEN AMOR, J. BEN SOUISSI

Vie Milieu, 2021, 71

Size at first sexual maturity occurred in females

at 4 mm in length. The fecundity ranged from 5

to 14 eggs. Larger females carried more eggs than

smaller ones; the number of eggs was positively

correlated with female body length (r = 0.972)

(Fig. 4). An important correlation between the

number of ovigerous females and the water tem-

perature was highlighted in the Tunis south-

ern lagoon (the Pearson correlation coefficient

r = 0.8652, P = 0.002592) (Table III).

Ovigerous females were collected from March

to July and reproductive activity showed with

a peak in May (44 %) and a minimum in July

(8.80 %) (Fig. 5).

Fig. 3. - Paracerceis sculpta. Monthly size class distribution (November 2016-July 2017).

Table II. – Monthly sex ratios of Paracerceis sculpta in Tunis Southern

Lagoon.

Month/year Females Males % F % M Sr χ²

Nov 2016 180 75 70.58 29.41 0.41 *43.23

Dec 2016 171 65 72.46 27.54 0.38 *47.61

Jan 2017 58 16 78.38 21.62 0.27 *23.84

Feb 2017 107 57 65.24 34.75 0.53 *15.24

Mar 2017 130 175 42.62 57.38 1.34 *6.64

Apr 2017 100 127 44.05 55.95 1.27 3.21

May 2017 110 120 47.83 52.17 1.09 0.43

June 2017 135 114 54.21 45.78 0.84 1.8

July 2017 145 75 65.90 34.09 0.52 *22.27

Fig. 4. – Paracerceis sculpta. Relationship between the number

of eggs and the total body length of ovigerous females.

Fig. 5. – Monthly distribution of ovigerous, non-ovigerous

females and reproductive activity of Paracerceis sculpta col-

lected from Tunis Southern Lagoon (November 2016- July

2017).

PARACERCEIS SCULPTA FROM TUNIS SOUTHERN LAGOON 59

Vie Milieu, 2021, 71

DISCUSSION

Prior to the lagoon’s ecological rehabilitation, the abi-

otic parameters fluctuated considerably altering the water

quality of the lagoon; the average monthly salinity ranged

between 31 and 48.9 (Abidi et al. 2018). Measurements

of abiotic variables after the restoration work showed that

the water quality of Tunis Southern Lagoon was consider-

ably improved (Ounifi Ben Amor et al. 2019b). Improve-

ment of the physico-chemical parameters of the lagoon

waters have favored the establishment of new species in

the region. The two alien isopods Sphaeroma walkeri and

S. venustissimum are the best instances (Ounifi Ben Amor

et al. 2018). The sampling of the alien isopod P. sculpta

in the Tunis southern lagoon reveals its high tolerance for

fluctuating salinities, such as the ones encountered in bal-

last tanks during transoceanic voyages, typically ranging

between 4 and 30 ‰ (Ellis & MacIsaac 2009). The influ-

ence of several factors as salinity, temperature, turbidity,

chlorophyll-a, phosphate and nitrate concentrations on

crustacean distribution in Tunis southern lagoon, includ-

ing P. sculpta, have been previously determined (Ounifi

Ben Amor et al. 2017). These results showed that temper-

ature plays an important role in the population changes of

other well established alien species in the area by increas-

ing the growth rate (Ounifi Ben Amor et al. 2018). Fur-

thermore, in winter, decline in salinity and temperature

may explain the mortality and dispersion of Paracerceis

populations, a phenomenon already observed in the ther-

mophilic isopod species (Ounifi Ben Amor et al. 2015,

2018) and in amphipods such as Monocorophium acheru-

sicum (Costa, 1853) and Ericthonius punctatus (Spence

Bate, 1857) (Rumbold et al. 2016).

In Tunis southern lagoon, P. sculpta is always sampled

essentially among algae, which constitute their essential

food, and in the empty tests of barnacles unlike its com-

mon habitat, calcareous sponge, Leucandra losangelensis

(Shuster, 1987). Habitat change in this species proves its

tolerance to a wide range of environmental conditions

such as inhabiting hard substrates and even soft-sedi-

ment areas (Espinosa-Perez & Hendrickx 2001, Hewitt

& Campbell 2001). Paracerceis sculpta share the same

habitat as the established alien isopod Sphaeroma venus-

tissimum in the lagoon unlike S. walkeri which inhabits

different bottoms and is found among fouling communi-

ties (Ounifi Ben Amor et al. 2018). Sphaeroma walkeri

colonizes empty barnacle shells of balanoids, such as

Balanus amphitrite amphitrite Darwin, 1854, oscula of

sponges, especially Chondrosia reniformis (Nardo, 1847),

and ascidians Phallusia mammillata (Cuvier, 1815), Ect-

einascidia turbinata (Herdman, 1880), Ascidiella aspersa

(Müller, 1776) and Ciona intestinalis (Linnaeus, 1767)

(Ounifi Ben Amor et al. 2015).

The sampled population is composed of males,

females and juveniles. Only alpha males, possessing the

conspicuously elongated uropods, were found, in accor-

dance with observation of invasive populations from

Europe, Australia, Asia, North, and South America (Bra-

zil and Argentina) (Loyola e Silva et al. 1999, Hewitt &

Campbell 2001, Munguia & Shuster 2013, Marchini et al.

2017). Beta-males are smaller and do not exceed 4.3 mm

and the gamma-males are tiny, not exceeding 3 mm; all

three male morphs are sexually mature (Shuster 1992).

The long lifespan of alpha males allows them to survive

long voyages and ballast water since ships take at least

six weeks and could even take up to two or three months

to cross the Atlantic Ocean from the American continent.

This could explain their invasion compared to beta and

gamma males that have a shorter lifespan (Munguia &

Shuster 2013, Marchini et al. 2017). Transporting indi-

viduals across large bodies of water could create genetic

bottlenecks, which can result in large populations (Shus-

ter 1989). However, perhaps only alpha males were able

to colonize some new sites (Hewitt & Campbell, 2001).

The variation in the sex ratio of P. sculpta remains with-

out detailed explanation since those studies have showed

that the sex ratio varies considerably through time, both

within and between populations (Shuster & Wade 1991).

The early maturation at small size of females (4 mm)

of the Tunisian population requires detailed genetic study

to explain it. Geburzi & McCarthy (2018) attributed this

change, among other mechanisms, to a phenotypic plasit-

icity, a key feature of successful invaders.

Paracerceis sculpta present a discontinuous spawn-

ing period throughout the year, however, no reproductive

activity was observed during cold months. This suggests

that temperature may play an important role in the gonad

maturity. This discontinuity in reproductive activity is

also explained by the lack of alternative mating morphs,

which could have rapid and profound changes in mating

strategies and life history traits (Shuster 1989). However,

the alien isopod sphaeroma venustissimum has continu-

ous spawning period (Ounifi Ben Amor et al. 2018).

Larger females carried more eggs than smaller ones;

the number of eggs was positively correlated with female

body length. The same trend was observed in S. walkeri

and in S. venustissimum (Ounifi Ben Amor et al. 2015,

Table III. – Number of ovigerous females and water tempera-

tures in the Tunis southern lagoon.

Month/year Ovigerous females Water temperature (°C)

Nov-16 0 18.18

Dec-16 0 14.56

Jan-17 0 11.88

Feb-17 6 13.46

Mar-17 17 16.01

Apr-17 19 18.66

May-17 30 22.7

Jun-17 45 26.45

Jul-17 28 27.1

60 K. O. BEN AMOR, M. M. BEN AMOR, J. BEN SOUISSI

Vie Milieu, 2021, 71

2018). The fact that fecundity is positively related to

female body length is a feature common to many marine

isopods (Guarino et al. 1993, Garcia-Guererro & Hen-

drickx 2005).

The similarities observed also in the dynamics popula-

tion of P. sculpta in the Tunis southern lagoon with those

of Mexico (Munguia & Shuster 2013) and the South-

west Atlantic (Rumbold et al. 2018), where the species is

declared definitively established, could explain the inva-

sive capacity of this species linked, for example, to their

phenotypic plasticity. Broad environmental tolerance

is also likely to be a favorable trait for successful estab-

lishment, introduced species must be able to cope with

a range of contrasting environmental conditions, which

are often different from those encountered in native areas

(Geburzi & McCarthy 2018).

Among several alien crustaceans introduced into the

lagoon, only a few species have successfully established.

Several ecological and life-history traits are associated

with their success, many of these traits are associated with

reproduction, an essential factor determining whether a

species successfully establishes and spreads (Geburzi &

McCarthy 2018). Therefore, a study of population dynam-

ics and reproductive traits of P. sculpta was required.

High densities and occurrence of all developmen-

tal stages of the intertidal isopod Paracerceis sculpta in

Tunis Southern Lagoon suggest that the species is estab-

lished in its new environment. Similar patterns were

reported in the same area for the lessepsian isopods Spha-

eroma walkeri and S. venustissimum (Ounifi Ben Amor et

al. 2015, 2018). Such settlement could be considered as a

main consequence, which displays the successful ecolog-

ical rehabilitation of the area by introduction of the spe-

cies previously unknown in the region (Ben Souissi et al.

2005). Lagoon ecosystems also provide favorable trophic

conditions for alien growth and reproduction and a rela-

tively low biotic resistance due to impoverished native

communities (Azzurro et al. 2014). Ounifi Ben Amor

et al. (2016, 2019b) noted that most of the first records

of alien species in Tunisia stem from ports and lagoons

appeared as favorable ‘transit sites’. Tunis Southern

Lagoon could be considered at present as a prosperous

recipient ecosystem for the settlement of a viable popu-

lation and a potential spread zone of invasive species of

P. sculpta.

The location of the Tunis southern lagoon near the

ports has always played an important role in the bioinva-

sion phenomenon (Ounifi Ben Amor et al. 2019b).

The present study represents an additional contribu-

tion, highlighting how the life history traits can be the

key in determining the success or failure of the settle-

ment process of an introduced species. Studies of genetic

diversity and evolutionary changes should be useful for

understanding the lack of the two morphs in introduced

invasive populations, the potential for colonization and

establishment, geographic patterns of invasion and range

expansion.

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Received on November 11, 2020

Accepted on June 3, 2022

Associate editor: J Orignac