Distribution of four isopods (Peracarida: Crustacea) in shallow waters of the Bizerte Lagoon (Tunisia, SW Mediterranean Sea)

Figures & data

Table I. Monthly variation of isopod abundance and values of Shannon–Wiener (H′) and evenness indexes (J′) from October 2009 to September 2010. Di%, dominance; F%, frequency; N, total abundance of isopods; Ni, total abundance of species.

	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Ni	Di%	F%
Species
Idoteidae
Idotea balthica basteri (Pallas, 1772)	306	102	45	136	147	206	272	413	322	188	163	310	2610	61	100
Idotea chelipes mediterranea Charfi-Cheikhrouha, 1996	6	0	0	2	2	0	19	3	9	5	1	4	51	1.2	75
Sphaeromatidae
Sphaeroma serratum (Fabricius, 1787)	86	168	47	29	24	67	90	83	62	279	420	161	1516	35.4	100
Paracerceis sculpta (Holmes, 1904)	4	12	4	6	2	5	2	3	0	21	41	3	103	2.4	91.66
N	402	282	96	173	175	278	383	502	393	493	625	478	4280
H’ (bits)	0.93	1.16	1.20	0.94	0.75	0.91	1.09	0.74	0.78	1.25	1.16	1.03
J’	0.46	0.73	0.76	0.47	0.37	0.58	0.54	0.37	0.49	0.62	0.58	0.51

Figure 1. Water characteristics at Menzel Jemil station during the sampling period.

Figure 2. Relationship between isopod density and plant biomass at Menzel Jemil.

Figure 3. Distribution of isopods at Menzel Jemil station during sampling period. (a) Temporal distribution; (b) Greatest abundance of I. b. basteri in winter and spring and at 80 cm.

Figure 4. Temporal dynamics of the two dominant species in the algae and seagrass at Menzel Jemil. (a) Idotea balthica basteri, (b) Sphaeroma serratum. The shaded area represents species density, as total number of individuals per square metre; the darker area indicates juveniles.

Figure 5. Abundance of the two dominant species of isopods according to different (a) temperatures and (b) salinity conditions.

Figure 6. Multidimensional scaling (MDS) based on quantitative (square root transformation data) similarity (Bray–Curtis measure) amongst the isopods during sampling period.

Table II. Multiple stepwise regression analysis of isopod abundance (N) in relation to environmental variables from October 2009 to September 2010. SE, standard error. Only environmental variables accepted in each regression model (p < 0.05) have been listed. G. long, Gracilariopsis longissima; G. b. pas, Gracilaria bursa-pastoris; U. lac, Ulva lactuca; t, t-statistic.

	Coefficient	SE	t	R^2	p
Idotea balthica basteri
Constant	615.32	246.9	2.49		0.018
Temperature (°C)	140.48	4.83	2.9	0.39	0.012
G. long	118.67	28.93	4.1	0.7	0.03
G. b. pas	123.82	29.02	4.2	0.69	0.03
Idotea chelipes mediterranea
Constant	901.09	265.09	3.39		0.015
G. long	159.9	31.06	5.14	0.64	0.02
G. b. pas	180.84	31.16	5.80	0.66	0.002
Sphaeroma serratum
Constant	500.01	175.99	2.84		0.015
U. lac	2.73	0.81	3.36	0.81	0.04
Paracerceis sculpta
Constant	40.53	9.85	4.11		0.02
Temperature (°C)	0.95	0.19	4.95	0.75	0.015
U. lac	0.44	0.04	9.79	0.78	0.002

Table III. Results of biotic and environmental linking (BIOENV) analysis displaying the three highest Spearman rank correlations using square root quantitative data of isopods collected from October 2009 to September 2010. T, temperature (°C); U.lac, Ulva lactuca biomass (g.m⁻²); G.b.p, Gracilaria bursa-pastoris biomass (g.m⁻²).

Variables	Quantitative square root
T, U.lac	0.536
T, G.b.p	0.534
T, U.lac, G.b.p	0.534

Charfi-Cheikhrouha F. 1996. Idotea chelipes (Pallas, 1766), espèce polytypique: Description de ses sous-espèces (Isopodes Valvifères). Crustaceana 69:187–203. DOI:10.1163/156854096X00501.

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