Theodoxus is a neritid snail succeeded in invading freshwater habitats. These snails are usually subglobose or hemispherical, have few whorls, very reduced spires and very large body whorl. Two freshwater species were reported from Jordan, T. jordani and T. macri (Burch, 1985). Dagan (1971) contended that both species of Theodoxus are the same, a conclusion based on details of the opercular apophyses and the central teeth of the radula. However, T. macri is uniformly black or dark purple in color, while, T. Jordani is zebrated having red or brown to deep purple or dark black zig-zag stripes on a white or yellow background (Burch, 1985). Both species are common in all freshwater habitats of Jordan. In Azraq Oasis, T. macri is mostly found in spring pools, ponds and swamps. The present research aims at providing more information on the population structure and shell morphometrics of T. macri from Azraq Oasis.
MATERIALS AND METHODS
Study area: Azraq Oasis is a semi-desert area in the East Jordanian Desert 85 km east of Amman, the capital of Jordan. It is the only permanent water in this region. Its water is supplied by two springs, one in North Azraq and the other is in South Azraq. Azraq pools are infested with several snail species. Most predominant are Melanopsis praemorsa, Melanoides tuberculata and Theodoxus macri.
Sampling and analysis: A total of 371 Theodoxus macri (Sowerby, 1849) (Neritidae: Prosobranchia) snails were collected by hand picking from Azraq Oasis pools. In the laboratory, shell length, shell width, aperture length and aperture width were measured using vernier calipers, to the nearest 0.1 mm. The shells of about 310 snails were separated from soft tissues and dried to constant weights at 100°C. The dry weights of both the shell and soft body were weighed using an electrical digital balance accurate to 1 mg. Measured snails were divided into size groups of length intervals which were estimated using histogram plots that show normal distribution of length. Normality was tested at 95% confidence level. These histograms used to estimate the possible life span of T. macri snails.
The growth in length of T. macri was theoretically calculated using the Von Bertalanffys Growth Formula (VBGF) (Lt = L∞ [1-e k (t-t0)]) and the Richards Growth Formula (RGF) ( Lt = L∞ [1-A*e kt]). Lt is the length at age t, A = 1 (minimum length/maximum length), k is the growth coefficient and t0 is the length at which the age is theoretically nil. The constants L∞, k and t0 were calculated using the Quasi-Newton method (Ostle and Mensing, 1975) to the age-length data. For comparison with VBGF and RGF, the observed age length data were fitted to a linear regression equation. The relationships of the shell length to dry body weight and shell weight were described using a power regression equation (Ott, 1984).
The relative coefficient of condition (Kn) which measures the degree of fitness
and suitability of the environment with regard to feeding condition was also
estimated (Le Cren, 1951). Kn was calculated as: Kn = X/aLn where
X represents the dry body weight, L is the shell length, a and n are the coefficients
of the power regressing equation.
The relationships of the shell length to shell width, aperture length, aperture width, shell weight, dry body weight, total weight, ratio of aperture width to shell length, ratio of aperture length to shell length and ratio of shell width to shell length were studied using regression analysis.
Age and growth: The frequency distribution of shell length of the collected
snails which fits normal distribution indicates the presence of four age cohorts
for T. macri (Fig. 1). The mean observed length at
each of these age cohorts was 3.295, 4.984, 7.707 and 10.281 mm (Table
1). Calculated shell lengths using VBGF and the linear method were highly
correlated with the observed values (R2 >0.96) (Table
1). However, this correlation was 78.3% using RGF. Thus, it is expected
that this snail may survive to four years. The annual observed increase in length
ranges from 1.689 to 2.723 mm. Theoretically, the increase ranges from 1.835
mm in old snails to 2.861 mm in young ones using the VBGF and from 1.058 to
1.650 mm using RGF. The theoretical growth parameters of T. macri were
similar using VBGF and RGF (Table 2 ). The theoretical maximum
growth was 17.187 and 12.930 mm and the growth coefficients (k) were 0.218 and
0.222. The length at to was calculated to be 0.194.
Length-weight relationship: Collected T. macri snails range in
length from 2.20 to 11.00 mm, with a mean value of 5.628 mm.
||Frequency distribution of estimated length groups of Theodoxus
macri from Azraq Oasis showing expected normal distribution (n = 371)
Their shell and dry body weights range from 2 mg to 360 mg with an average
of 44.4 mg and from 1.0 mg to 150 mg with an average of 36.05 mg, respectively.
Increase in shell and body weights was slower in young snails than older ones.
Observed and calculated shell lengths (in mm ) of various
age groups of Theodoxus macri from Azraq Oasis using Von Bertalanffys
Growth Formula (VBGF) and the Richards Growth Formula (RGF) and Linear
Growth parameters of Theodoxus macri from Azraq Oasis
using Von Bertalanffys Growth Formula (VBGF) and the Richards
Growth Formula (RGF)
||The relationships of the shell length to shell, body and total
weight of Theodoxus macri from Azraq Oasis
||The relationship between the total weight and the dry body
weight of Theodoxus macri from Azraq Oasis
The relationships of shell length to shell width, aperture
length and aperture width of Theodoxus macri from Azraq Oasis
This produced curvilinear relationships of the shell length to both shell and
dry body weights (Fig. 2).
||Measurements (means in mm±SD) of length (SHL), shell
width (SHW) aperture length (APL) and aperture width (APW) of various age
groups of Theodoxus macri from Azraq Oasis
The relationships of the shell length to ratios of shell length
(SHL) to each of shell width (SHW), aperture length (APL) and aperture width
(APW) of Theodoxus macri from Azraq Oasis
The power regression equations for these relationships are:
X=0.281 (L) 3.199
X1=0.189 (L) 3.680
X2=0.114 (L) 2.712
Where X, X 1 and X2 represent the total, shell and dry body weights, respectively and L represents the shell length. The relationship of the total weight to dry body weight was linear (Fig. 3). The relative coefficient of condition (Kn) which measures the progression of body weight and length was measured for young and old snails (age groups 1 and 2 and 3 and 4, respectively ). It was 1.008 and 0.999 for both groups, respectively.
Shell morphometrics: The mean shell length of age groups 1 to 4 of T. macri ranges from 3.295 to 10.281 mm. The shell width ranges from 2.684 to 7.644 mm, aperture length from 2.203 to 5.713 mm and aperture width from 1.468 to 3.781 mm (Table 3). The relationships of the shell length (L) to shell width (W), aperture length (AL) and aperture width (AW) are linear (Fig. 4). The regression equations for these relationships are:
W=0.039 + 0.713L
AL=0.076 + 0.524L
AW=0.073 + 0.328L
The ratios of the shell width, the aperture width and the aperture length to shell length decrease with age (Fig. 5). This may indicate that this snail become more elongate with age.
The frequency distribution of the shell length of collected Theodoxus macri snails from Azraq Oasis indicated the presence of four age cohorts. These snails range in length from 3.3 to 10.3 mm. Theodoxus jordani snails collected from Yarmouk River ranged in size from 1 to 20 mm and 74-84% of them measured 6-13 mm (Abel-Hafez and Ismail, 1987). Two peaks of young snails (4 mm long) appeared during September and March. These snails grew at a rate of about 6 mm per year (Abel-Hafez and Ismail, 1987). In the present study this snail increased in length at a rate of 1.689 to 2.723 mm annually. Melanopsis praemorsa from Yarmouk River increased by 3-4 mm between January and December and ranged in size from 3-27 mm (Ismail and Abel-Hafez, 1987). Higher rates of increase were reported from Drupella cornus snails from the Gulf of Aqaba (Ismail et al., 2000). These snails grew at a rate of 3.7-6.8 mm annually. The limpet, Cellana radiata, from the Gulf of Aqaba grew at a rate of 4.0-9.0 mm annually (Ismail and Elkarmi, 1999).
Increase in shell and dry body weights of T. macri was slower in young
snails than older ones of in the present study. This produced curvilinear relationships
of the shell length to both shell and dry body weights. This was also true for
D. cornus (Ismail et al., 2000 ) and C. radiata (Ismail
and Elkarmi, 1999). However, power regression equations for these relationships
for different snails were different. Moreover, the relationship of the dry body
weight to shell weight was linear for both T. macri in the present study
and D. cornus from the Gulf of Aqaba.
The relationships of the shell length to shell width, aperture length and aperture width were linear for both T. macri from Azraq and D. cornus (Ismail et al., 2000 ). However, linear regression equations were different for both snails. Similarly, the shell length to shell width relationship of C. radiata was also linear (Ismail and Elkarmi, 1999).