Morphometric relationships of the siphon clam Panopea globosa ( Bivalvia : Hiatellidae ) in the southeasternmost of the Gulf of California , Mexico

The morphometric relationships of the siphon clam Panopea globosa from Sinaloa, Mexico, were analyzed from February 2014 to April 2015. A total of 370 specimens were collected, measured (shell length, width, and height) and weighed (body weight, tissue weight, and shell weight). The coefficient of determination indicated variation in the fitting of morphometric variables among measured traits. The type of relative growth for all morphometric relationships was negative allometry. This is the first report on morphometric relationships of P. globosa from Sinaloa, Mexico.


Introduction
Sustainable fishery management relies on obtaining morphological measurements of exploited organisms, and a complete assessment includes shell size (length, height and width) and body weight through the study of morphometric relationships among variables (Syda-Rao 2007, Grizzle et al. 2017).The bivalve Panopea globosa (Dall, 1898), commonly known as "siphon clam" or "geoduck", is a subtropical species endemic to the Gulf of California (Hendrickx et al. 2005) and constitutes one of the most recent and important clam fisheries in the northwestern of Mexico (Calderón-Aguilera et al. 2010).Aragón-Noriega et al. (2012) mentioned that landings increased from 49 mt in 2002 to 2,000 mt in 2011 in the states of Baja California and Sonora, Mexico.The available information on this species focuses basic aspects of wild populations specifically from the central Gulf of California and the Pacific coast of Baja California, Mexico.It deals with its biology (Aragón-Noriega et al. 2007), morphometrics, age, and growth (Calderón-Aguilera et al. 2010, Cruz-Vázquez et al. 2012), fishery status (Aragón-Noriega et al. 2012), spatial distribution and abundance (Cortéz-Lucero et al. 2014), density and population structure (Aragón-Noriega et al. 2016).Recently, Góngora-Gómez et al. (2016) reported the first record of P. globosa in Sinaloa's state, at the southeastern coast of the Gulf of California.Due to the (total body wet weight BwW, tissue wet weight TwW, and shell wet weight SwW) were obtained after separating the shell from the soft tissue and blotting them dry in absorbent paper prior to weighing.
The morphometric relationships of shell measurements and weighings of geoduck clams were estimated by fitting the power function W= aL b .The goodness of fit was described using the determination coefficient, r 2 (Sokal & Rohlf 1981).The relative growth between variables (isometry vs. allometry) was analysed through the allometric coefficient (regression slope -b) of the morphometric relationships (Vasconcelos et al. 2018).When comparing variables with the same units (both linear), isometry occurs when b is not statistically different from 1, but when variables have different units (linear and ponderal), isometry occurs when b is not statistically different from 3. The b values obtained in linear regressions were significantly different from the isometric value (b= 1 or 3) or allometric range (negative allometry: b < 1 or 3, or positive allometry or b > 1 or 3) when a t-test (H 0 : b= 1 or 3) with a confidence level of 95% was applied, expressed by the following equation (Lleonart et al. 2000): where, t is the t-test value, b is the regression slope and Sb is the standard error of the regression slope.

Results and discussion
The regression slope varied from 0.310 for the SH-SW relationship, to 1.562 for the TwW-SH relationship.Considering BwW versus shell measurements, the higher coefficient of determination (r 2 = 0.37) was obtained for the BwW-SW relationship, indicating that BwW-SW may be the most suitable morphometric relationship for describing this species instead of BwW versus SH or SL when sampling geoduck directly in the field and without an analytical balance.On the contrary of most of bivalves in which internal organs are totally covered by the valves, the inequilateral valves of adult P. globosa present a dehiscent periostracum that partially covers the mantle edge and prolongs into siphonal sheath (Coan & Valentich-Scott 2012).Therefore, BwW could also be affected by the different stages of gonad maturation expressed with measurement of SW.Based in the morphological traits, it is possible that similarly to other geoduck species studied by Aragón-Noriega et al. (2007) and Calderón-Aguilera et al. (2010), sexually mature specimens would be found within the population of P. globosa from Sinaloa.In fact, Aragón-Noriega (2015) estimated sizes at 50% maturity for P. globosa of 91.9 mm SL and 90.9 mm SL for females and males, respectively, coinciding with the size range analyzed in this study.Nevertheless, histological evidence is needed to further confirm this statement.
Shell measurements varied from 87.3 to 168.2 mm for SL, and from 267 to 1039.5 g for BwW (Table 1).Compared with the mean SL (147.7 mm) and BW (533.3 g) for the P. globosa population sampled by Aragón-Noriega et al. (2007) in Guaymas-Empalme Bay, Sonora, and for the P. generosa population studied by Calderón-Aguilera et al. (2010) in San Quintín-Isla Coronado Bay, Baja California (SL= 132 mm and BW= 763.6 g), the P. globosa population from Altata-Ensenada Pabellones lagoon, Sinaloa, comprised smaller specimens (SL= 123.1 mm and BW= 615.4 g).Since no juvenile geoducks were found, it is possible to state that the present population was dominated by adult individuals, which corroborates the aforementioned authors and Gribben & Creese (2003) who described similar population size structures.
The coefficient of determination (r 2 ) ranged from 0.085 for the SwW-SW relationship, to 0.411 in the SwW-TwW relationship (Table 2).Although there were significant correlations among all morphometric relationships, the relationships of SWI versus SL, SWE and TWM displayed the lower coefficients of determination indicating a reasonably higher variability between these variables, which could be partially explained by factors such as sediment type and water content within the tissues.Goodwin & Pease (1991) mentioned that gravel or shell substrata affected the shape of geoduck shells and the burial depth of P. abrupta, which could lead to lighter specimens with misshapen shells.We observed that sediment type at the Altata-Ensenada Pabellones lagoon system is constituted by a range of substrata from small shells and gravel, as well as fine sandy sediments.Thus, shell and tissue traits are susceptible to vary when geoducks are collected from heterogeneous substrata (Aragón-Noriega et al. 2007).Wet weights may have also contribute to trait variations since geoduck retain water inside the mantle cavity and in within the tissues (Gribben & Creese 2003).The present specimens were maintained alive in seawater and transported to the laboratory before weighing in order to avoid variability in total weight and tissue wet weight.The higher slope (b= 1.562) was obtained for the TwW-SH relationship, whereas the lowest slope (b= 0.310) was recorded in the SH-SW relationship.In the present study, the b values of all BwW -shell measurements relationships were between 1.324 to 1.531, being quite different (0.03 to 5.97) from those reported by Aragón-Noriega et al. (2007).These differences may be a consequence of distinct hydrological and sedimentological features between different geographical areas (Gaspar et al. 2002), which would affect morphological traits of the species.
Since b values for the morphometric relationships between the same type of variables (i.e., linear vs. linear and ponderal vs. ponderal) were below 1 (b= 0.254-0.589)and those relationships between different types of variables (i.e., linear vs. ponderal) were less than 3 (b= 0.563-1.562),a strong negative allometry describes the type of growth of the studied population of P. globosa.Specifically, the negative allometry (b < 1) in the relationships between shell measurements suggests that geoduck development is expressed more in terms of shell height and width than in shell length, which in practice means that bivalves become wider, reflecting a morphological adaptation that could improve burial depth on the bottom substrata (Lauzier et al. 1998), allowing geoducks to accommodate a longer siphon extending to the sediment surface.Calderón-Aguilera et al.
(2010) obtained a mean b value of 2.428 for P. generosa in the coast of Baja California state, suggesting that species and environmental conditions, among other factors, could influence the morphometric traits of this bivalve genus.
In conclusion, morphometric relationships suggested likely effects of external and internal factors.The geoduck population sampled in the Altata-Ensenada Pabellones lagoon system is constituted by large individuals, for which measurement of SW rather than SL and SH was more consistently proportional to BwW.Due to the increasing importance of this fishery in Sinaloa, more biological/ ecological information is needed to develop sustainable exploitation strategies.The present data provides valuable information for fisheries assessment and management, constituting the basis for future research.