Colour and Morphometric Variation of Donacid Bivalves from Nepa Beach , Madura Island , Indonesia

Donacid bivalves vary in colour, pattern, and the shape of the shells. A population of donacid bivalves had been found in the northern shore of Madura Island, Indonesia. This study aimed to identify the donacids found in northern shore of Madura Island as well as to describe their variation on morphometric and morphological characters. Colour and pattern of 215 shells were observed and carefully photographed. Morphometric of the shells including the shell height, shell length, shell width, dorsal length, and umbo-margin length were measured and then their patterns were analyzed using regression analysis. The results revealed that there were two species of donacid bivalves in Nepa Beach, namely Donax cuneata and Donax faba with different morphological characteristics, including twelve different patterns and colours of the shells. These variations were described completely in this paper. This study contributes to the research on bivalve taxonomy, and is useful for the identification of donacid bivalves based on morphological characteristics.

populations of Donax serra showed shapes and colour variations.They argued that those morphological plasticities related to their habitat condition.Tan & Low (2013) described that Singaporean Donax cuneata had white, cream, brown, and grey coloration, and also radial bands pattern.In addition, Tan & Low (2013) also described that Donax faba found in Singapore varied in shell shape, and coloration pattern.These findings highlight the possibility that a wide range of distinctive morphological characters of donacid bivalves can be related to their habitat.
Recently, a dense population of donacid bivalves has been found in sandy beach located on the northern shore of Madura Island.Madura Island is a relatively big island in East Java, Indonesia, located in Western Pacific area.The occurrence of donacid bivalves in Madura Island has thus far not been reported.Previous studies on this island worked on other groups of molluscs.Rahmasari et al. (2015) investigated the diversity and abundance of gastropods in the Southern shores of Madura, meanwhile Rochmawati et al. (2015) discovered the potency of razor clams taken from Madura.Hence, the research on donacid bivalves will be valuable input to the data of molluscs of this island.In addition, this paper describes these donacid bivalves and examines their morphological variation that will be useful for taxonomy work.

Sampling and Preservation
Samples were collected from Nepa Beach, Sampang, Madura Island, Indonesia.Living bivalves were relaxed by using MgCl 2. 6H 2 O 7.3% in sea water, and then preserved in Ethanol 70%.Shells were cleaned in fresh water before drying for storage.Exterior and interior of 215 shells were observed carefully under a magnifying lamp.

Identification and Measurement
Specimens were identified based on morphological characters of the shells.Morphometric measurements of the specimens were taken by calipers.Shell length (SL) was defined as the perpendicular distance between the anterior and posterior ends of the shells.Shell height (SH) was measured from the highest part of the dorsal side to the lowest part of the ventral side of the shells.Shell width (SW) was defined as the distance between the most prominence parts of the lateral side of the two shells.Dorsal height (DH) was measured from the highest part of the dorsal side to an imaginary line that was perpen-

INTRODUCTION
Donacids bivalves (Bivalvia: Donacidae) are widely distributed in sandy bottom ecosystems throughout the world.Donax truncus can be found in the Mediterranean Sea (La Valle et al., 2011) and in sandy beaches in Portugal (Pereira et al., 2012).Other members, namely Donax semistriatus, D. vittatus, and D. variegatus were found in Portugal (Pereira et al., 2012). Herrmann, et al., (2009) reported the occurrence of Donax hanleyanus in sandy beaches of Argentina.
Donacid bivalves not only have high diversity based on the number of species, but also high variation in morphological aspects including shape variation and colour pattern.Soares et al. (1998) noted that two geographically different dicular between the anterior and posterior ends of the shells.Umbo margin line (UML) was defined as the distance from the highest part of the dorsal side of the shell to the posterior end of the shell.Abbreviations used for collections are: MZB.PEL= bivalves collection of Museum Zoologicum Bogoriense (Research Center for Biology -Indonesian Institute of Science); and RA = collection of the first author.

Data Analysis
The morphological characters of the shell, such as colour, band pattern, and the shape of the shell, were descriptively analyzed.Meanwhile the patterns of shell morphometric were analyzed by regression analysis.

RESULTS AND DISCUSSION
Identification placed specimens of donacid bivalves found in Nepa, northern shores of Madura Island Indonesia into two different species, namely Donax cuneata and Donax faba.Systematic accounts of these two species were described as follow.

Donax (Latona) cuneata, Linnaeus 1758
Donax cuneata Linnaeus, 1758: 683;Prashad, 1932: 202;Lamprell & Whitehead, 1992: plate 51, 379;Hylleberg & Kilburn, 2002:  Shell: thick, compressed, distinctly inequilateral, trigonal-ovate in outline.Shell length to up 31.75 mm; shell height up to 28.1 mm; shell width up to 19.9 mm.Sculpture: smooth with fine concentric lines, radial sculpture distinct at posterior end, forming reticulate scale-like sculpture.Umbo: inflated, prosogyrate.Colour: white, cream, brown, grey; usually with radial bands of varying thickness and prominence; interior white to purple, often with tinted yellow, brown or purple blotch.Dentition: heterodont with anterior and posterior lateral teeth.Interior of the shell: elongated anterior adductor scar and rounded posterior adductor scar; deep palial sinus (approximately ½ of shell length); obvious palial line (Figures 1-2).Habitat: these bivalves were buried shallowly in the sand of intertidal zone.Shell: thick, compressed, inequilateral, triagonal-ovate in outline.Shell length up to 26.8 mm; shell height up to 19.9 mm; shell width up to 18.4 mm.Sculpture: surface smooth with fine concentric lines that are usually more pronounced and appear as ridges on the posterior side.Umbo: inflated, prosogyrate.Colour: white, cream, brown, purple; often with one or more radial bands and random wide spots; interior white, often with shaded yellow or with purplish blotches, to purple, usually with whitish blotches and/or radial bands.Dentition: heterodont with anterior and posterior lateral teeth.Interior of the shell: elongated anterior adductor scar and rounded posterior adductor scar; deep palial sinus (approximately ½ of shell length); obvious palial line (Figures 3-4).Habitat: these bivalves were buried shallowly in the sand of intertidal zone.
Donax cuneata and Donax faba can be distinguished on the basis of their morphological characteristics.Donax cuneata have radial ridges at the posterior end of the shells, which can be seen as reticulate scale-like sculpture, while D. faba lack of these sculpture, but they have fine concentric lines at the posterior and ventral part of the shell.Furthermore, these two bivalves can be differentiated based on their morphometric patterns.Donax cuneata collected from Madura Island were bigger than D. faba.The length of Donax cuneata could reach 31.75 mm, while the length of Donax faba was up to 26.8 mm.Moreover, the anterior part of the former was more likely to become wider than the latter.In addition, most of Donax cuneata shells were thick, and this could be observed in the width of the shells.The shell width of Donax cuneata could reach 19.9 mm and the ratio of shell width and shell height (SW/ SH) was 0.5, while shell width of Donax faba was up to 18.4 mm and the ratio of shell width and shell height (SW/SH) was 0.48 (Table 1).The relationship of shell height and shell length of D. cuneata can be shown by regression equation SH= -1.234+0.772SL,whereas equation for D. faba was SH= -4.795+ 0.4795SL.Moreover, the relationship of shell shell width and shell length of D. cuneata was shown by regression equation SW= -2.924+0.482SL,whereas equation for D. faba was SW= -0.716+ 0,374SL (Table 1).However, the size and colour of sampled shells collected from Madura Island was highly varied.Both of Donax cuneata and Donax faba varied among the individuals in the population, including colour, shape, and size.There were twelve types of colour patterns of Donax cuneata.White colour on both the exterior and interior was the most frequent pattern in this species and, followed by dark purple with tinted yellow exteriorly and dark purple interiorly (Table 2).Similar with D. cuneata, D. faba was found in twelve types of colour pattern.The pattern of light brown with brown maculation exteriorly and yellow with purple maculation interiorly was the most frequent pattern that could be found, followed by Type 5 which was white exteriorly with brown maculation and the interior was white tinted purple (Table 3).
Colours and patterns of Donax cuneata and Donax faba were almost the same, however they had different tendency in pattern.The shell of Donax cuneata was mostly dark purple and white, often found with radial bands; while Donax faba was frequently found in cream shell with random brown maculation formed "batik" pattern.In addition to colour, Donax cuneata and D. faba could be distinguished on the basis of the sculpture of their shells.The former had prominent radial ribs at the posterior side, while the latter only had strong concentric line at this side.This was also confirmed by Tan & Low, (2013).
Moreover, the shapes of these two species were different.Although Donax cuneata and Donax faba had a common shape known as "wedge" (McLachlan et al., 1995) because the average ratio of shell width and shell length were 0.36 and 0.34 respectively.However, their shapes were not the same.The posterior side of D. cuneata was rounded, while posterior side of D. faba was often found elongated to the ventral side, such as found in the Type 2, 3, 6, 9, 10, and 12.In addition, D. cuneata looked more inequilateral since the umbo margin line was longer (Table 1).The shell of Donax faba looked more slender than D. cuneata because the ratios of shell width and shell length as well as the ratio of shell width and shell height of D. faba were smaller than D. cuneata.This nature of the shell facilitated their burrowing activities in the sandy beach.Stanley, (1970) explained that the wedge-shaped shell of donacid bivalves was an adaptation for fast burrowing and movement between tide-levels.
There was high variation among the individuals of D. cuneata collected from Madura Island that could be grouped into twelve types of variations.In a previous study, Tan & Low (2013) reported this kind of variation among the individuals from Singapore.Among the individual collected from Madura Island, Type 5, which was white exteriorly and interiorly, was the most dominant, followed by Type 1 with dark purple appearance.The morphometric characteristics of those types varied, but, generally they revealed a wedge-shaped appearance, namely the range of SW/SL ratio was 0.34-0.38, the ratio of SW/SH ranged 0.49-0.54,and the ratio of SH/SL was 0.66-0.74.The similarly high variation was also found among the individuals of D. faba collected from the same site.Dharma (2005) also recorded five-shell variation of D. faba collected from Panimbang, the Indonesian Province of West Jawa.Previous study also reported the variation of D. faba in Singapore (Tan & Low, 2013).Among the individuals collected from Madura Island, Type 1, which had "batik" appearance (the exterior was light brown with brown maculation) was the most dominant.The morphometric characteristics of D. faba types also varied, but, they commonly had slender wedge-shaped appearance, namely the ratio of SW/SL ranged 0.31-0.4,the ratio of SW/SH ranged 0.43-0.65,and the ratio of SH/SL was 0.64-0.70.
Various coloration of Donax faba and Donax cuneata often lead to misidentification.This research discovered a quantitative as well as qualitative variation of those two species, which has been described completely in this paper.Hence, it will be useful for the identification of donacid bivalves based on the morphological characteristics.

CONCLUSION
Donax cuneata and Donax faba were found at Nepa Beach, the northern shores of Madura Island, Indonesia.Each of these species had twelve types of shells, based on coloration and morphometric patterns.

Table 1 .
Comparison of morphometric features of Donax cuneata and Donax faba