Morphological diversity of setae and chela in accordance to habitat within of (, Brachyura, ) from intertidal flats of Pakistan

Item Type article

Authors Aziz, Uroj; Saher, Noor Us

Download date 29/09/2021 07:03:55

Link to Item http://hdl.handle.net/1834/40805 Pakistan Journal of Marine Sciences, Vol. 25(1&2), 83-91, 2016.

MORPHOLOGICAL DIVERSITY OF SETAE AND CHELA IN ACCORDANCE TO HABITAT WITHIN CRABS OF GENUS MACROPHTHALMUS (DECAPODA, BRACHYURA, MACROPHTHALMIDAE) FROM INTERTIDAL FLATS OF PAKISTAN

Uroj Aziz and Noor Us Saher APWA Govt. College for Women Karimabad, Karachi (UA); Centre of Excellence in Marine Biology, University of Karachi (NUS). email: [email protected]

ABSTRACT: Sentinel crabs of genus Macrophthalmus are deposit feeders inhabiting sandy muddy substrate. The diversity and distribution of these crabs depend upon physical, biological and environmental factors including tidal exposure, larval recruitment, salinity, pH, organic matters, air and water temperature, nutrient availability and sediment structure in which they survive. These showed various morphological adaptations to cope with their environment and to extract food from their surroundings. Current study aimed to study structural variation in 2nd maxilliped and chela and correlate it with their habitat. The substrate structure appears as one of the most important controlling factor in the distribution of sentinel crabs. These crabs utilize their spatulate chela to gather the soft mud and sandy substrate for extraction of organic matter and specialized setae on their mouth appendages to manipulate sediment particles and to extract food from it. Current study describes the setae of the second maxillipeds within the five Macrophthalmid spp. known from the Pakistan coast by using scanning electron microscopy. In all the studied species plumose setae is present on meropodite, ischium and exopod flagellum, whereas spoon tipped setae are restricted to the palp region. Structural variations in these spoons tipped setae have also been reported during the present study. Comparatively, more spoon- tipped setae were present in M. sulcatus, a sand dwelling species than mud dwelling forms, provides evidence that the crabs from sandy habitats possess more spoon-tipped setae (an adaptation for processing sandy sediments) than those living in muddy habitats.

KEYWORDS: Morphological diversity, Macrophthalmus, 2nd maxilliped, scanning electron microscopy, meropodite, spoon-tipped setae.

INTRODUCTION

Ocypodoid are semi terrestrial crabs that are distributed along Indo West Pacific (IWP) regions of the world. They are one of the most diversified fauna living from sub-tidal to supra-tidal zones having a variety of feeding modes, including deposit feeding, herbivore, scavenging and predation. Species of genus Macrophthalmus are among the important members and ecological engineers of intertidal shores as they perform surface activities including feeding, burrowing, and performing social behavior during ebb tide. They are mainly detritivore crabs that extract, organic debris through filter feeding mechanisms. During feeding, they ingest sediment containing dead organic matter which is sorted within 84 Pakistan Journal of Marine Sciences, Vol. 25(1&2), 2016. the buccal cavity. These feeding modes require appropriate mechanisms and special modified feeding appendages to acquire and process food. The morphology of the filtering appendages and the mechanism to transfer food particles to the mouth and its suspension by filter feeding crabs was studied by many workers (Ono, 1965). Studies showed that the third maxilliped (outermost mouthparts), helps to retain water and sediment within the buccal region as a result particles becomes suspended in the water during the sorting process. The 2nd maxilliped aids in the manipulation and sorting of larger sediment particles, they have spoon tipped setae with cuticular projections present on the inner face of endopodite. These spoon tipped setae holds sediment grain against the brush like setae of the first maxilliped while inorganic, organic matter, diatoms and bacteria are sorted and moved towards the mouth. The rejected particles rolled between two maxillipeds and expelled out as feeding pellets (ball) (Miller, 1961; Ono, 1965). Studies suggested that the distribution of brachyuran crabs depends upon to the ability of their mouthpart which manipulate different particle sizes and extract food in sediments with different grain-size compositions (Miller, 1961; Crane, 1975; Icely and Jones, 1978; Maitland, 1990; Al-Khayat, 1996; Yamaguchi and Ogata, 2000; Costa and Negreiros- Fransozo, 2001; Lim, 2004; Lim and Kalpana, 2011). Icely and Jones (1978) studied the feeding appendages in fiddler crabs and reported that second maxilliped showed important structural variations and correlate the mouthparts to the substrate texture on which members of these species feed. Setae of 2nd maxilliped displayed a variation in size and shape within . In recent years, many researchers have been studied the structural diversity of 2nd maxilliped to understand the filter feeding mechanisms amongst brachyuran crabs including (Lim and Kalpana 2011; Wortham and LaVelle, 2016). Morphology of setae is a distinctive character that contains a large amount of information which enables taxonomic and phylogenetic comparisons within brachyurans. Therefore, the objectives of this study are to describe the different types of setae in sentinel crabs and to correlate the functional morphology of the mouthparts to the distribution patterns and zonation of these crabs.

MATERIALS AND METHOD

Pakistan is a subtropical country having a coastal belt of about 1,050 Km long. Crabs sampling was conducted from Sandspit (240450N, 67034 0E), Korangi Creek (24◦79 0N, 67◦20 0E), Sunhera Beach (240530N, 660420E), Sonmiani Bay lagoon (250260N, 660350E) between May to September (2013-2015) randomly by hand during low tide periods. Specimens were ice killed and shipped to the laboratory and stored in freezer up to further analysis. The entire structure of the second maxilliped was carefully removed using a pair of fine forceps. The samples were cleaned and dehydrated in a 70% ethanol. The material was fixed on a metal stub with the inner face upward, and sputter coated with gold as the inner side of the site, manipulate sediment and showed greater diversity (Yamaguchi et al., 2005) and scan under scanning electron microscope (SEM) using JOEL model JSM-6380 LV.

Aziz and Saher: Diversity of setae and chela of genus Macrophthalmus 85

RESULTS AND DISCUSSION

To investigate the structure of 2nd maxilliped and correlate it with substrate type and zonation, crabs of family Macrophthalmidae were randomly collected during low tide periods along the intertidal habitats, specimens from the different locations were collected in plastic bags by hand, identified using available identification keys (Tirmizi and Ghani, 1996; Naderloo, 2011; Davie, 2012). Total 5 species of sentinel crabs collected, including single species from sub genus Mareotis (Macrophthalmus) depressus, two from sub genus Macrophthalmus (M) sulcatus and (M) laevis and Venites (M) dentipes and (M) latreillei. Macrophthalmus (M) depressus is a medium-size species common in low to mid intertidal muddy/ sandy habitat. Macrophthalmus (V) dentipes is the largest sentinel crab species, recovered from water logged soft muddy/sandy substrate from mid to low intertidal zones. Macrophthalmus (V) latrellei is a sub-tidal species, having fine grain size. Macrophthalmus (M) dilatatus sulcatus a medium-sized species inhabits the mid intertidal zone, found buried in sandy substrate mixed with mud. Macrophthalmus (M) laevis, a medium-sized species that collected mainly from the mid littoral intertidal flats within sandy/muddy substrate. A. Descriptive characters of 2nd maxilliped: 1. Macrophthalmus (Mareotis) depressus 2nd maxilliped not expended (Fig. 1) elongated, endopodite ischium small, provided with serrate setae; medriporite elongated, bears plumose setae, setae shaft smooth having long setules entirely; carpus with few simple and spoon tipped seate; remaining portion of palp with spoon tipped setae especially along the margins and few simple setae scattered on inner median face, distal end of the spoon-tipped setae moderate to slightly curve, tip rounded, two lateral rows of pointed denticles directed forward along distal portion;

Fig. 1. Macrophthalmus (Mareotis) depressus: a. 2nd maxilliped (inner view); b & c. palp setae; d & e. merus setae. 86 Pakistan Journal of Marine Sciences, Vol. 25(1&2), 2016.

exopodite concealed, flagellum of exopod provided with simple and plumose setae. 1. Macrophthalmus (Macrophthalmus) sulcatus 2nd maxilliped of Macrophthalmus (Macrophthalmus) sulcatus is somewhat broader than other four macrophthalmid crab species (fig. 2); ischium bears long simple setae having a smooth shaft with pointed tip; meropodite inner margin bears long plumose setae; palp densely covered with spoon tipped setae especially propodal and dactylar portion. Spoon tipped setae’s shaft long and smooth, distal region with deep cup shaped grooved having broad tipped denticles arranged laterally, a shaft of spoon tipped setae on carpus short. The flagellum of exopod with plumose setae.

Fig. 2. Macrophthalmus (Macrophthalmus) sulcatus: a. 2nd maxilliped (inner view) b & c, Palp setae d & e, merus setae.

2. Macrophthalmus (Macrophthalmus) laevis The 2nd maxilliped longer than broad (Fig. 3), ischium with having simple and few plumose setae on the medial margin; merus is elongated, inner lateral margin fringed with long plumose setae; palp (carpus, propodus and dactylus) covered with simple and spoon tipped setae, proximal end of setae shaft smooth while the distal end grooved lined with lateral rows of denticles; carpus bends inward bears another type of setae having serration on one site. 2nd Maxilliped exopod long, flagellum bears plumose setae. 3. Macrophthalmus (Venites) dentipes Second maxilliped is elongated (Fig. 4), not broad; ischium with a thick mat of simple and plumose setae; inner merus margin bears long plumose setae along the entire length; the palp of 2nd maxilliped is covered with long spoon tipped setae, the proximal half of setae shaft smooth while distal half curved, broad and spoon shaped with two lateral rows of pointed tipped denticles; palp carpus also bears long spinose setae which bear setules in two opposite rows. Exopod flagellum covered with plumose setae. 4. Macrophthalmus (Venites) latreillei

Aziz and Saher: Diversity of setae and chela of genus Macrophthalmus 87

Fig. 3. Macrophthalmus (Macrophthalmus) laevis: a. 2nd maxilliped (inner view); b & c. palp setae; d & e, merus setae.

Fig. 4. M. (Venites) dentipes: a. 2nd maxilliped inner view; b & c, palp setae; d & e, merus setae.

These crabs have elongated 2nd maxilliped which is broader than long (Fig. 5); ischium with simple setae, tips of setae serrate; meropodite margin is fringed with two type of setae’s i.e. a simple setae and a (simple pappose setae) setae with smooth shaft with distal portion bears a lateral row of setules; carpus of second maxiliped also bears simple setae; the propodus and dactylus (2nd maxilliped palp) is fringed with spoon tipped setae, 2/3 portion of setae devoid of setules, distal half denticulate, curved, having shallow groove, tip pointed; exopod flagellum with plumose setae. 88 Pakistan Journal of Marine Sciences, Vol. 25(1&2), 2016.

Fig. 5. Macrophthalmus (Venites) latreillei: 2nd maxilliped inner view b & c, Palp setae d & e, merus setae.

Fig. 6. Chela structure in different species of Macrophthalmid crab: a, Macrophthalmus (Mareotis) depressus: b, Macrophthalmus (Macrophthalmus) sulcatus; c, Macrophthalmus (Macrophthalmus) laevis; d, M. (Venites) dentipes; e, Macrophthalmus (Venites) latreillei.

Aziz and Saher: Diversity of setae and chela of genus Macrophthalmus 89

B. CHELA MORPHOLOGY The Morphological characters of chelae of 5 Sentinel crab species (male only) were examined (Fig. 6). 1. M. (Mareotis) depressus The chela of male crabs equal, longer than broad, covered with dense setae on inner surface; movable finger relatively narrow, slightly curved with flat spatulate tip, cutting margin with subproximal tooth followed by small teeth. Propodus with relatively large teeth on cutting edge, medially elevated, tip flat, finger margin with setae. 2. M. (Macrophthalmus) sulcatus Chela elongated; movable finger long, relatively narrow, curved inward, cutting edge with small teeth, becoming larger large in proximal half. Immovable finger short, with median large tooth granular sinuous. 3. M. (Macrophthalmus) laevis Chela equal, elongated, relatively long, inner surface without setae; movable finger long, curved, cutting edge with differentiated sub proximal tooth and small denticles distally. Immovable finger short, with median tooth, cutting edge of immovable finger with small denticles. 4. M. (Venites) dentipes Chela equal, elongated; palm longer than broad, upper portion covered with patches of dense setae; dactylus spatulate, flat with long, dense sites along the inner surface, upper and outer surface, cutting edge with subproximal tooth and small teeth distally; immovable finger spatulate cutting edge bears a tooth, inner margin having long setae. 5. M. (Venites) latreillei The chela of male crab symmetrical, inner surface with a row of long hairs, cutting the margin of dactylus with very large, rectangular, crenulated tooth near proximal end, with flat teeth distally, index cutting the margin with a series of linearly arranged, low, crenulated tooth extending from the tip almost to base. DISCUSSION: Distribution patterns, abundance and habitat preference of intertidal marine organisms, mainly govern by many biological and ecological factors. Among them the availability of food resources and adaptations for the procurement of food from the environment is a basic requirement for survival, which restrict the habitat-type in which any organism keeps itself alive. Substrate texture is one of the main factors that restrict the distribution of Ocypodid crabs e.g. Most of the species of genus Macrophthalmus living along the coastal areas of Pakistan are inhabitant of muddy, sandy habitat from mid to low tide level, which are rich in organic content. In these species modification in mouth parts, form and armature of chelae had occurred in relation to the type of substratum utilized during feeding (Alkhayat, 1996). The results of the study showed that in these species only few modified spoon tipped setae are present that were restricted in the palp region of 2nd maxilliped while the remaining portion is provided with plumose and simple setae. Results showed that species live in the silt, where there is greater availability of food they neither required an efficient food sorting mechanism or an efficient morphological adaptation, as a result they have only few modified spoon-tipped setae. 90 Pakistan Journal of Marine Sciences, Vol. 25(1&2), 2016.

Hartnoll (1973) reported that crabs of genus Scopimera and Dotilla inhabit on sandy substrate, they have specialized 2nd maxillipeds having broad merus, containing the spoon shaped setae that enable them to sort food from sand with a high efficiency. Ono (1965) correlated the specialized setaes of the mouth parts with feeding mode and habitat in different Ocypodoid crab species (S. globosa, U. lactea, M. japonicus and P. cristatum) and concluded that species living in sandy texture have more spoon shaped setae which are less developed and replaced by "woolly" type in muddy dwelling species. Altevogt (1957) previously studied feeding appendages of fiddler crabs and showed that second maxilliped modification is correlated with habitat. Species inhabited in muddy substrate have more ‘plumose’ setae on the second maxilliped having feather-like appearance which is used to, which provide a large surface area to brush fine sediment particles and trap food material from the sediment whereas species inhabit predominantly in sandy habitats have spoon shaped setae, which enhance sorting ability and assist in scouring food material bound to sand particles. Miller (1961) also studied the mouthparts adaptation in fiddler crabs and concluded that U. minax, a mud living species have few modified spoon tipped setae as compared to U. pugilator which lives in sandy habitat. As deposit feeders, sentinel crabs pick surface mud from the substrate with the help of chelae. Modification in chelae form also occurred in relation to the type of substratum as it is utilized during feeding (Jones, 1984). These crabs have flattened spoon-shaped chela which efficiently sweeps (scoop) the mud particles. Icely and Jones (1978) suggested that the chela structure is also correlated to the substrate in which an live e.g. The mud dwelling species (S. leachii, N. dotilliformis and M. depressus) have flattened spatulate tips chela and dense hairs, which are helpful for scooping finer sediment while Scopimera sp. living in sandy substrate (coarse) specialized fine forceps like chelae to pick organic materials from coarse sand. The variability in shape of the spoon-shaped setae is the most important morphological differences in the second maxillipeds among five studied species of Macrophthalmus that can be utilized for species identification.

ACKNOWLEDGEMENT

This work is funded by the PSF project no. KU-456 (Bio) and gratefully acknowledged.

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