Copeia, 2004(4), pp. 903±907 Ontogeny of Scale Feeding in the Asian Glass®sh, Chanda nama (Ambassidae) ARCHIS R. GRUBH AND KIRK O. WINEMILLER Dietary analysis of a population of the glass®sh Chanda nama from a wetland in southwestern India revealed facultative scale feeding (lepidophagy). In addition to ®sh scales, microcrustacea and aquatic insects were consumed by all size classes, with juvenile diets containing larger fractions of invertebrates. More scales were consumed during the wet season, the period when abundance of juvenile ®shes was greatest. Aquarium observations revealed how the glass®sh uses stealth and ambush tactics. Scales are dislodged by raking the extended lower jaw, distally armed with four curved conical teeth, across the ¯anks of prey. PECIALIZED scale feeding, or lepidophagy, (Major, 1973; Sazima and Uieda, 1980; Sazima, S is de®ned as the ingestion of scales without 1983). bones or ¯esh (Sazima, 1983). Lepidophagy has Roberts (1989) brie¯y described the denti- been described for seven Neotropical characid tion of two Asian glass®shes (Ambassidae, Per- genera (Roberts, 1970; Sazima, 1983; Vari, ciformes), Chanda nama from India and Para- 1986), three Neotropical trichomycterid genera doxodacna piratica from the Malay Peninsula and (Baskin et al., 1980), at least six African cichlid Western Borneo, presumed to feed on scales. genera (Fryer et al., 1955; Eccles and Lewis, Although the species were described as scale 1976; Liem and Stewart, 1976), an Australian feeders, no assessment of the degree of lepido- ariid cat®sh (Roberts, 1978), an Australian ter- phagy was provided for either species. Here we aponid (Whit®eld and Blaber, 1978), two caran- report for the ®rst time ontogenetic diet shifts gid genera (Major, 1973; Sazima and Uieda, involving scale feeding in C. nama from a fresh- 1980), and two ambassid genera (Roberts, water wetland in the western Ghats region of 1989). Lepidophagous Neotropical characids India. have morphological and behavioral specializa- tions for removing scales (Sazima, 1977, 1983; MATERIALS AND METHODS Sazima and Machado, 1990). Catoprion mento has everted teeth and a long lower jaw that opens From August 2000 to July 2001, ®shes were to a . 908 angle to rake scales from the ¯anks collected from a natural wetland (approximate- of ®shes (Sazima, 1983; Nico and Taphorn, ly 40 ha) at Manavalakurichy (88109120N, 1988). Exodon paradoxus, Roeboexodon guyanensis, 778189240E) in southern region of the western Serrabrycon magoi, and Roeboides species possess Ghats, Tamil Nadu, India. Annual rainfall in the external teeth on the snout that are used to dis- region is 1.2±1.5 m, with a period of relatively lodge scales (Roberts, 1970; Sazima, 1983; Vari, low rainfall from December through March 1986). Morphological and behavioral mecha- (mean monthly rainfall 35 mm) and high rain- nisms for scale feeding are less apparent in most fall from April through November (mean rain- noncharacid ®shes (but see Hori, 1993). fall 134 mm). During most years, low rainfall in In lepidophagous characids (Catoprion mento September (53 mm rainfall during 2000) brie¯y and Roeboides spp.), the juvenile diet shifts from interrupts the long rainy season. Manipulation microcrustacea to aquatic insects before switch- of inlet and outlet sluice gates maintains rela- ing to scales in the adult diet (Sazima and Ma- tively constant water level. Water level increases chado, 1982; Nico and Taphorn, 1988; Peterson during unusually wet periods. The wetland con- and Winemiller, 1997). Frequency of scales in tains 28 ®sh species (ARG, unpubl. data) and stomachs of Reoboides dayi and Reoboides af®nis in lacks exotic carps (e.g., Catla catla, Labeo rohita, the Venezuelan llanos was greater during the Chirrhina mrigala) commonly introduced into dry season when aquatic insect abundance is surface waters of the country. Fishes were cap- lower and ®sh densities are higher (Peterson tured with seines (6 3 2 m, 3 mm mesh), cast and Winemiller, 1997; Peterson and MacIntyre, nets (3 m radius, 10 mm mesh) and experimen- 1998). Among most marine lepidophagous ®sh- tal gill nets (150 3 3 m, with 2, 3.5, 5, 8, and es, scale eating is performed by juveniles and 15 cm mesh). Sampling was conducted from ceases during later ontogeny in association with 0600±1400 h. All aquatic habitats were surveyed modi®cation of jaw structure and dentition (i.e., vegetation, open water, edge, and mid- q 2004 by the American Society of Ichthyologists and Herpetologists 904 COPEIA, 2004, NO. 4 pool). Specimens were preserved in 15% for- malin, then rinsed in freshwater prior to ex- amination in the lab. Of 3361 C. nama captured, 3034 were mea- sured for standard length (SL) to the nearest 0.1 mm, and 382 individuals representing all size classes (grouped by 5 mm intervals), months, and habitats were dissected for stom- ach contents. Food items in the anterior half of gut were examined under a dissecting micro- scope and identi®ed to order (invertebrates), species (®shes), or general category (e.g., algae, detritus, scales) yielding a total of 22 prey cat- egories. Food items were blotted dry on paper Fig. 1. Volumetric percentages of three major re- source categories in diets of size classes of Chanda towel, and their volumes measured by water dis- nama (values in parentheses are sample sizes). placement in graduated cylinders. Resolution for the smallest food items was approximately 1 ml. To estimate the approximate size of matu- ration, gonads of dissected specimens were ex- was20.30 (Spearman's rho, df 5 355, P , amined under a dissecting microscope, and ova- 0.0001), between SL and the volume of aquatic ries were staged as immature (lacking visible oo- insects in the diet was20.26 (P , 0.0001), and cytes or oocytes within 0.10 mm of the maxi- the correlation between SL and the volume of mum diameter observed for the population) or scales in the diet was 0.86 (P , 0.0001). Con- mature (containing at least some oocytes of the sumption of scales was facultative, with no size maximum diameter). class averaging more than 50% scales in the diet by volume. Nonetheless, scales were the most RESULTS important diet category for adult size classes 5 Chanda nama were captured during each (average volumetric percentage 38%, range 5 monthly survey (average sample 5 110, range among size-class averages 35±50%). The 5 15±892). Approximately half of all males and mean number of scales in individual stomachs females had mature gonads by 25 mm SL; thus was 8.15 for juveniles (n 5 114) and 13.94 for we consider this the size at maturation. In every adults (n 5 242). The maximum number of monthly sample, females . 26.4 mm SL con- scales in individual stomachs was 32 for juve- tained ripe ovaries. Ripe ovaries contained mul- niles and 65 for adults. Only eight juveniles and tiple oocyte cohorts, and juveniles were present 17 adults in our sample had empty stomachs. in every sample, which suggests that some seg- Diets of both juveniles and adults varied sea- ment of the population probably spawns during sonally (Fig. 2). Juveniles (SL , 25 mm) fed any given month. March through July (early wet predominantly on aquatic insects from late wet season) samples were dominated by juveniles to early dry seasons (October to January). Mi- (10±25 mm SL) and probably re¯ect peak crocrustacea were the most important dietary spawning during this period. component for juveniles during other periods, Analysis of volumetric percentages of major the only exception being July (middle wet sea- diet categories for different size classes of C. son) when scales were the dominant item in ju- nama revealed a strong pattern of ontogenetic venile diets. Scales were signi®cant dietary ele- diet shift (Fig. 1). Juveniles fed heavily on aquat- ments for adults throughout the year, compris- ic insects (mostly Ephemeroptera but including ing from 15±90% of the monthly mean volume Odonata and Coleoptera) and microcrustacea of adult stomach contents (Fig. 2). Greatest con- (calanoid copepods, Cladocera, Isopoda, and Amphipoda). Terrestrial insects appeared in the sumption of scales by adults was during the pe- diet at 25 mm SL and remained a minor dietary riod April to August (end of dry season±early component (5±17%) for all subsequent size wet season). Microcrustacea (zooplankton) classes. Scale feeding was observed among in- comprised the greatest percentage of the adult dividuals as small as 15 mm, and the volume of diet during the period November to February scales in stomachs increased gradually with size (end of wet season±early dry season). Aquatic until reaching an asymptote at approximately and terrestrial insects were important diet ele- 35 mm (Fig. 1). The correlation between SL ments for adults during the late dry season and the volume of microcrustacea in the diet (March to April). GRUBH AND WINEMILLERÐSCALE FEEDING IN CHANDA 905 ultative, and adults also consume insects and microcrustacea. In a manner similar to Neo- tropical characids in the genus Roeboides, C. nama undergoes an ontogenetic diet shift from feeding primarily on invertebrates as juveniles to scale feeding as adults. Roeboides dayi in streams of the Venezuelan llanos fed extensively on seasonally abundant microcrustacea and aquatic insects during the wet season (Peterson and Winemiller, 1997). In contrast, Roeboides prognathus from the Cuiaba River in Brazil ap- pears to have a diet largely restricted to scales (Sazima, 1983). Seasonal resource ¯uctuations could have been associated with temporal vari- ation in the degree of lepidophagy observed in C. nama in the Manavalakurichy wetlands. Daily operation of sluice gates maintains a fairly con- stant water level in this ecosystem. Nonetheless, environmental parameters, such as tempera- ture, pH, DO, turbidity and conductivity, vary in response to seasonal rainfall.