Morphology and Ultrastructure of Possible Integumentary Sense Organs in the Estuarine Crocodile (Crocodylus porosus)

The skins of crocodylids and gavialids can be distinguished from those of alligatorids by the presence of darkly pigmented pits, known as integumentary sense organs (ISOs), on the postcranial scales. The structure of ISOs, in Crocodylus porosus, was studied using light microscopy and scanning and transmission electron microscopy. The stratum corneum of the epidermis in the area of the IS0 is thinner, while the stratum germinativum is thicker, relative to other regions of the integument. Beneath the epidermal layer the IS0 region has a paucity of collagen fibers relative to the rest of the dermis. Widely dispersed fibrocytes, nerve terminals, and chromatophores occur throughout the IS0 region of the dermis, but these elements are concentrated in the area immediately beneath the stratum germinativum in the IS0 region. The morphology of the ISOs suggests that they are sensory organs. It has traditionally been assumed that sensory organs on the amniote integument have a mechanosensory function. However, alternate functional interpretations of this structure are possible, and a resolution awaits further work. o 1996 Wiley-Liss, Inc. Integumentary pits are present, one per scale, on the postcranial scales of some crocodilians, and these have been used extensively as taxonomic characters in the identification of crocodilian skins (King and Brazaitis, ’71; Wermuth and Fuchs, ’78; Brazaitis, ’87). The pits are present on the postcranial scales of crocodylids (Fig. la) (including Tomistoma) and gavialids, but are absent from the postcranial scales of alligatorids (Fig. lb). The presence o r absence of pits can therefore be used to distinguish alligatorids from the other two crocodilian families. The pits have been referred to in the taxonomic literature as “follicle pores” (King and Brazaitis, ’71), “Poren” (Wermuth and Fuchs, ’781, “follicle glands” and “follicle pits” (Brazaitis, ’87), and “integumentary sense organs” or “ISOs” (Brazaitis, ’87). In this study, the term I S 0 will be used throughout, for reasons to be explained below. Although the ISOs have been well studied as taxonomic characters, their structure and function are not known. Brazaitis (’87) says that they are thought to be mechanosensory, while Grigg and Gans (’93) speculate that they may be either sensory structures or secretory pores. A search of the literature reveals no detailed study of their structure or function. In contrast, detailed morphological and ultrastructural studies have been performed of mechanoreceptors or “touch papillae” on the cranial scales of the alligatorid, Caiman crocodilus (von During, ’73, ’74; von During and Miller, ’79). These touch papillae are confined to the cranial scales and are found in all crocodilians (Fig. 2). The touch papillae on the cranial scales of Caiman crocodilus are not the postcranial ISOs referred to in the taxonomic literature, as alligatorids lack ISOs on the postcranial scales. However, von During’s work may nonetheless be the source of the idea that the ISOs have a mechanosensory function. Guibe (’70) reported that the abundance of ISOs decreases as the animal ages. The source of this information, however, is a study which compared juvenile Crocodylus with adult Alligator (Hulanicka, ’13), and it seems likely that this difference may have more to do with phylogeny than with ontogeny. The objective of the present study was to investigate the general morphology and the ultrastructure of the ISOs of Address reprint requests to Kate Jackson at her current address: Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138. o 1996 WILEY-LISS, INC. Fig. 1. A: Ventral scales of a crocodylid, Crocodylus porosus, showing the ISOs (arrows). B: Ventral scales of an alligatorid, Caiman crocodilus, which lacks ISOs. Fig. 2. Touch papillae (sensu von During, '73) from the cranial scales of Crocodylusporosus. Head of a crocodile with touch papillae indicated (arrow). INTEGUMENTARY SENSE ORGANS OF CROCODILES 317 Crocodylus porosus as one of the means we will use to determine their function. MATERIALS AND METHODS