Monogeneans rely on firm attachment to often flexible and uneven surfaces and are renowned for their effective posterior attachment structures in the form of adhesives, clamps, hamuli and suckers. Polystomatids do not secrete adhesives and do not have clamps. While only some have hamuli, all have suckers in the adult form. Three different types of haptoral suckers have been described based on basic morphology but have never been studied in depth. Using enzyme digestion and light (differential interference contrast), confocal and scanning electron microscopy, we examined representatives and propose four sucker types. Haptoral sucker Type I are symmetrical soft, flexible, cup- to disk-shaped suckers and are found in all polystomes infecting frogs and salamanders. Type II suckers are symmetrical soft, flexible, cup-shaped suckers with a hollow continuous skeletal ring and no other skeletal elements. They are found in species of Nanopolystoma Du Preez, Wilkinson et Huyse, 2008 infecting caecilians. Type III suckers are symmetrical firm, cup-shaped suckers with elaborate skeletal elements that contribute to a secure grip on the host tissue. This type of sucker is found in all polystomes infecting freshwater turtles and the common hippopotamus. Type IV suckers are asymmetrical with an elaborate series of long, thin sclerites with terminal spines or hooks. This type of sucker is only known from Concinnocotyla australensis (Reichenbach-Klinke, 1966) infecting the Australian lungfish. These different sucker types are crucial for the survival of polystomatid flatworms within their respective microhabitats.
Neopolystoma liewi sp. n. is described from the conjunctival cavity of the Malayan box turtle Cuora amboinensis (Daudin, 1802), in Peninsular Malaysia, This is the first record of Neopolystoma in Malaysia and the fourth polystomatid species described from C. amboinensis. Of the 27 Malayan box turtles examined, 8 were found to be infected. A maximum of 2 parasites per eye and 4 individuals per host was recorded. N. liewi sp. n. differs from all other members of the genus by possessing few and short genital spines and small marginal hooks. The oncomiracidium has 64 ciliated cells arranged symmetrically about the sagittal axis.
While the majority of polyopisthocotylean monogeneans feed on blood, some polystomatids infecting chelonians do not. This study examined the gastrodermis of two polystomatids, one - Neopolystoma spratti Pichelin, 1995 - from the conjunctival sac of a chelonian and the other - Concinnocotyla australensis (Reichenbach-Klinke, 1966) - from the oral cavity, gill arches and primary gill lamellae of a fish, and also found no evidence of blood feeding. However, the gastrodermal architecture in both species basically resembles that found in blood feeding polyopisthocotyleans, with alternation of lamellated digestive cells and an intervening syncytium. In C. australensis, oesophageal secretion appeared to be responsible for initial extracellular digestion and this was followed by pinocytotic uptake of partly degraded material in pits between the numerous apical lamellae of digestive cells. Posterior dorsolateral gut pockets unique to C. australensis were shown to be blind sacs separated from the external environment by a narrow cytoplasmic bridge, composed of a thin layer of tegument apposed to a thin layer of pocket syncytial epithelium. The pockets are lined with greatly folded syncytium and set in a “capsule” of tissue in which numerous pro-tonephridial flame cells are embedded. It is suggested that the pockets have an osmoregulatory function related to the particular environment and evolutionary history of the host, the primitive lung fish (Dipnoi) Neoceratodus forsteri (Krefft, 1870).