Larval Hysterothylacium (=Thynnascaris) (Nematoda: Anisakidae) from Fishes and Invertebrates in the Gulf of Mexico

Abstract

Numerous fishes and invertebrates in the northern Gulf of Mexico contain larvae of four forms of Hysterothylacium: ?//. reliquens (=type MA, Mississippi A), type MB, H.fortalezae (=type MC), and type MD. The third-stage larvae (possible second-stage larva of type MB) of all are described and illustrated; furthermore, corresponding fourth-stage larvae of all but type MB are described. A key to all these inshore and nearshore larvae is included. Biological differences among related species are exemplified using H. reliquens and Iheringascaris inclines. The second-stage larva of /. inclines survives much longer in cold water than that of H. reliquens, and the fourth-stage larva of /. inclines is unique because it invades alimentary tissue of the final host. Adult nematodes of the genus Hysterothylacium Ward and Magath, 1917 are restricted to the digestive tract, with one exception (Machida et al., 1978), of fishes, whereas their larvae parasitize various tissues of numerous fishes and invertebrates (e.g., Norris and Overstreet, 1976). Deardorff and Overstreet (1981) reviewed the genus, previously known as Thynnascaris Dollfus, 1933 or Contra- caecum sensu lato, and examined adults of six species from the Gulf of Mexico. In this paper, we describe the larvae of four forms of Hysterothylacium fol- lowing the nomenclature initiated by Norris and Overstreet (1976) and provide a listing of hosts. The number of species in the inshore northern Gulf of Mexico may be greater because more than one species could have morphologically similar larvae. However, some known adults from the Gulf of Mexico are rare and not found in inshore northern regions where most larval specimens were collected. On the other hand, two forms may be different phases of the same species, and we may have included larvae of the related Iheringascaris inquies (Linton, 1901) Deardorff and Overstreet, 1981 which is not Hysterothylacium sensu stricto, even though placed in the genus Thynnascaris by most recent workers (see Deardorff and Overstreet, 1981). Our studies of the ascaridoid fauna in the Gulf of Mexico have differentiated at least one species that is a potential public health hazard. Norris and Overstreet (1976) established that Hysterothylacium type MB rapidly penetrated the alimen- tary tract of white mice. That study has been expanded (Ebert and Norris, per- sonal communication), and the worm also causes lesions and diffuse mucosal hemorrhaging in the rhesus monkey (Overstreet and Meyer, 1981). Fetter (1969a, b) hypothesized that H. aduncum (Rudolphi, 1802) Deardorff and Overstreet, 1981 (as Thynnascaris adunca) from Sardina pilchardus (Walbaum) was the caus- ative agent for several cases of anisakiasis in man; however, Vermeil et al. (1975) determined that the suspected nematode did not penetrate the rabbit stomach and questioned the hypothesis. Under stated experimental conditions, Hysterothy- lacium type MA did not penetrate tissues in mice (Norris and Overstreet, 1976).