Polymorphus is a genus of obligate endoparasitic worms that belongs to the phylum Acanthocephala. There are currently 36 extant species within the genus Polymorphus characterized by their endoparasitic lifestyle, and red colouration 3.

Species Polymorphus actuganensis Petrochenko, 1949 Polymorphus acutis Van Cleave and Starrett, 1940 Polymorphus arctocephali Smales, 1986 Polymorphus ariusis (Bilqees, 1971) Polymorphus biziurae Johnston and Edmonds, 1948 Polymorphus boschadis (Schrank, 1788) Polymorphus brevis (Van Cleave, 1916) Polymorphus chongqingensis Liu, Zhang and Zhang, 1990 Polymorphus cincli Belopolskaya, 1959 Polymorphus contortus (Bremser in Westrumb, 1821) Polymorphus corynoides Skrjabin, 1913 Polymorphus corynosoma Travassos, 1915 Polymorphus crassus Van Cleave, 1924 Polymorphus cucullatus Van Cleave and Starrett, 1940 Polymorphus diploinflatus Lundström, 1942 Polymorphus fatimaae Khan, Dharejo, Birmani and Bilqees, 2008 Polymorphus fulicai Birmani, Dharejo and Khan, 2011 Polymorphus gavii Hokhlova, 1965 Polymorphus inermis Travassos, 1923 Polymorphus karachiensis (Bilqees, 1971) Polymorphus kostylewi Petrochenko, 1949 Polymorphus magnus Skrjabin, 1913 Polymorphus marchii (Porta, 1910) Polymorphus marilis Van Cleave, 1939 Polymorphus mathevossianae Petrochenko, 1949 Polymorphus meyeri Lundström, 1942 Polymorphus miniatus (von Linstow, 1896) Polymorphus minutus (Goeze, 1782) Polymorphus mohiuddini Muti-ur-Rahman, Khan, Bilqees and Khatoon, 2008 Polymorphus mutabilis (Rudolphi, 1819) Polymorphus nickoli Khan and Bilqees, 1998 Polymorphus obtusus Van Cleave, 1918 Polymorphus paradoxus Connell and Corner, 1957 Polymorphus paucihamatus Heinze, 1936 Polymorphus phippsi Kostylew, 1922 Polymorphus piriformis (Bremser in Rudolphi, 1819) Polymorphus pupa (von Linstow, 1905) Polymorphus sichuanensis Wang and Zhang, 1987 Polymorphus sindensis Khan, Ghazi and Bilqees, 2002 Polymorphus spindlatus Amin and Heckmann, 1991 Polymorphus striatus (Goeze, 1782) Polymorphus strumosoides Lundström, 1942 Polymorphus swartzi Schmidt, 1965 Polymorphus trochus Van Cleave, 1945

Morphology A distinguishing feature of Polymorphus species is their vivid red to orange pigmentation, which becomes particularly pronounced within transparent intermediate hosts, such as amphipod crustaceans 5. This conspicuous coloration enhances the visibility of infected hosts to predatory definitive hosts, thereby increasing the efficiency of trophic transmission 5. The pigmentation arises from the parasite's unique ability to sequester carotenoids from its amphipod hosts, resulting in the characteristic reddish coloration of the organism 5.

General morphological features

Proboscis: The proboscis is a defining structure, armed with 20 to 22 longitudinal rows of 11 to 13 hooks per row, enabling firm attachment to the intestinal wall of the definitive host 4. Trunk: The body is cylindrical, with anterior expansions. The fore-trunk bears spines, and the neck region is conical in shape 4. Internal anatomy includes a prominent ganglion, clavate (club-shaped) lemnisci, and simple roots anchoring the proboscis apparatus 4.

Sexual dimorphism and reproductive morphology

Males

Possess well-developed testes 4. The posterior extremity is dramatically rounded, terminating in a terminal genital pore 4. A distinctive copulatory bursa is present, characterized by numerous sensory pits. The length of the bursa is variable and depends on the degree of eversion, playing a crucial role in reproductive behaviour 4.

Females

The reproductive system features a vaginal complex, ovarian balls, and developing eggs 4. Eggs are elongated with a polar prolongation of the middle membrane, a taxonomically significant trait within the genus 4.

Endoparasitic lifestyle Worms within the genus Polymorphus are well known for their obligate endoparasitic lifestyle, particularly their ability to manipulate intermediate hosts 1. Polymorphus species utilize invertebrates, including amphipods and decapods, as intermediate hosts and vertebrates such as waterfowl and fish-eating birds as their final hosts. An example of this interaction is the relationship between Polymorphus paradoxus and its intermediate host, Gammarus lacustris. These parasites manipulate G. lacustris by modifying its behavior, causing the amphipod to be drawn to light and swim toward the surface 3. Such manipulation ultimately enhances the likelihood that P. paradoxus will be consumed by the parasite's final host. Additionally, Polymorphus species exhibit the ability to steal carotenoids from their amphipod hosts, leading to the characteristic red colouration of the organism 3. By leaching carotenoids from G. lacustris, this causes the amphipod to appear blue opposed to brown, which once again increases the probability of being detected by waterfowl or fish-eating birds.

Previous names Polymorphus used to be called by many names such as: Profilicollis Meyer, 1931, Falsifilicollis Webster, 1948, Parafilicollis Petrochenko, 1956, Subfilicollis Hoklova, 1967, Subcorynosoma Hoklova, 1967, Arhythmorhynchus Lühe, 1911, Hexaglandula Petrochenko, 1950 or as subgenera (Profilicollis, Hexaglandula).

References

García-Varela M, Pérez-Ponce de León G, Aznar FJ, Nadler SA. 2013. Phylogenetic relationship among genera of Polymorphidae (Acanthocephala), inferred from nuclear and mitochondrial gene sequences. Mol Phylogenet Evol. 68(2):176–184. doi:10.1016/j.ympev.2013.03.029. ‌Garey JR, Near TJ, Nonnemacher MR, Nadler SA. 1996. Molecular evidence for Acanthocephala as a subtaxon of Rotifera. J Mol Evol. 43(3):287–292. doi:10.1007/BF02338837. ‌Bethel WM, Holmes JC. 1973. Altered Evasive Behavior and Responses to Light in Amphipods Harboring Acanthocephalan Cystacanths. The J Parasitol. 59(6):945. doi:10.2307/3278623. Presswell B, Bennett J. Description and molecular data for a new acanthocephalan parasite, Polymorphus circi n. sp. (Polymorphidae) from the Australasian harrier (Circus approximans Peale) in New Zealand. Syst Parasitol. 2023;100(6):725-733. doi:10.1007/s11230-023-10120-5 William M. Bethel and John C. Holmes. 1977. Increased vulnerability of amphipods to predation owing to altered behavior induced by larval acanthocephalans. Canadian Journal of Zoology. 55(1): 110-115. https://doi.org/10.1139/z77-013 McAllister CT. 2012. Acanthocephalans. Encyclopedia of Arkansas. https://encyclopediaofarkansas.net/entries/acanthocephalans-8362/