Hemioniscus Buchholz 1866

Hemioniscus Buchholz, 1866

(Fig 4K)

= Leponiscus Giard, 1887 n. syn.

Diagnosis: Cryptoniscus larva/male body tear-drop shaped with prominent cuticular striations. Head wider than long, oral cone directed anteriorly; eyes present. Antennule article 1 with 7 bluntly rounded teeth. Coxal plates with or without teeth. Pereopods 1 and 2 gnathopodal, 3–7 ambulatory; propodi of 3–5 quadrate distally, 6 and 7 tapering distally. Pleotelson with smooth evenly rounded distal margin. Mature females with anterior segments unmodified from cryptoniscus larval form; posterior segments fused into enlarged marsupium.

Included species and subspecies: Hemioniscus anatifae (Giard, 1887) n. comb.; H. balani balani Buchholz, 1866 (type species); H. balani japonica Ogawa & Matsuzaki, 1985; H. pagurophilus Williams & Boyko, 2006.

Distributions: Hemioniscus anatifae n. comb.: Roscoff, Concarneau, and Wimereaux, France, intertidal (Giard 1887).

Hemioniscus balani balani: Northern hemisphere: East Atlantic: (River Gironde, France to northern Norway, United Kingdom (Vader 1983; Arnott 2001); western Atlantic: Halifax, Nova Scotia, Canada (Crisp 1968); east Pacific: Alaska to Baja California Norte, Mexico (Cornwall 1955; Campos-González & Campoy-Favela 1987; Blower & Roughgarden 1989). Intertidal at all localities.

Hemioniscus balani japonica: Futaba County, Fukushima Prefecture, Japan, intertidal (Ogawa & Matsuzaki 1985).

Hemioniscus pagurophilus: Philippines, subtidal (<5 m) (Williams & Boyko 2006).

Hosts: Cirripedia: Acrothoracica: Tomlinsonia mclaughlinae Williams & Boyko, 2006 (H. pagurophilus, Philippines) (Williams & Boyko 2006). Cirripedia: Thoracica: Austrominius modestus (Darwin, 1854) (H. balani balani, Norway) (Vader 1968); Balanus glandula Darwin, 1854 (H. balani balani, east Pacific) (Blower & Roughgarden 1989); Chirona hameri (Ascanius, 1767) (H. balani balani, Irish Sea) (Crisp 1968); Chthamalus challengeri Hoek, 1883 (H. balani japonica, Japan) (Ogawa & Matsuzaki 1985); Chthamalus dalli Pilsbry, 1916 (H. balani balani, east Pacific) (Cornwall 1955; Blower & Roughgarden 1989); Chthamalus fissus Darwin 1854 (H. balani balani, east Pacific) (Campos-González & Campoy-Favela 1987; Blower & Roughgarden 1989); Lepas anatifera Linnaeus, 1758 (H. anatifae n. comb., France) (Giard 1887); Semibalanus balanoides (Linnaeus, 1767) (H. balani balani, east and west Atlantic, Alaska) (Crisp 1968; Vader 1968, 1983; Coyle & Mueller 1981)

Remarks: We herein synonymize Leponiscus Giard, 1887 with Hemioniscus Buchholz, 1866. Giard (1887) placed two “species” in Leponiscus: L. anatifae Giard, 1887 and L. pollicipedis Giard, 1887, both listed as “nov. gen. et nov. sp.” The latter name is a nomen nudum as Giard (1887) provided data on the host (Pollicipes cornucopia Leach, 1824 = Pollicipes pollicipes (Gmelin, 1791)) and locality (Concarneau, France) but did not provide morphological characters. Giard (1887) explicitly referred to figures in Hesse (1867) as representing L. anatifae and this name is therefore available from Giard’s (1887) work (ICZN Article 12.2.5). Giard (1887) did not designate a type-species for Leponiscus, but as only one of the originally included species is an available name, the type species is L. anatifae by monotypy.

As pointed out by Buhl-Mortensen et al. (2020), it is clear from the illustrations of Hesse (1867, pl. 3, fig. 8–26), on which L. anatifae was, in part, based, that this species is close to if not identical with Hemioniscus balani Buchholz, 1866 (Hemioniscidae). However, the figures of L. anatifae are stylized and inaccurate in many characters (e.g., large teeth on the second antennular segment which are certainly not present) and the count of the antennular basal segment teeth is suspect. It is also possible that there are two species represented in Hesse’s (1867) figures. Giard (1887) did not provide a written description of any characters for L. pollicipedis, but he did make a drawing of the ventral anterior end of a cryptoniscus larva he examined which remained unpublished until it was included in Bocquet-Vedrine & Bocquet’s (1972) paper (Fig. 4K herein). This illustration shows that L. pollicipedis is nearly identical with H. balani (see Goudeau 1970) and as Giard considered L. pollicipedis and L. anatifae to be congeneric, it can be assumed that L. anatifae also belongs to Hemioniscus. Giard (1887) likely placed the parasites of pedunculate barnacles in a different genus than those of sessile barnacles following his theory that parasites found on different hosts must belong to different taxa (Kuris 1974). Despite the possibility that H. anatifae n. comb. is synonymous with H. balani, we conservatively retain the two species as distinct until topotypic specimens of the pedunculate barnacle parasite can be collected and examined.

Several websites (e.g., https://www.marlin.ac.uk/species/detail/1376#biology) list Hemioniscus balani balani as occurring as far south in the western Atlantic as Massachusetts but this is based on an erroneous interpretation of statements in Crisp (1968) that only apply to the host barnacle. The parasite has only been reported in the western Atlantic from sites in Halifax, Nova Scotia, possibly as the result of a relatively recent introduction of the parasite from eastern Atlantic waters (Crisp 1968). Populations of the host barnacle in Canada, Semibalanus balanoides, have been shown to have high gene flow in both regional and trans-Atlantic populations (Holm & Bourget 1994; Flight & Rand 2012) and it would be interesting to see if similar homogeneity exists in the parasite across the Atlantic Ocean.

Cornwall (1955) stated that Hemioniscus balani balani (as “ Homioniscus ” [sic] balani) was found infesting up to 90% of Chthamalus dalli in British Columbia and incorrectly stated that these infestations were “usually fatal to the host”; the parasite can cause castration of hosts but it has not been documented to negatively impact the life span of barnacles.

Hemioniscus balani balani has been reported from the southern hemisphere in South Africa parasitizing Chthamalus dentatus Krauss, 1848 and Notomegabalanus algicola Pilsbry, 1916 (Sandison 1954); however, Vader (1983) thought it likely that these were actually records of Crinoniscus sp. (Cryptoniscoidea: Crinoniscidae), as the host barnacles were not castrated as typically occurs with parasitism by H. balani balani. On the other hand, the subspecies H. balani japonica has not been reported in Asian waters subsequent to its description and it remains to be determined if this is a naturally occurring population of H. balani, in which case there should be other populations nearby, or a distinct subspecies or species of Hemioniscus. The restricted geographic range is possibly the result of introduction in a manner similar to that postulated for H. balani balani in Atlantic Canada.