Chun (1900) provided the first description of a very large larvacean from specimens collected during the Valdivia expedition (1898–1899). He named the new species Bathochordaeus charon after the mythical figure who ferries the souls of the dead across the river Styx. During the Valdivia expedition two individuals were collected from the South Atlantic and two smaller specimens, from the Indian Ocean. When he studied them later, Lohmann (1914) placed the two largest Valdivia specimens in the family Oikopleuridae. Garstang (1936) later referred to them as “veritable giants among Appendicularians, the depressed body being as large as a walnut and the broad tail almost 3 ins. in length” and his comment may be responsible for them being referred to today simply as “giant larvaceans”. The two smaller Indian Ocean specimens were not described until much later (Lohmann 1931). Not only were the Indian Ocean specimens smaller at <20 mm total length, they appeared to differ appreciably from the larger specimens collected in the Benguela Current (Lohmann 1914; Lohmann 1931; Garstang 1936). When Garstang (1936, 1937) collected two specimens of Bathochordaeus in very good condition from surface waters near Bermuda, he at once observed that they differed appreciably from Chun’s. Garstang’s specimens were smaller in size than the two collected by Chun in the South Atlantic but similar in size to the larger of the two collected in the Indian Ocean. However, three features struck Garstang (1936, 1937) as different: the Bermuda specimens lacked the prominent “obconical gill-pouches” (spiracles), and the crop-like esophageal expansion of Bathochordaeus charon and they possessed an oikoplastic region that was more comparable to other oikopleurids than B. charon as described by Chun (1900) and Lohmann (1914).
Consequently, Garstang wondered if the original description was hampered by misinterpretation or poor preservation (Garstang 1936, 1937). He could not fathom a purpose for either the seemingly counter-productive funnel-shape of the spiracles or the improbable and capacious esophageal expansion. Having no access to the original specimens, and unable to account for the absence of these conspicuous features, Garstang somewhat reluctantly described a new species: Bathochordaeus stygius, and greatly expanded the slowly growing body of work surrounding these enigmatic animals (Garstang 1937).
Since its description, Bathochordaeus charon has appeared in the literature a few times (Thompson 1948; Bückmann and Kapp 1973; Barham 1979; Galt 1979; Castellanos et al. 2009; Lindsay et al. 2015), without specimen collections to accompany them. The lack of specimens combined with Garstang’s (1937) concerns about the characteristic features in the original description, have cast doubt on the legitimacy of B. charon (Chun) as a species distinct from B. stygius (Garstang). The first probable record of B. charon since Chun (1900) appeared in 1948, from the Pacific Ocean off Australia, although it was a single, small (trunk 3.2 mm, tail 7.5 mm) specimen (Thompson 1948). Time passed and the lack of specimens caused Fenaux (1966) to synonymize the two species. Subsequently, the second potential specimen(s) of B. charon were collected by Galt (1979): five animals acquired during three cruises off southern California, that he called B. charon. Unfortunately, those specimens are no longer available and Galt may have used the name B. charon for all his specimens in lieu of B. stygius, since the latter was suggested to him as applying to juvenile specimens (Bückmann and Kapp 1975; Galt 1979 and pers. comm.) and Galt’s specimens were on the order of 3–6 cm in total length. In his description of B. charon Galt wrote “The present specimens conform generally to published accounts of B. charon, detailed descriptions of which were given by Chun (1900), Lohmann (1931)), Garstang (1937 as B. stygius),” indicating that Galt likely deferred to Fenaux’s synonomy of the species in referring to them all as B. charon. More recently, a specimen called B. charon was collected by Castellanos et al. (2009)) but no description was provided and, although a photograph was included neither the large spiracles nor esophageal expansion are visible. Lindsay et al, (Lindsay et al. 2015) provide an in situ ROV image of the house of “Bathochordaeus sp. A” observed off the Nansei Island chain of Japan, but the structure of the inner filter differs markedly from the B. charon and B. stygius we have observed, and that larvacean was not collected.
Giant larvaceans, like other species of larvaceans, use their oikoplastic cells to secrete complex filters or ‘houses’ that allow them to concentrate and feed on particles (Lohmann 1933; Alldredge 1977; Morris and Deibel 1993; Flood et al. 1998). A house consists of a large, diaphanous outer structure as well as a smaller, more convoluted and bi-lobed inner structure that functions as a filter. Together these serve to concentrate appropriately-sized food particles from the surrounding water. The outer part of the house excludes larger material that would clog the inner filter. Thus, the outer structure often acquires a covering of marine ‘snow’ that can alter its size and shape (Hamner and Robison 1992; Silver et al. 1998). The inner filter concentrates food particles of ingestible size and are ultimately connected to the animal’s mouth via a tube made of the same material as the rest of the structure. However, the inner filter is less diaphanous and more stereotypical in shape, often retaining that shape long after the animal has left its house (Robison et al. 2005).
In situ, larvaceans in the genus Bathochordaeus are often visible from several meters away because their houses may span a meter in longest dimension (Hamner and Robison 1992; Robison et al. 2005). When Barham (1979) made the first observations of the occupied houses of large larvaceans he was diving by bathyscaph and saucer in the Pacific Ocean, off southern California and Mexico. He called them “giant” or “large” larvacean houses and inferred they were probably “Bathochordaeus charon”. However, he recalled, “seeing at least five types of large larvacean houses” and gave “general descriptions of three types”. It is not clear he thought all types belonged to B. charon. One animal was collected, identified by Donald P. Abbott as a “larvacean” and the specimen was subsequently lost. Intraspecific differences in Bathochordaeus’ house structure were not known at the time. Resolving the shape of the spiracles and esophagus of Bathochordaeus spp. from the portholes of his submersibles seems unlikely, since the larvaceans themselves were often invisible. Barham’s dives occurred 10 years prior to his publication and it seems more likely that he used the name B. charon because Fenaux (1966) had just synonomized the species and it was the correct name to use.
Almost 30 years after he synonymized the two species, Fenaux apparently acquired specimens collected by manned submersibles that proved to him the validity of both species (Fenaux 1993, 1998). Regrettably, he never published that proof and in the current literature it remains unclear if more than one species of Bathochordaeus exists (Hopcroft 2005; Flood 2005).
Using remotely operated vehicles (ROVs) we have carefully observed and collected B. charon (Chun) as well as B. stygius (Garstang). A combination of morphological features, house structures and molecular evidence clearly distinguish the two species and provide the first records of B. charon from Monterey Bay as well as off the coast of Oregon, expanding its range into the eastern North Pacific Ocean (Fig. 1).
