A New Species of Caprella (Crustacea: Amphipoda: Caprellidae) from Gippsland Lakes, Australia, with a Redescription of Caprella acanthogaster Mayer, 1890 from Northern Japan

Caprella acanthogaster Mayer, 1890 sensu lato (Crustacea: Amphipoda: Caprellidae) has previously been recorded from temperate and cold waters of Far East Asia and Australia. A detailed morphological comparison of specimens collected from Gippsland Lakes (Victoria, Australia) and Otsuchi Bay (Tohoku District, Japan) revealed that C. acanthogaster sensu lato from these regions comprises two separate species. Caprella acanthogaster Mayer, 1890 sensu stricto is confirmed to be distributed in northern Japan. Caprella tamboensis sp. nov. is present in Victoria and Tasmania, Australia, and most noticeably differs from C. acanthogaster sensu stricto in having a relatively short triangular projection on the palm of propodus of gnathopod 2 and a relatively longer peduncle article 2 of antenna 1 (being longer than article 3).


Introduction
The Amphipoda is a highly diverse order of crustaceans with approximately 10,000 species so far recorded from marine to terrestrial environments worldwide (Arfianti et al., 2018). Within this order the genus Caprella is one of the most diverse genera containing more than 180 species with a peak in Far East Asia (Arimoto, 1976;Vassilenko, 1976;Takeuchi, 1999;Horton et al., 2021). Caprella occupies the most apomorphic position in the Caprogammarus-Caprella line of the Caprellidea due to the loss of pereopods 3 and 4, and absence of a mandibular palp in the mouthparts (Takeuchi, 1993), which makes members of this genus easy to recognize. The species belonging to Caprella range in size from 1-3 cm in body length, and are mostly found in algal communities, or on buoys in port areas, and in aquaculture facilities with euryhaline environments (Takeuchi et al., 2001(Takeuchi et al., , 2003.
In the present study, we conducted a detailed morpho logical comparison of C. acanthogaster sensu lato from Otsuchi Bay in Iwate Prefecture, Tohoku District of northern Japan, and from Gippsland Lakes in Victoria, Australia. The comparison revealed that these are different species. We propose and describe a new species C. tamboensis sp. nov. for the Victorian specimens and earlier Australian records from Tasmania, and also provide a detailed redescription of C. acanthogaster sensu stricto based on the material from northern Japan.
Type locality. Mouth of Amur River, at Strait of Tartary (Mamiya Strait) between the Sea of Okhotsk and the Sea of Japan.

Remarks
Taxonomy, previous records. Caprella acanthogaster was first reported by Mayer (1890) from the mouth of the Amur River, at the Strait of Tartary (Mamiya Strait) between Sakhalin Island and the Eurasian Continent, based on 4 males (25 mm maximum body length) and 1 female. Mayer (1890) provided a brief description of the species, with figures of gnathopod 2 and pereonite 3, but was mainly focused on its differences from C. eximia Mayer, 1890. The type specimens were deposited at the Hamburg Museum, Germany, according to McCain and Steinberg (1960) and Marelli (1981). Mayer (1890) also reported a male specimen of C. acanthogaster at the Museum Godeffroy, Hamburg, Germany. The sampling location of this specimen is unknown, and Mayer (1890) concluded that it most likely originated from Australia after consultations with G. Pfeffer, although Mayer (1890) also speculated that the specimen was collected from De Castries Bay, Sakhalin Island in Far East Asia or from the Le Maire Strait at the southern tip of South America before contacting G. Pfeffer (see remarks under C. tamboensis sp. nov. for further discussion of this specimen). Mayer (1903) also reported this species from the Sea of Japan near Vladivostok, Russian Far East, from a collection deposited in the Moscow Museum without a specified sampling location, and from Nakabuta, Hokkaido, Japan deposited at the Hamburg Museum. In the same report, Mayer (1903) provided a brief description and the lateral view of entire C. acanthogaster based on the male specimen, 42 mm in body length, deposited in the Moscow Museum. Schurin (1937) reported and briefly described a specimen of C. acanthogaster from Peter the Great Bay, and cited the differences between C. acanthogaster and C. eximia following the notes in Mayer (1890Mayer ( , 1903. However, in the lateral view Schurin (1937) provided for C. acanthogaster minute setae are present on pereonite 2 and gnathopod 2, a character typical for C. mutica. Therefore, Vassilenko (1974) concluded that C. acanthogaster reported in Schurin (1937) was a misidentified sample of C. mutica. Caprella acanthogaster s.l. recorded from Onagawa Bay, Miyagi Prefecture, northern Japan (Utinomi, 1943a) is similar to the above C. acanthogaster s.l. from Peter the Great Bay. Vassilenko (1974) also described and provided figures of C. acanthogaster s.l. that was collected from Peter the Great Bay. The male specimen in Vassilenko (1974) possesses paired posterodorsal projections on pereonite 1 and paired mid-lateral and posterodorsal projections on pereonite 2. These features make the C. acanthogaster s.str. determination questionable. Therefore, further detailed study on these C. acanthogaster s.l. specimens from Peter the Great Bay and Onagawa Bay is needed. Vassilenko (1974) noted that C. acanthogaster differs from C. eximia by the smaller body size and presence of two to four projections around the base of gills on pereonites 3 and 4.
Regarding C. acanthogaster s.l. from Japan, Arimoto (1976) described C. acanthogaster collected from Kesennuma Bay, Miyagi Prefecture, northern Japan, and illustrated specimens that possessed intermediate features or a mixture of traits between C. acanthogaster and C. mutica, as pointed out by Marelli (1981) and Vassilenko (2006), and it is considered that there were both of these species among his material. Marelli (1981) studied a syntype of C. acanthogaster deposited at the Hamburg Museum, Germany, and presented its description and a lateral view of the specimen. Mayer (1890) noted the presence of paired projections on the head, but the corresponding projections are absent in the description and figure provided by Marelli (1981).
Material examined from Japan in the present study matches the description and figure of C. acanthogaster in Marelli (1981) in the following characteristics: smooth head, elongate pereonite 1, elongate pereonite 2 with several dorsal spines, setae on pereonite 2 absent, large triangular projection present on the middle of propodus of gnathopod 2, and elongated gills on pereonite 4.
In addition to the Siberian coast of the Sea of Japan and northern Japan, C. acanthogaster s.l. has been widely reported from South Korea (Hong, 1988;Lee & Lee, 1993;Lee & Hong, 2011;Heo et al., 2020) to China (Wang et al., 1989;Cao et al., 2012;Wei et al., 2016;Chen et al., 2018) in East Asia.
For the Korean records Lee & Hong (2011)  To the best of our knowledge, there are no detailed descriptions and/or figures of C. acanthogaster from the Chinese coastline. However, the record from fouling assemblages in the sub-tropical area of Hainan Island in southern China (Chen et al., 2018) is questionable because this location is currently disjunct from other records in the more temperate regions contiguous with the type locality. Furthermore, recent studies have confirmed that the huge amount of freshwater from the Yangtze River entering the East China Sea forms a barrier to several species of Mollusca and Crustacea where the duration of the pelagic larval stage is < 10 days (Ni et al., 2017). Since the Amphipoda including Caprella spp. lack a pelagic larval stage, the outflow from the Yangtze River may act as a distinct barrier for the species distribution along the coast of China in a similar way.
We therefore conclude a detailed comparison of C. acanthogaster from northern China and Korea with those from other localities should be conducted. Particularly as recent detailed comparisons of common species of Caprella and related genera in the Caprellidae and the Phtisicidae, previously regarded as cosmopolitan examples from different geographical localities, shows that these records are separated into region-specific species (Takeuchi & Lowry, 2007, 2019Takeuchi & Oyamada, 2013;Hughes & Takeuchi, 2016;present study). Similarly, Takeuchi & Oyamada (2013) conducted a detailed comparison of C. californica Stimpson, 1857 s.l. from Japan and California, and proposed C. scauroides Mayer, 1903 as the correct name for the Japanese specimens.
Mouthparts, similar to those described in the current study for Caprella acanthogaster.
Etymology. The species name refers to the type locality of the species Tambo Bay, Gippsland Lakes, Victoria.

Remarks
The highly dense aggregations of Caprella attached to fisheries nets at Tambo Bay, Gippsland Lakes, Victoria, were first discovered by M. Jenkins, a local fisherman (Fig.  8), and the specimens were sent to the Australian Museum for identification.
While the body length of the male specimen of the C. acanthogaster redescribed here from Japan is double that of the Caprella from Gippsland Lakes the former has less articles in the flagellum of the antenna 1. The number of antenna 1 flagellar articles has been reported to increase in Caprella danilevskii Czerniavski, 1868 by one or two in every moult as individuals mature (Takeuchi & Hirano, 1991). Taking this into account it appears both male specimens described here from Japan and Gippsland Lakes are at approximately the same stage of development. Therefore, comparison of Caprella from Gippsland Lakes and C. acanthogaster s.str. from Far East Asia (Marelli, 1981; present study) based on large mature males has revealed that they are similar but the former differs in: i) the small midprojection on the palm of propodus of gnathopod 2; ii) the relatively longer antenna 1 peduncle article 2, being longer than article 3; and iii) the relatively higher number of articles in the antenna 1 flagellum. To accommodate these specimens, we establish a new species, Caprella tamboensis sp. nov. The specimens of C. acanthogaster reported form Mercury Passage, Tasmania, Australia, by Guerra- García & Takeuchi (2004) are also identified as belonging to this new species.
The male specimen reported by Mayer (1890) as C. acanthogaster found in the collection from the Museum Godeffroy, Hamburg, Germany, possibly originated from Australia (see remarks under C. acanthogaster above). The Museum Godeffroy was founded by Mr Cesar Godeffroy, one of the merchant magnates of Hamburg, Germany, whose ships had sailed around every ocean for over half a century (Ward, 1876). Included in these travels were expeditions concentrated around the South Sea Islands to collect crustaceans, molluscs, starfish, sea urchins, holothurians, corals, sponges, sea fans, and other organisms (Ward, 1876). Ward (1876) noted that Mr Darnel (probably staff of Mr Cesar Godeffroy), while working in Eastern Australia, passed through Queensland and penetrated three hundred miles into the interior, obtaining strange forms of molluscs and fishes. Therefore, there is a possibility that the specimen of Caprella in the Museum Godeffroy collection cited by Mayer (1890) was obtained in Australia and may be C. tamboensis or another species related to C. acanthogaster or C. tamboensis. However, with the closure of the Museum Godeffroy in 1885 the collection was scattered among various institutions (Wikipedia, 2021) and we have been unable to locate the specimen to resolve this.
Evidence provided here suggests a different conclusion to that of Guerra- García & Takeuchi (2004) who reasoned that Caprella acanthogaster s.l. (= C. tamboensis sp. nov.) from Tasmania was possibly introduced from Far East Asia as the specimens were found among a scallop aquaculture facility, including on the invasive alien macro-alga Undaria pinnatifida (Harvey) Suringar, 1873, and suggested the likely transport vector as associated with spat of scallop from the north-eastern Pacific (Edgar, 2000;Guerra-García & Takeuchi, 2004). While we do not discount the possibility that the caprellid recognized here from Gippsland Lakes and Tasmania as C. tamboensis may have been introduced to those areas we do not find it to be the same species as C. acanthogaster and it is equally likely it may be a native species, widely distributed but as yet largely unrecorded in southern Australia. For example, Guerra-García et al. (2020) have noted examination of the Caprellidae from South Australia and Victoria would also assist in filling gaps in understanding the biodiversity and biogeographical patterns of the Australian fauna.
Our observations also highlight that further detailed morphological study, possibly combined with genetic analysis, may be necessary for reconstructing the phylogeny and to better understand the distribution pattern for species reported as C. acanthogaster and other similar related species from the temperate to cold waters of the northeast Asia. These include C. mutica and C. eximia, both of which were reported to be closely related to C. acanthogaster (Vassilenko, 1993), as well as C. centroda Vassilenko, 1993 from the Chishima Islands (Kurile Islands) located between Hokkaido and the Kamchatka Peninsula.