Deltamysis Bowman & Orsi, 1992

Daneliya, Mikhail E., 2023, On the Mysid Crustacean Genus Deltamysis (Mysidae: Heteromysinae), with a New Species and a New Record from Australia, Records of the Australian Museum (Rec. Aust. Mus.) 75 (4), pp. 413-430 : 415-419

Deltamysis Bowman & Orsi, 1992

Deltamysis Bowman & Orsi, 1992: 733 .— Jaume & Garcia, 1993: 234.— Müller, 1993: 218.—Bravo & Murano, 1996: 483.—Smith, 2001: 549.— Price, 2004: 68.— Petryashev, 2005: 15.— Panampunnayil & Biju, 2007: 1957.—Thorp & Rogers, 2010: 155.—Wittmann et al., 2014: 341.— San Vicente & Monniot, 2014: 333–334, 341.— Price, 2016: 702.—Scripter et al., 2020: 501, 504.

Kochimysis Panampunnayil & Biju, 2007: 1955–1957 , [synonymized by Scripter et al., 2020].

Type species. Deltamysis holmquistae Bowman & Orsi, 1992 ; by monotypy.

Diagnosis. Male process of antennula rather small, tuberclelike, sometimes barely visible, with setae. Mandibular palp segment 2 with few (up to three) short medial setae in middle part. Pereopodal endopods 1 and 2 slightly stronger than other pereopods; their carpopropodi 3-segmented, with serrated setae. Pereopodal endopods 3–6 are 4–5-segmented. Penis longer than basis of pereopodal endopod 6. Marsupium with two pairs of oostegites. Uropodal endopods without spiniform setae.

Comparison. Deltamysis is uniquely distinguished from other genera of Mysidetini by the very modest mandibular palp setation, with the characteristic two or three setae in the median part of segment 2. The genus is most similar to Burrimysis of Jaume & Garcia (1993), clearly differing by having only a few medial setae on the mandibular palp segment 2 (numerous long setae in Burrimysis ), the pereopodal carpopropodus 1 and 2 being 3-segmented (6-segmented in Burrimysis ), the longer penis (shorter than the pereopod 6 basis in Burrimysis ), the unarmed uropodal endopod (with one spiniform seta in Burrimysis ), and the telson cleft rather shallow or absent (rather deep in Burrimysis ).

Distribution.Western Indo-Pacific. Originally recorded from Sacramento—San Joaquin Estuary in California (Bowman & Orsi, 1992), and later also along the Atlantic US coast (Scripter et al., 2020), where it was supposed to be introduced. With discovery of D. lowryi sp. nov. on the New South Wales coast, and the transfer of the former Kochimysis pillai from India (Scripter et al., 2020), the former Heteromysoides nana from northern Australian coast and H. songkhlaensis from Thailand to Deltamysis ( Daneliya, 2021), it is now clear that the genus is naturally distributed in the Western Indo-Pacific Region ( Fig. 1 ).

Habitat. Marine-estuarine genus, confined to warm tropical and subtropical shallow waters.

Remarks. The genus was described by Bowman & Orsi (1992) for D. holmquistae , distinguished by the presence of the distal suture on the antennal scale, the pereopod 1 (mentioned as pereopod 2) not stronger than other pereopods, the rudimentary pleopods in both sexes, the unarmed uropodal endopod, the penis of moderate size, reaching basis of the pereopod 6 (indicated as pereopod 7) and the entire telson with two, short, apical spiniform setae, flanked by three pairs of longer spiniform setae. From the original generic diagnosis, I excluded the shape of the antennal scale and the pleopods for being general characters for the subfamily and the tribe, and the armature of the telson as species specific. Describing here D. lowryi sp. nov., which has the telson cleft, I also exclude the shape of the telson from the generic diagnosis. Jaume & Garcia (1993) expressed hesitation that the distinguishing characters between Deltamysis and Burrimysis were of generic level. In my opinion, the distinct setation pattern of the mandibular palp, somewhat enlarged endopods of the pereopods 1 and 2, with their 3-segmented carpopropodus, together with other characters make clear gap between Deltamysis and Burrimysis . Another two potential generic diagnostic characters, found in D. lowryi sp. nov., but not studied in other heteromysids, except in the genus Heteromysis , are the overlap of the posterolateral lobes of the abdominal segment 6 ( Figs 2F , 6E ), not overlapping in Heteromysis ( Daneliya, 2021) , and the presence of the subapical wing-like extensions on the maxilliped 1 dactylar claw ( Figs 4A , 7E ).

Panampunnayil & Biju (2007) distinguished the genus Kochimysis from Deltamysis on the basis of the male lobe presence on the antennular peduncle, the articulation absence on the antennal scale, the presence of the lacinia mobilis on the mandible and the wide segment 2 of the mandibular palp, equal size of the maxilla 2 endopod segments and the oval shape of its segment 2. Such discrimination was possible partly due to confusion (Scripter et al., 2020), because Bowman & Orsi (1992), describing Deltamysis , 1) did not notice the male lobe on the antennulae; 2) mentioned the presence of the lacinia mobilis on the left mandible and its absence of the right mandible (p. 737); 3) it was also the aspect of the mandibular palp illustration that did not show the width of the segment 2; 4) the maxilla 2 was mistakenly illustrated as 3-segmented with rather short segment 2, and the aspect did not clearly show the width of the segments. The presence or absence of the articulation on the antennal scale is a common intraspecific variation in different groups of mysids.

San Vicente & Monniot (2014) mentioned in their key that Deltamysis is distinguished from Kochimysis by the cornea being as wide as the eyestalk, the antennal peduncle as long as the antennal scale, the peduncular segments subequal and the maxilliped 2 without the notches in males. Some individuals of D. holmquistae , D. lowryi sp. nov., D. nana and D. songkhlaensis , have the cornea narrower than the stalk, the antennal peduncle longer than the antennal scale, with the peduncular segments subequal. The males of D. holmquistae and D. lowryi sp. nov. have the notches (or processes) on the maxillipeds 2. In all, I support Scripter et al. (2020) in their synonymization of Kochimysis with Deltamysis .

Scripter et al. (2020) updated the diagnosis of the genus, based on the redescription of D. holmquistae , the single known species at that time. The shape of the carapace, apically rounded in D. holmquistae , D. lowryi sp. nov. and D. nana , but pointed or blunt in D. songkhlaensis , is no longer diagnostic. As discussed earlier, the presence or absence of the suture in the antennal scale is a generally variable feature and not suitable for the generic diagnosis. They also included the presence of the processes (“papillations” in their terminology) on the maxilliped 2 carpus and propodus to the diagnosis. This character is absent in D. nana (although only a female has been known so far) and D. songkhlaensis , and should be excluded from the diagnosis. It is also mentioned that the pereopod 1 endopod (“thoracic endopod 3”) is not stouter than other endopods. Though not prehensile like in the members of Heteromysini , I find that both pereopod 1 and 2 endopod are rather similar and clearly stouter than the endopods 3–6. The latter have 4-segmented carpopropodus in D. holmquistae and D. songkhlaensis , but 4- or 5-segmented in D. lowryi sp. nov. and 5-segmented in D. nana . The rudimentary nature of the pleopods is the tribal character and not necessary in the generic diagnosis. The telsonal characters are also rather variable, particularly with the presence of the telson cleft in D. lowryi sp. nov. and, thus, should be excluded from the diagnosis.

In an updated diagnosis above, I have added new common features, particularly in the setation of the mandibular palp and the similarity of the pereopod endopods 1 and 2.

Except for Deltamysis , all the genera of the subfamily Heteromysinae contain exclusively marine species. Deltamysis is the only heteromysine genus, which has been penetrating estuaries and diversifying in brackish water.

Deltamysis holmquistae was occasionally sampled even in fresh water.

Composition. The genus includes four species: D. holmquistae , D. lowryi sp. nov., D. nana and D. songkhlaensis .

Key to the species of the genus Deltamysis

1 Anterior margin of carapace triangular, apically pointed or blunt. Telson apical spinules as long as or longer than lateral terminal spiniform setae. Pereopod 3–6 carpopropodus, segment 1 longer, as long as or slightly shorter than other segments combined ........................................ D. songkhlaensis (Yolanda, Sawamoto & Lheknim, 2019)

—— Anterior margin of carapace apically rounded ( Figs 2B , 6B ). Telson apical spinules clearly shorter than lateral terminal spiniform setae ( Figs 2C, D , 6C, D ). Pereopod 3–6 carpopropodus, segment 1 shorter than other segments combined ( Figs 5B–E , 8E, F ) .................................................................................................... 2

2 Telson rather narrow, 1.5–1.6 times as long as wide anteriorly; terminal spiniform setae 0.12 of telson length. Maxilla 2, exopod and endopod segment 2 with lateral setae ............................................ D. nana ( Murano, 1998)

—— Telson rather wide ( Figs 2C , 6C ), 1.1–1.4 times as long as wide anteriorly; terminal spiniform setae 0.2–0.4 of telson length. Maxilla 2, exopod and endopod segment 2 without lateral setae ( Figs 3E , 8A ) .................................................................................................................... 3

3 Telson apically without cleft ( Fig. 2C, D ), armed with two (rarely three) spinules; lateral subterminal spiniform setae not shorter than preceding lateral spiniform setae. Maxilla 2 exopod rather small, barely reaching half of endopod segment 1 ( Fig. 3E ). Pereopod dactylus rather thick, nearly as long as wide ( Figs 4C, D , 5A–E ) ....................................................................... D. holmquistae Bowman & Orsi, 1992

—— Telson with cleft ( Fig. 6C, D ), armed with seven spinules; lateral subterminal spiniform setae significantly shorter than preceding lateral spiniform setae. Maxilla 2 exopod rather big, nearly reaching endopod segment 2 ( Fig. 8A ). Pereopod dactylus rather thin, about twice as long as wide ( Fig. 8C–G ) ..................................................................................................... D. lowryi sp. nov.

Figures

Figure 1. Distribution of the species of the genus Deltamysis in the West Indo-Pacific.

Figure 2. Deltamysis holmquistae Bowman & Orsi, 1992, female, 4 mm, New South Wales, Yamba, AM P.98703: (A) habitus, lateral; (B) head, dorsal; (C) posterior part of body with telson and uropods, dorsal; (D) telson posterior part; (E) posterolateral lobe of abdominal segment 6; (F) posteroventral margin of abdominal segment 6, and uropodal protopod. Scales (mm): A = 1; B, C, E = 0.5; D, F = 0.25.

Figure 3. Deltamysis holmquistaeBowman & Orsi, 1992, female, 4 mm, New South Wales, Yamba,AM P.98703:(A) antennular peduncle distal part, dorsal; (B) antennal peduncle and scale, ventral view; (C) mandibular palp, posterior; (D) labrum, ventral; (E) maxilla 2, posterior; (F) pleopod 3, anterior; (G) pleopod 5, anterior. Scales (mm): A, B, F, G = 0.5; C–E = 0.25.

Figure 4. Deltamysis holmquistae Bowman & Orsi, 1992, female, 4 mm, New South Wales, Yamba, AM P.98703: (A) maxilliped 1 endopod, anterior; (B) maxilliped 2 endopod, anterior; (C) pereopod 1 endopod, anterior; (D) pereopod 1 endopod distal part, anterior. Scales (mm): A, B, D = 0.25; C = 1.

Figure 5. Deltamysis holmquistaeBowman & Orsi, 1992, female, 4 mm, New South Wales,Yamba,AM P.98703:(A) pereopod 2 endopod, anterior; (B) pereopod 3 endopod, anterior; (C) pereopod 4, anterior; (D) pereopod 5 endopod, anterior; (E) pereopod 6, anterior; (F) pereopod 6 basis distal part, anterior. Scales (mm): A–E = 0.5; F = 0.25.

Figure 6. Deltamysis lowryi sp. nov., holotype, male, 4 mm, New South Wales,Yamba,AM P.98699. (A) habitus, lateral; (B) head, dorsal; (C) posterior part of body with telson and uropods, dorsal; (D) telson posterior part; (E) posteroventral margin of abdominal segment 6, and uropodal protopod; (F) antennular peduncle distal part, dorsal; (G) antennal peduncle and scale, ventral view. Scales (mm): A = 1; B, C, G = 0.5; D–F = 0.25.

Figure 7. Deltamysis lowryi sp. nov., holotype, male, 4 mm, New South Wales, Yamba, AM P.98699: (A) mandibular palp, anterior; (B) mandibles, ventral; (C) labrum, ventral; (D) maxilla 1 outer ramus, posterior; (E) maxilliped 1, anterior; (F) pleopod 2, anterior; (G) pleopod 4, anterior; (H) pleopod 5, anterior. Scales (mm): A–D, F–H = 0.25; E = 0.5.

Figure 8. Deltamysis lowryi sp. nov., holotype, male, 4 mm, New South Wales,Yamba,AM P.98699:(A) maxilla 2, posterior; (B) maxilliped 2 endopod, anterior; (C) pereopod 1 endopod, posterior; (D) pereopod 1 distal part, posterior; (E) pereopod 4 endopod, anterior; (F) pereopod 6, posterior; (G) pereopod 1 distal part, posterior. Scales (mm): A, D, G = 0.25; B, C, E, F = 0.5.