Jassafalcata ( Montagu, 1808 )

Conlan, Kathleen E., Desiderato, Andrea & Beermann, Jan, 2021, Jassa (Crustacea: Amphipoda): a new morphological and molecular assessment of the genus, Zootaxa 4939 (1), pp. 1-191 : 81-87

Jassafalcata ( Montagu, 1808)

( Table 11 , Figs 42–47 )

Synonyms: see Conlan (1990).

Diagnosis.

Both sexes:

Mandibular palp: article 2, dorsal margin witha fringe of setae.

Maxilla 1: without a seta or setal cluster at the base of the palp article 1.

Gnathopod 1: basis, anterolateral margin with a few very short setae; carpus without a single or small cluster of setae at the anterodistal junction of the propodus.

Gnathopod 2: basis with a few minute setae along the anterolateral margin but without long filter setae (setal lengths <20% of the basis width); carpus and propodus, setae on the anterior margin short and simple (setal length <basis width).

Pereopods 5–7: propodus not expanded anteriorly.

Uropod 1: ventral peduncular spinous process underlying about 1/2 of the longest ramus.

Uropod 3: inner ramus with 1–2 spines mid-dorsally in addition to the usual single apical spine.

Telson: tip without apical setae, only the usual short setae at each dorsolateral cusp.

Thumbed male:

Antenna 2: with plumose setae on the flagellum and peduncular article 5.

Gnathopod 2: propodus, palmar defining spines absent except in small males. Thumb distally acute or squared in minor males and squared in major males. Dactyl centrally toothed in minor forms and shallowly expanded proximally in major forms.

Adult female:

Antenna 2: with plumose setae on the flagellum and peduncular article 5.

Gnathopod 2: propodus, palm concave, palmar defining angle bulbous, distal to and fairly distant from the defining spines.

The basis of pereopod 3, which holds the tube spinning gland, was also measured as the width appeared to be greater in the females and juvenile males than in the adult males ( Fig. 46b ). Basis width was measured at the widest part of the basis which tended to be in the central part. The same individuals as for Gn2 propodus length were measured, with the addition of 6 additional adult males for which propodus length was lacking. Fig. 46b suggests that basis width was greater in the females and juvenile males (which included one subadult male) than in the adult males. Pairwise comparisons by Dunn’s Method showed a significant difference between major form adult males (n = 20) and juvenile males (n = 8) (Q = 2.905, p<0.05). Differences of adult females from both adult and juvenile males were not significant (Q = 1.913 and 0.454, respectively), although Fig. 46b suggests a transitional difference.

Because of the large variation in male thumb morphology in Jassa as well as the synonymy of previously recognized species to J. falcata by Sexton and Reid (1951), many references to J. falcata in the literature are mis-identifications. Conlan (1990) lists corrected identifications for those specimens that could be obtained. The specimens noted in Lobo et al. (2017) as being J. falcata are confirmed as this species (specimens examined 4 March 2019). So too are the specimens described by Walker (1911) (NHM).

Remarks. Minor forms have a distinct tooth on the inner surface of the dactyl which inserts into the palmar incision on the propodus ( Figs 42 and 45 ). In major forms, the thumb is long and the propodus deeply incised ( Figs 43– 45 ). The dactyl lacks the obvious tooth of the minor form, though it is expanded proximally. These differences are so great that this lead Leach (1814) to describe the major form as J. pulchella ; Montagu (1808) had based his description of J. falcata on a minor form male. Walker (1911) was of the opinion that both forms occurred within the species.

Aplot of thumb length vs body length for a population sampled in summer at Audrassalas, France ( Fig. 45 ) shows the shorter thumb length and dactyl toothing in the minor form and the very long thumb of the major form. Comparing mean thumb length between the minor form group (n = 2) and the major form group of the body length range (5.6–6.2 mm, n = 10) found that the differences were not great enough to exclude the possibility that the differences were due to random sampling variability (ANOVA, F = 4.289, p = 0.065). Minor forms were rare in this population and were uncommoninothercollectionsalso, despite 4,000 specimenshavingbeenexamined ( Table 4 ).

Aplot of gnathopod 2 propodus length for the same population ( Fig. 46a ) showed a significant difference between the major form adult male (n = 14), juvenile (including subadult) male (n = 8) and adult females (n = 5) within the body lengthrange whereeach overlapped (5.08–6.67 mm) (ANOVA, F = 22.112, p <0.001). Adultmaleshada significantly longer propodus than adult females (t -test, t = 5.631, p <0.001) or juveniles (t = 5.107, p <0.001). Propodus length for major form adult males averaged 0.794 ± 0.104 mm while for juvenile males it averaged 0.595 ± 0.0657 mm. Adult females had an average propodus length of 0.536 ± 0.0623 mm which was not significantly different from the juvenile males (t = 1.175, p = 0.251).

Figures

FIGURE 42. Jassa falcata (Montagu, 1808), type species for the genus Jassa. Holotype, adult male, minor form, 6.3 mm. Torcross, South Devon, England, date unknown, collector unknown, NHM 603a. Lateral views: whole body and right uropods 1-3; dorsal view: telson; other views medial. Scale 0.1 mm. Illustration after Conlan (1990).

FIGURE 43. Jassa falcata (Montagu, 1808). Adult male, major form, 5.7 mm, lectotype for the series 296a-296g of Jassa pulchella Leach, 1814. Devon, England, date unknown, collector unknown, NHM 296e. The tip of the thumb on gnathopod 2 was probably damaged while the animal was alive; other damage was probably incurred while it was dry mounted. Adult male 1, major form, 11.0 mm; juvenile male 2, 7.0 mm; adult male 3, major form, 6.5 mm; juvenile male 4, 4.5 mm; and adult female, 7.0 mm. Stoer Bay, Sutherland, Scotland, 22 October 1937, D. M. Reid, coll., very abundant on the floats of coastal salmon nets (SNM). Lateral view: whole body; other views medial. Appendages are of the lectotype unless indicated otherwise. Scale 0.1 mm. Illustration after Conlan (1990).

FIGURE 44. Jassa falcata (Montagu, 1808). Adult male 1, major form, 11.0 mm; juvenile male 2, 7.0 mm; adult male 3, major form, 6.5 mm; juvenile male 4, 4.5 mm; and adult female, 7.0 mm. Stoer Bay, Sutherland, Scotland, 22 October 1937, D. M. Reid, coll., very abundant on the floats of coastal salmon nets (SNM). Developing new cuticle is visible inside the propodus and dactyl of the juvenile male 2 gnathopod 2, not yet at the terminal molt as the new cuticle lacks a thumb. All views medial. Scale 0.1 mm.

FIGURE 45. Jassa falcata (Montagu, 1808). Variation in thumb length relative to body length in males in a single collection at Audrassalas, Pas-de-Calais, France, 1 July 1985, K. E. Conlan, coll., in tubes and and actively walking over Ceramium rubrum (mainly) and Polysiphonia, even though out of contact with seawater at low tide, IZ 1985-093 (CMN). Arrows refer to the associated gnathopod illustrations. The subadult male had a thumb visible inside the cuticle, indicating that it would molt next into a thumbed adult. Setae omitted except for those around the thumb and spines in order to landmark position changes with growth. All views lateral. Scale 0.1 mm. Linear regression assumptions passed for the adult major form male. Linear regression statistics: Major form, thumb length = -1.219 + 0.281 body length, r2 = 0.773, n = 38. Illustration after Conlan (1990).

FIGURE 46. Jassa falcata (Montagu, 1808). Variation in gnathopod 2 propodus length and pereopod 3 basis width in males and females from a population at Audrassalas, Pas-de-Calais, France (1 July 1985, K. E. Conlan, coll., in tubes and and actively walking over Ceramium rubrum (mainly) and Polysiphonia, even though out of contact with seawater at low tide, IZ 1985-093 (CMN). Linear regression assumptions passed for the juvenile male. Linear regression statistics: juvenile male, propodus length = -0.152 + 0.126 x body length, r2 = 0.916, n = 13.

FIGURE 47. Jassa falcata (Montagu, 1808). Holotype, adult male, minor form, 6.3 mm. Mouthparts. Frontal view: upper lip; lateral views: maxilla 1; other views medial. Scale 0.1 mm. Illustration after Conlan (1990).