Brissus cf. agassizii Döderlein, 1885

Arachchige, Gayashan M., Jayakody, Sevvandi, Mooi, Rich & Kroh, Andreas, 2019, Taxonomy and distribution of irregular echinoids (Echinoidea: Irregularia) from Sri Lanka, Zootaxa 4541 (1), pp. 1-100 : 70-75

Brissus cf. agassizii Döderlein, 1885

Figures 62–64

1885 Brissus Agassizii Döderlein : p. 108–109. 1951 Brissus (Allobrissus) Agassizii Döderlein. —Mortensen: p. 520–522; pl. 33: fig. 7; pl. 34: fig. 2; pl. 35: figs. 1, 6–8; pl. 62: figs. 9, 10, 12, 16–21, 23–28.

Material studied. Six denuded tests: WUSL/EI/75, EI/76, EI/77, EI/79, and EI/81 from Silavathurai, WUSL/EI/80 from Baththalangunduwa 2.

Description. Shape and size —Test medium, 27.9–52.1 mm TL; width 72.9–73.8% TL; height 52–59% TL; outline elliptical, without anterior depression; anterior end rounded in lateral view; posterior end of test ovoid and not pointed when viewed from above.

Apical system —Ethmolytic, madreporite extending beyond posterior oculars; with four gonopores; posterior pair larger than anterior pair; situated anteriorly, c. 20% TL (SD=0.8) from anterior margin.

Ambulacra —Ambulacrum III narrow and flush, pore pairs small, simple isopores; paired ambulacra distinctly petaloid; petals sunken, narrow, closed distally; anterior paired petals extending laterally almost 180° to each other, spanning c. 46% of TW; posterior paired petals 126% longer than anterior paired petals; mean petal lengths of anterior paired petals and posterior paired petals 31% and 39% TL respectively; interporiferous zones very narrow; anterior paired petals slightly curved anteriorly at the tips; posterior paired petals slightly divergent at an angle of 44° and curved laterally at the tips; inner and outer pores elliptical, more or less equal in size; distinctly conjugate; ridges between consecutive pore pairs low with 3–5 small tubercles; anterior paired ambulacra bear well-developed phyllodes; periplastronal areas narrow and naked, but also forming phyllodes near peristome.

Interambulacra —Aboral side convex; posterior interambulacrum raised but not keeled, not projecting above periproct; posterior end nearly vertically truncated; densely packed with perforate, crenulate primary tubercles, larger tubercles inside peripetalous fasciole especially along ambulacrum III and anterior part of test.

Fascioles —Peripetalous fasciole well-developed, closely outlining petals with strong indentation between petals; subanal fasciole broad, bilobed and kidney shaped, width of fasciole 43–47% TL concavity in outline directed towards periproctal region; usually encloses five pore pairs on each side; no anal branches ( Fig. 63 ).

Plastron —Wide, 39–45% TL.

Peristome —Kidney-shaped; about twice as wide as long; moderately large, length 9–13% TL, width 18–21% TL; located anteriorly, anterior margin of peristome c. 15% TL (SD=0.4) from anterior margin of test; labrum short and wide, thickened, rounded and downturned.

Periproct —Vertically elongate; teardrop-shaped, outline pointed aborally and orally; large, 1.5 times longer than wide, length 18–22% TL, width 12–15% TL.

Geographic range. Indo-West Pacific, from Green Head to Swain Reefs ( Miskelly 2002), Japan ( Mortensen 1951) and Philippine (van Noordenburg 2008).

Bathymetric range. Intertidal zone to 120 m ( Miskelly 2002).

Observed occurrence in Sri Lanka. Denuded specimens were collected from a sandy bottom with sea grass beds in Silavathurai and Baththalangunduwa, north-western coast of Sri Lanka at a depth range of 9–20 m. In addition, this species was observed in Kalmunai, on the eastern coast of Sri Lanka ( Fig. 64 ).

Remarks. In examined specimens, relative peristome size decreases during growth. This indicates negative allometry for the peristome, which is common throughout the Irregularia. Test height of this species was observed to decrease with growth. In addition, width of the subanal fasciole decreases relative to test length. Average ratio between the subanal fasciole width and periproct width is 3.5 for the studied specimens (SD=0.12, N=4). This ratio was higher than that in specimens of B. agassizii (3.0 [SD=0.02, N=3, based on NHMW 2008z0168/0028, 0 0 29 and 0030]), or B. unicolor (2.8 [SD=0.44, N=7, based on NHMW 2006z0300/0002a, 0002b, 0002c, 0002d, 0002e, 2009z0207/0001 and 2015/0529/0001] examined at the Natural History Museum Vienna).

The largest specimen (collected from Hiriketiya, southern coast of Sri Lanka) with a TL of 80.9 mm had six pore pairs in the right lobe of the subanal fasciole and five in the left lobe, indicating that the number of pore pairs in the subanal fasciole can vary with test length. In addition, this specimen’s apical system was very close to the anterior margin (9% TL from anterior edge) and the test broader (81% TL).

The Sri Lankan Brissus ( B. cf. agassizii ) specimens clearly do not belong to B. latecarinatus as the periproct is not overhung by the posterior interambulacrum. The posterior end is almost vertically truncated, so that the periproct is not fully visible from the oral side. In addition, aboral interambulacrum 5 is not strongly carinate. In the Sri Lankan material, the periproct is not broadly oval, but elongate and pointed, and the plastron is about half the test width. B. cf. agassizii also tends to have five pore pairs on each side of the subanal fasciole even in the smallest (27.8 mm) specimen. The apical system is placed more anteriorly in the Sri Lankan specimens than in B. latecarinatus .

Although, the specimens collected in Sri Lanka are more similar to B. agassizii , the test is high compared to that species. However, this feature appears to be quite variable and relative test height decreases with growth in the studied specimens. In addition, the posterior petals are shorter than in typical B. agassizii .

A specimen (WUSL/EI/74, TL 53.1 mm, Fig. 65 ) collected from Hikkaduwa Beach had the major features of a Brissus but differed from typical Brissus by the confluent nature of its posterior paired petals (giving a similar appearance as in Metalia sternalis or Brissalius ). Different interpretations for this specimen are possible: 1) the peculiar morphology could be the result of abnormal growth, or 2) this could be a new species of Brissus . Until additional specimens showing this peculiar feature are found, we favour interpretation 1 and refrain from identifying the specimen to species.

Figures

FIGURE 62. Brissus cf. agassizii: A, aboral view (WUSL/EI/81); B, oral view (WUSL/EI/81); C, lateral view (WUSL/EI/81); D, posterior (WUSL/EI/81); E, sternal view (WUSL/EI/75).

FIGURE 63. Subanal fasciole of Brissus cf. agassizii (WUSL/EI/75).

FIGURE 64. Distribution map of Brissus cf. agassizii recorded in this study.

FIGURE 65. Brissus sp. (WUSL/EI/74): A, aboral view; B, oral view; C, lateral view, right side.