Isodiametra westbladi (Marcus, 1949)

(Fig. 18)

Convoluta westbladi: Marcus 1949 (p 12); Ax 1961 (p 8); Dörjes 1968 (p 84). Conaperta westbladi: Antonius 1968 (p 309); Dörjes and Karling 1975 (p 178); Hooge and Tyler 2005 (p 102).

Isodiametra westbladi: Hooge and Tyler 2005 (p 112).

Material. Living specimens in squeeze preparation; whole mounts for fluorescence imaging of musculature.

Localities. Praia da Fazenda, Ubatuba, from intertidal fine sand (23°21’37.8”S, 44°51’00.3”W); Praia da Enseada, Caraguatatuba, from intertidal fine sand (23°39’58.8”S, 45°25’47.0”W); Praia Pedra do Sino, Ilhabela, from intertidal coarse sand (23°44’49.8”S, 45°20’52.9”W).

Description. Living specimen ~ 500 m in length and ~ 100 m wide (Fig. 18 A). Body mostly without coloration by transmitted light except for reddish-brown material in chordoid vacuoles at edges of body. Rhabdoid glands arranged in rows. Mouth opening ventral, middle of body.

Ovaries and testes paired. Common gonopore opens caudally into male copulatory organ and rostrally into vagina. Vagina surrounded by weakly muscular sphincter; opens anteriorly to walled bursa that leads to short bursal nozzle (Fig. 18 B). Short, muscular penis surrounded by glandular secretions and sperm that lie within seminal vesicle (Fig. 18 B).

Remarks. Isodiametra westbladi was found in large quantities at three sites, with thousands of specimens emerging from our sediment extractions. The short, muscular penis appears to lack glandular secretions within the lumen and can be difficult to see in squeeze preparations. The musculature of the male copulatory organ of I. westbladi, including the penis, was revealed clearly in our whole mounts stained for fluorescence imaging of musculature (Hooge & Tyler 2005).