Isodiametra bajaensis Hooge & Eppinger 2005, sp. nov.
Creators: name: Hooge, Matthew D.
name: Eppinger, Neil
Publisher: name: Zenodo
Year: 2005
Subjects: Biodiversity
Taxonomy
Animalia
Xenacoelomorpha
Acoelomorpha
Acoela
Isodiametridae
Isodiametra
Isodiametra bajaensis

description: Isodiametra bajaensis sp. nov. (Figs. 4–6)   
 Type Material. Syntypes. AMNH PLATY 1661 and AMNH PLATY 1662, one set of 1.5­ µm­thick serial sagittal sections and one set 1.5­µm­thick serial frontal sections of epoxyembedded specimen stained with toluidine blue. Paratype. AMNH 1663, epoxy­embedded whole mount.   
 Type Locality. San Felipe, Baja California, from subtidal medium grained sand from the south side of San Felipe Bay (3058’35.1” N, 114°48’22.6” W).   
 Other Material Examined. Whole mounts for fluorescence imaging of musculature; photographs of living specimens in squeeze preparations, three sets of 1.5­µm­thick serial sagittal sections and one set of 1.5­µm­thick serial frontal sections of epoxy­embedded specimens.   
 Etymology. Species name refers to the type locality of Baja California, Mexico.   
 Description. Examined specimens 280 to 330 µm long and 75 to 100 µm wide (Figs. 4A, B, 5A). Anterior and posterior ends rounded. Body cylindrical.   
Epidermis completely ciliated. Many mucoid glands present. Body colorless in transmitted light, but digestive syncytium has brown and slight green coloring.  
Body­wall musculature with circular muscles that encircle the body along entire length of animal; straight longitudinal muscles present between frontal organ and anterior edge of mouth; longitudinal muscles with a longitudinal orientation anteriorly that bend medially to cross diagonally over the body; longitudinal­cross­over fibers present in dorsal and ventral body walls; longitudinal muscles in the anterior half of the body that wrap around the posterior rim of the mouth (U­shaped muscles) present in ventral body wall (data not shown).  
Frontal organ well developed. Cell bodies of frontal glands positioned 75 µm behind frontal pore (Fig. 4B).  
Mouth opening on ventral surface, middle of body. Digestive central syncytium extends from frontal glands posteriorly to male copulatory apparatus.  
Ovaries paired, ventral; extend from frontal glands posteriorly to bursal nozzle (Figs. 4A, B).  
Testes paired, lateral to eggs; separate from ovary. Testes extend from frontal gland posteriorly to level of the bursal nozzle (Figs. 4A, B).  
Common genital pore opens with posterior side opening to male copulatory apparatus, and anterior side opening to vagina (Figs. 4A, B, 5C).  
Vagina is positioned directly anteriorly to the male copulatory organ (Figs. 4A, B, 5C). Vagina surrounded by a large muscular sphincter; leads to seminal bursa with well­developed sclerotized bursal nozzle. Lateral extensions of bursal nozzle cells extend posteriorly to form anterior and lateral portions of seminal bursa wall (Figs. 4B, 5C, 6A).  
Gonopore opens directly to well­developed, C­shaped tubular penis with outer nonanastomosing longitudinal muscles and inner circular muscles (Figs. 4B, 5C, 6B). Circular muscle fibers are thicker than spaces between fibers. Lumen filled with large spherical glandular secretions (Figs. 4A, B, 5B, C). Penis invaginated into muscular seminal vesicle filled with sperm and small spherical granules that surrounds proximal end of penis.  
 Remarks. The members of the genus Isodiametra are distinguished from the other 18 genera of the Isodiametridae by the presence of a true seminal vesicle, along with a bursa bearing a single sclerotized bursal nozzle.   
 The genus Isodiametra contains thirteen described species, seven of which have in common with I. bajaensis the characteristic of a common gonopore. Of these, I. bajaensis is most similar to I. divae (Marcus, 1950) and I. vexillaria (Marcus, 1948), all of which have a male copulatory organ with a C­shaped tubular penis containing glandular secretions in the lumen. The body length of I. bajaensis is similar to that of I. vexillaria (~ 300 µm), but shorter than that of I. divae (~ 500 µm). The bursal nozzle of I. bajaensis is like that of I. divae and I. vexillaria in having lateral extensions that extend posteriorly to form the anterior and lateral portions of the bursal wall; however, I. divae ’s sclerotized portion of the nozzle is short and thick, and I. bajaensis has a long and thin sclerotized nozzle similar to that of I. vexillaria . Isodiametra bajaensis has a wider and thicker vaginal sphincter compared to those of I. vexillaria and I. divae. In addition, the circular muscle fibers in the penis of I. bajaensis and I. divae are thicker than the spaces between the fibers; this is not the case in I. vexillaria.   
 Diagnostic characters of I. bajaensis include: a C­shaped tubular penis containing glandular secretions in the lumen of the penis, a thick vaginal sphincter, a single, welldeveloped sclerotized bursal nozzle with lateral extensions that extend posteriorly to form anterior and lateral portions of seminal bursa wall, and circular muscle fibers in the penis that are thicker then the spaces between the fibers.
description: Published as part of Hooge, Matthew D. & Eppinger, Neil, 2005, New species of Acoela (Acoelomorpha) from the Gulf of California, pp. 1-14 in Zootaxa 1009 (1) on pages 6-8, DOI: 10.11646/zootaxa.1009.1.1, http://zenodo.org/record/5048876