Genus Sinantherina Bory de St. Vincent, 1826

Sinantherina is composed of five morphospecies, but unlike Lacinularia, Sinantherina possess an oviferon and they lack a gelatinous matrix.

Diagnosis. Corona either heart-shaped or quadrangular; dorsal gap in coronal ciliation limited or absent. Dorsal and ventral antennae small. Some species with distinctive elliptical bodies (warts) located just below the corona on the ventral and/or dorsal sides. One species with ventral spines, but lacks warts. Foot of various lengths depending on species. Pigmented eyespots in juveniles (larvae) and planktonic adults, but lacking in adults of sessile species. Embryos brooded by adult females, fastened to specialized egg-bearing structure (oviferon) located on foot always below anus. No extensive gelatinous matrix (perhaps very limited amounts at base of foot). Adult females usually colonial with small (5–12 individuals) to large colonies (>200 individuals), but occasionally solitary. Sessile with newborn females free-swimming until attachment or remaining within parent colony or planktonic. (NB: The oviferon is shared with Pentatrocha gigantea, but the morphology of the coronae are distinctly different (Segers & Shiel 2008).) Oviparous.

Of the five valid species, three possess glandular structures called warts for which there is evidence that they possess unpalatability factors (Felix et al. 1995; Hochberg et al. 2015; Wallace et al. 2023; Walsh et al. 2006) and one species has spines that may provide defense against fishes with a small mouth gape (Wallace 1987). Information on colony formation in a few species of Sinantherina is provided by Bhardwaj and Dattagupta (1984) and (Garcia 2004). A review of this genus that includes a key to the species is provided by Dioni (1966). Table 3 provided a compilation of the important characteristics of this genus.

Dichotomous key to species of genus Sinantherina

1 Warts (1 or 2 pairs) present on the anterio-ventral surface..................................................... 2 [Depending on the lighting conditions, these glands may appear to be white (epi- illumination) or dark (transmitted light).]

1’ Warts absent......................................................................................... 4

2(1) One pair of warts; corona quadrilateral, flexed dorsally; shallow ventral sinus; ventral antennae small; adults with 2 eyespots; planktonic, but may secrete a thread and adhere to a hydrophyte; colonial (~25–100 individuals); total body length ~1250 µm. (Fig. 6A–C).................................................................... semibullata (Thorpe, 1889) [Adult females with 2 eyespots in the upper edge of the corona between the trochus and cingulum; males possess two eyespots. 2’ Two pairs of warts; corona with deep ventral sinus; two ventral antennae......................................... 3

The trophi may be slightly asymmetrical (Fig. 6C). Trophi also are presented by Banik (1996) and Dioni (1966). Temporary attachment by a common thread has been observed (Gunter & Knight Jr. 1978). Unlike the illustration of Koste (1978) and Vidrine et al. (1985), the coronae of all individuals in a free swimming colony point in the direction of the colony movement. Colony size in a population in Wisconsin (43.9086417; -89.0952805) was recorded to be as follows: n=24, x =57.2, SD=17.6 (RLW, pers. obs.). A detailed analysis of this species was provided by Canella (1952).]

3 Foot long (¾ the length of the whole animal); colonial (~40–50 individuals); oviferon small, rudimentary (mere protuberance topped with 3 knobs); total body length <1000–2500 µm; colonial (large). (Fig. 6D–F)............ procera (Thorpe, 1893) [Eyespots present in larvae and male; absent in the adult.An adult and trophi are documented by Banik (1996), who reports that colonies comprise individuals of the same age (size), with numbers varying between 12 and 65 individuals.]

3’ Foot not long; total body length ≤ 2000 µm; colonial (15 to ≥200 individuals), all adults about same size (age). (Fig. 7).................................................................................... socialis (Linnaeus, 1758) [Trophi are documented by Melone et al. (1998). The biology and ecology of this species has been explored by many researchers (Champ 1978; Champ & Pourriot 1977a; Champ & Pourriot 1977b; Dioni 1966; Felix et al. 1995; Garcia 2004; Hochberg et al. 2015; Hochberg & Lilley 2010; Rico-Martínez & Walsh 2013; Wallace 1993; Wallace et al. 2023; Walsh et al. 2006). NB: We have noticed that young larval colonies may join (fuse) with an established colony thereby forming an integrated unit of mixed ages. Resting eggs (diapausing embryos) may be too large to pass through the cloaca and may be seen in the foot and even on the corona (Edmondson 1940).]

4(1’) Foot wide and short; adults lacking eyespots; solitary or small to large colonies (≤200 individuals); total body length ~ 650 µm; sessile; colonial. (Fig. 8)......................................................... ariprepes Edmondson, 1939 [The trophi are documented by Dioni (1966) and Koste (1978). Dioni (1966) also comments on the behavior of both the larvae and adults, and on the morphology of this species. Edmondson (1939) described S. ariprepes as “… a very restless species, throwing itself about from side to side, rapidly contracting and expanding; its activity is broken by short periods of repose.” As seen in S. socialis, diapausing embryos may be present in the foot.]

4’ Foot long; numerous, tiny spines present on anterio-ventral surface numerous just below the corona (small, irregularly placed, thorn-like shapes); ventral sinus shallow; dorsal gap small; eyespots present on upper edge of corona; planktonic, but may secrete a thread and adhere to a hydrophyte; spherical colonies (20–100 individuals); total body length 800–1000 µm; colonial. (Fig. 9)........................................................................... spinosa (Thorpe, 1893)