Loxosomella diopatricola Williams, 2000

Bharathidasan, Veeraiyan, Borisanova, Anastasia O., Sarathy, Palanivel Partha & Murugesan, Perumal, 2022, New entoproct-polychaete association recorded in the Bay of Bengal, southeastern coast of India, Zootaxa 5100 (4), pp. 482-500 : 484-485

Loxosomella diopatricola Williams, 2000

The species was originally described by Williams (2000) from the shallows of southeast Australia as epibionts of Diopatra sp. Our specimens were collected in Bay of Bengal (Tamilnadu, Southeastern coast of India). Samples were collected from the depth of 15 m. Water temperature was 30°C, salinity range 30 psu, pH 8.1. Numerous loxosomatids were found on the body surface and notopodia of Diopatra sp .

Description: Medium-sized species with average body length 396 µm (from 288 to 565 in measured specimens) (measurements of several specimens are given in Table 1 ). Calyx length 200 - 369 µm, usually close 300 µm. Average calyx width 185µm. Calyx oval in shape, without prominent compression. Calyx bears 8 tentacles faced disto-frontally or distally in contracted state ( Figs. 4C, F, G ). Stomach large, roundish. Papillae absent. Stalk shorter than calyx, with variable length, average length 116 µm. In some specimens stalk very short, less than 60 µm ( Figs. 4A, B, D ). Foot reduced, but pedal gland persists in adults. Pedal gland large, occupies almost entire width of base of stalk.

Reproduction: Only one specimen with buds observed. Two buds originate from latero-frontal budding areas located at mid-stomach level, one bud on each side ( Fig. 4C ). Both buds young, without developed foot. Many specimens with developing embryos. Up to five embryos can develop in calyx of one specimen ( Fig. 4K ).

Ecology: The loxosomatids were abundant on host’s body surface. Total number of specimens on one polychaete about 550. They attach to anterior part of antennae, palps, mouth region and to parapodia and parapodial gills all over body, except for the ventral surface ( Figs. 2A ). Gills most populated by loxosomatids, up to 10 specimens on one gill ( Fig. 2E ).

Remarks. The specimens from the Bay of Bengal closely match the original description of Loxosomella diopatricola in general appearance, tentacle number, and stalk base structure, but they have some differences in sizes. Indian specimens are slightly larger than Australian. The body length up to 565 µm in Indian specimens and up to 450 µm in Australian specimens. Average calyx length is also bigger in Indian specimens – 280 µm against 230 µm of females and 200 µm of males from Australian waters. The stalk of Australian specimens is between one-half and two-thirds that of the calyx, when in Indian specimens it is much more variable in length and can be from 1/5 to 9/10 of the calyx. Williams (2000) mentions that L. diopatricola has only one ovary but we cannot confirm or reject this description for our specimens because the ovaries are not clearly distinguishable. The main difference between Indian and Australian loxosomatids is the structure of the adhesive organ of specimens attached to the gills. Australian specimens attach to gills with expanded attachment disc. Indian specimens attached to the gills lack the attachment disc and have the same attachment structure as specimens from other parts of polychaetes body—the pedal gland at the base of the stalk.

Figures

FIGURE 2. Diopatra sp. light micrographs. A, anterior end, dorsal view; B, posterior end, dorsal view; C, parapodium from the first chaetiger, anterior view; D, parapodium from the second chaetiger, anterior view; E, parapodium from the abdomenal chaetiger with the specimens of Loxosomella diopatricola on the gill (arrowheads), anterior view; F, the chaeta of the first parapodium which penetrates the dorsal cirri; G, limbate chaetae of the first parapodium; H, aciculae of the first parapodium; I, upper pseudocompound hook of the first parapodium; J, median pseudocompound hook of the first parapodium; K, limbate chaetae of the second parapodium; L, upper pseudocompound hook of the second parapodium; M-N, median pseudocompound hook of the second parapodium; O, ventral pseudocompound hook of the second parapodium; P, pectinate chaetae of the fifth parapodium; List of abbreviations: A, ul, upper lips; cr, ceratophore rings; pa, palps; an, antennae; arrowheads point on the specimens of Loxosomella diopatricola. C-D, dc, dorsal cirrus; prl, Prechaetal lobe; ptl, Postchaetal lobe; vc, ventral cirrus. Scale bars; F-P= 50 μm.

FIGURE 4. Various morphological views of Loxosomella diopatricola, light micrographs. A, lateral view of a specimen with a short stalk; B, frontal view of a specimen with a short stalk; C, lateral view of a specimen with two buds; D, frontal view of a specimen with a short stalk; E, lateral view of a specimen; F, frontolateral view of a specimen; G, frontolateral view of a specimen; H, frontal view of a specimen attached to the gill; I, lateral view of a specimen; J, lateral view of a specimen; K, frontolateral view of a specimen; L, lateral view of a specimen.