Anatomy of finfish and shellfish (2+1)

Unit 12 - Arthropoda

12.1.6. Reproductive System
In the white shrimp the most conspicuous components of the female reproductive system are two ovaries that extend, partially fused, from the anterior of the foregut pos¬teriorly to the tail fan. The portion of each ovary that is within the cephalothorax consists of a forward-projecting lobe, which lies close to the esophagus and chambers of the foregut, and seven finger-like lateral lobes, which are situated above the midgut gland and beneath the heart. This arrange-ment makes the heart resemble a saddle straddling the ovaries. The abdominal portion of the ovaries consists of two lobes, lying above and to the sides of the intestine and below and to the sides of the dorsal abdominal artery.

Emerging from each ovary at the sixth lateral lobe is an oviduct. Coursing ventrally, each oviduct opens to the ex¬terior a genital pore situated medially on the basal segment of the third thoracic leg. The opening is concealed within an ear-shaped protuberance covered with setae.
Externally and posterior to the genital openings of the female lies a structure that is adapted for receiving a packet of sperms, or spermatophore (sper-MA-t5-phôre; from the Greek, sperma, seed; phoros, bearing) from the male during mating. Known as thelycum (THEL--cüm; from the Greek, thelys, female), this structure consists of several lobes and protuberances bearing stiff bristles.
The male reproductive system of the white shrimp includes a pair of partially fused testes that lie in a position quite similar to that of the ovaries in the female. Each testis has an anterior lobe projecting forward over the chambers of the foregut and six lateral lobes that lie over the midgut gland and under the heart. In place of a long abdominal lobe as in an ovary, each testis has a short posterior lobe.
A pair of ducts known as the vasa deferentia emerge from the main axis of the testes at their posterior margin, course ventrally, and open to the exterior at the genital pores situated medially on the basal segment of the fifth pair of thoracic legs. Each vas deferens has four distinct regions: a short, narrow proximal portion; a thickened, doubly flexed medial portion; a long, narrow tubular portion; and a much dilated, muscular terminal ampoule. Within the terminal ampoule the sper¬matophore is formed.

The spermatophore of the white shrimp roughly resembles a pod. It consists of two halves, each of which contains sperms enclosed within a sheath and surrounded by chitin. The thoracic legs of the male shrimp presumably assemble the spermatophore immediately after each half is expelled from the terminal ampoule of the corresponding vas deferens. The legs place the spermatophore within the trough of the petasma, a structure that results from modification of the first pair of pleopods. The petasma consists of stiffened longitudinal rods and folds of soft chitin that, when unfolded, result in a broadly inflated male copulatory organ.

During mating, the male shrimp uses the petasma to thrust the spermatophore against the thelycum of the female. Here bristles on protuberances of the thelycum overlap the spermatophore, thus helping to secure it. Two lobes known as "wings" on the spermatophore become anchored in a groove on the ventral surface of the female between her third and fourth thoracic legs. Despite these devices for securing the spermatophore, it is easily dislodged, and spermatophore ¬bearing females of white shrimp are not commonly caught in shrimp trawls. A pair of light-colored, pad-like structures situated just posterior to the thelycum are believed to play no role during impregnation.