Question

Describe the life cycle of Salpa.

[Detailed answers required]
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Answer 1

The chain of salps is the 'aggregate' portion of the life cycle. The growing oozoids are eventually released from the parent blastozooids, and then continue to feed and grow as the solitary asexual phase, thus closing the life cycle of salps.

Schematic Representation of Salp Life Cycle with Alternation of Generations The oozooid gives rise by budding to the blastozooid, in which the embryo develops after fertilization of the egg by sperm of older blastozooids into a newly oozooid. a. Oozooid; b. chain of blastozooids; c. blastozooid with embryo.

The salp Iasis zonaria is often found over the Argentine continental shelf of the southwestern Atlantic Ocean and, occasionally, its high densities dominate over other groups of zooplankton. Therefore, a better understanding of the basic aspects of its life history is essential for understanding the mechanisms underlying bloom formation. In this study, I. zonaria was collected during the austral winters of 1999, 2000 and 2001. Additional data were obtained from this area from May to November 1978. The most widespread distribution and largest catches occurred in 1999 (≈55 100 ind. 10 m−2). Densities decreased markedly in 2000 and the species was virtually absent in 2001. The growth of the solitary stage blastogenetic stolon and formation of several blocks of aggregates buds are described. Eight development stages were characterized and it is estimated that each solitary can produce at least four blocks and a total of ≈ 420 aggregates. The average number of buds per block increases from the older (first formed) to the youngest. The total number of buds (y) was related to solitary length (x) as: y = 4.94 x – 134.57. Four developmental stages were described for aggregate individuals. Based on monthly samples collected in 1978, the estimated lifespan of aggregate individuals varies between 11 and 14 months.

Salps are holoplanktonic tunicates widely distributed throughout the world's oceans. They are capable of ingesting food particles from <2 µm to 1 mm with high efficiency (e.g. Kremer and Madin, 1992), significantly contributing through their fecal pellets to the vertical flux of materials and carbon to deeper waters (e.g. Caron et al., 1989, Phillips et al., 2009). Typically, salps are found at low densities but, at times, they form dense swarms which dominate over other zooplankton taxa. Swarm development of several species (e.g. Salpa thompsoni, Thalia democratica, S. aspera and S. fusiformis) has been described in various locations (e.g. Wiebe et al., 1979; Loeb et al., 1997; Madin et al., 2006; Daponte et al., 2011). The swarms are formed presumably in response to favorable conditions and result from the rapid growth (e.g. Heron and Benham, 1985), efficient filter feeding (e.g. Madin and Kremer, 1995) and a life cycle that involves a highly productive asexual reproductive stage.

During swarming events, ecosystem functioning is altered mainly due to modification of the epipelagic food web (Landry and Calbet, 2004), mediated by decreased organic matter available to higher trophic levels (e.g. Daponte et al., 2011) and enhanced export flux of organic matter to the deep layers (e.g. Fortier et al., 1994). The complex life history of salps involves an alternation between the solitary (oozooids) and aggregate (blastozooids) generations. Oozooids reproduce asexually by budding producing blastozooids that reproduce sexually; it is asexual reproduction that is responsible for explosive population increases. In this context, it is essential to know which species are capable of producing swarms and to better understand the reproductive mechanisms underlying swarm formation. Aggregate stage formation by the solitaries varies between species, and is either by budding of a blastogenic stolon that continuously forms buds of aggregates or by the stolon forming blocks of buds which contain a variable number of equally sized aggregates. Except for S. thompsoni and S. gerlachei (Foxton, 1966; Casareto and Nemoto, 1986; Daponte et al., 2001), S. fusiformis (Braconnot et al., 1988), Thalia democratica (Braconnot, 1963; Heron, 1972a,b; Deibel, 1982; Heron and Benham, 1984, 1985; Tsuda and Nemoto, 1992; Daponte et al., 1996) and Cyclosalpa bakeri (Madin and Purcell, 1992), there are no estimates of the minimum number of blocks and of buds per block that can cause massive population increases. The species under consideration, Iasis zonaria, appears to be capable of swarm formation. It has been occasionally found in high densities at the Bay of Bengal (Nagabhushanam, 1960), off of South Africa (Pakhomov et al., 1994) and in some areas over the Argentinian Continental Shelf (Mianzan et al., 2001; Daponte et al., 2011). This is an oceanic, cosmopolitan and eurythermal species (van Soest, 1975).

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Answer 2

Answer:

The sexually reproducing blastozooids and the asexually reproducing oozoids are the two life stages that must alternate in the usual salp life cycle.

Explanation:

• The "aggregate" part of the life cycle is the chain of salps. The collective individuals are also known as blastozooids; while swimming and feeding, they stay connected while each individual enlarges.
• The growing embryo oozooid is linked to the parent's body wall during the sexual reproduction of each blastozooid in the chain (blastozooids are sequential hermaphrodites that first mature as females and are fertilised by male gametes produced by older chains).
• The parent blastozooids are finally separated from the developing oozooids, which continue to grow and feed as the salps' solitary asexual phase.
• Both lone individuals and aggregate chains live and eat together in the water thanks to the alternation of generations, which speeds up the generation time.
• This rapid reproduction produces relatively brief salp blooms when phytoplankton is abundant, and the salps gradually filter out the majority of the phytoplankton.
• When there is not enough food to support the massive population of salps, the bloom comes to an end.
• Occasionally, salps are reported to be consumed by mushroom corals and members of the species Heteropsammia during blooms.

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