Early life history of a seahorse, Hippocampus mohnikei , in Tokyo Bay, Japan

Abstract

The early morphological development, seasonal and spatial occurrence patterns, and food habits of a seahorse, Hippocampus mohnikei, in offshore waters of Tokyo Bay, central Japan, were studied on the basis of 206 juvenile and young specimens (6.0-65.3 mm TL) collected between August 1995 and January 1999. All the specimens were collected within the period from May to January, inclusive, each year. In the least developed specimen (6.0 mm TL), the number of dorsal, anal, and pectoral fin rays had attained the adult complement, whereas the minute caudal fin, consisting of two rays, was present in juveniles of 6.0-26.4 mm TL. Hippocampus mohnikei м35 mm TL, being larger than settlement size (ca. 30 mm TL), had very low gut fullness index values (GFI ϭ 0, Ͼ70% of specimens), whereas those of 15-29 mm TL had higher values (GFI ϭ 2-4, Ͼ80% of specimens). In addition, larger individuals selectively fed on larger planktonic animals (species of Brachyura), which occurred naturally at low densities, although smaller food items, such as Oithona davisae and Penilia avirostris, occurred abundantly, being consumed by smaller H. mohnikei individuals (15-34 mm TL). These results indicated that food availability for H. mohnikei in offshore waters of Tokyo Bay is significant for individuals larger than settlement size, because their food preference would shift from smaller food items to larger food items, which would be scarce in their environments.

Assessing diet composition of seahorses in the wild using a non destructive method: Hippocampus reidi (Teleostei: Syngnathidae) as a study-case

“…The occasional consumption of insects (ants) observed in this study is unique, and is possibly related to the massive presence of mangrove trees along the margins of the estuary. While Harpacticoida was the principal item consumed by Hippocampus zosterae (Tipton & Bell, 1988), H. subelongatus (Kendrick & Hyndes, 2005) and H. reidi (this study), the other studied seahorse species mostly consumed Cyclopoida copepods, amphipods and caridean shrimps (see Kanou & Kohno, 2001;Kendrick & Hyndes, 2005;Teixeira & Musick, 2001;Burchmore et al, 1984;Woods, 2002). These results highlight the importance of crustaceans as a prey category to seahorses.…”

“…while mid size and larger specimens fed almost exclusively on amphipods (Teixeira & Musick, 2001). Additionally, Tipton & Bell (1988) found an increase in the consumption of amphipods by larger specimens of H. zosterae, despite the dominance of harpacticoid copepods in all size classes, and Kanou & Kohno (2001) reported that juveniles H. mohnikei exhibited a division of prey items between size classes, with the smaller seahorses feeding mainly on cyclopoid copepods smaller than those consumed by larger specimens. Some authors state that the reproductive system of Syngnathidae may result in differential reproductive costs, which may be reflected in different prey selection by males and females (Svensson, 1988;Steffe et al, 1989;Lyons & Dunne, 2003;Garcia et al, 2005).…”

“…Published data on the diet composition of seahorses from field-based studies, however, are limited to a few species (Tipton & Bell, 1988;Kanou & Kohno, 2001;Texeira & Musick, 2001;Woods, 2002;Kendrick & Hyndes, 2005), and typically based on destructive sampling.…”

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“…Digestive enzyme activities had been an effective tool for identifying particular components of Kanou et al, 2000). Digestive processes in fish aren't well known as in mammals, although the data obtained in fish so far show that the digestive enzymes studied are qualitatively similar to those observed in other vertebrates.…”

The life history of weedy seadragons, Phyllopteryx taeniolatus (Teleostei:Syngnathidae)

“…Additionally, fin development begins during stage III of the incubation period rather than during the larval period. However, one syngnathid (Hippocampus mohnikei) has been reported to possess a minute caudal fin with two vestigial fin rays during development, which later disappear (Kanou and Kohno 2001). The examination of larval and juvenile captive-bred weedy seadragon embryos revealed no evidence of a caudal fin or caudal fin rays.…”