Nutrition of the southern sea garfish Hyporhamphus melanochir: gut passage rate and daily consumption of two food types and assimilation of seagrass components

Abstract

The southern sea garfish Hyporhamphus melanochir fed on the seagrass Heterozostera tasmanica for 13 h during daylight, ingesting an estimated 3 gut volumes of plant matter, equivalent to 500 kJ d-I kg-' dry wt. No cellulase was detected in garfish digestive tissues, and gut pH was near neutral, but the pharyngeal mill efficiently triturated the seagrass tissue. Seagrass frond was assimilated at an efficiency of 38 % (organic matter). 28 % (energy). 50 % (protein) and 76 % (lipids), these being equivalent to the total components available in seagrass cell contents. Nocturnally emergent crustaceans were preyed upon by garfish over a period of 6 h and were processed at half the rate of seagrass. The daily energy intake, and probably assimilation, from this source was 223 W kg-' dry wt. Although the crustaceans consumed were only a third the quantity of seagrass, it was reasoned to be an essential source of protein, and at least as important as seagrass in satisfying the energy requirements of H. melanochir. Part TV in the series 'Study of Food Chains in Seagrass Communities' gut is used to estimate the ingestion rates for each of ' ' Present address: Biologische Anstalt Helgoland, Notke-these food types. We discuss the relative importance of straDe 31, D-2000 Hamburg 52 seagrass and animal food to the nutrition of this fish.

Seagrass selection by omnivorous and herbivorous consumers: determining factors

“…In particular, seagrasses often have elevated C:N ratios, due to high levels of structural carbohydrates (lignin and cellulose) that cannot be easily digested by most marine organisms (e.g. Klumpp & Nichols 1983) and might influence preference and/or ingestion rates by consumers. Other factors such as caloric content, amount of organic matter, total protein and total lipid content may also be detected by grazers and result in enhanced consumption rates of higher quality plants (Montgomery & Gerking 1980, Targett & Targett 1990.…”

The trophic ecology, functional morphology and phylogeny of the hemiramphidae (beloniformes)

“…They are particularly abundant in coastal waters, some species penetrate estuaries into freshwater, and a fe w complete their life-cycle in freshwater (Collette et al, 1984). Herbivory is widely reported for the coastal species (Talwar, 1962;Randall, 1967;Thomson, 1959;Robertson, 1978Robertson, , 1980Blaber, 1980;Blaber and Blaber, 1980) and they have been implicated in nutrient recycling (Klumpp & Nichols, 1983).…”

“…This is especially important when one considers that hemiramphids have a relatively small gut length to total body length ratio (Klumpp & Nichols, 1983), which is commonly considered to be a fe ature of carnivorous fish (Kapoor, 1972). Since it has been reported that their pharynx plays an important role in food treatment (Klumpp & Nichols, 1983) and no functional morphological examination of the structure has been forthcoming it seemed advisable to carry out this work. Certain modem analytical techniques are available and the application of these might not only throw light upon the unusual alimentary specializations of hemiramphids but, also on certain anomalies (e.g.…”

“…Robertson (1976) report� Hyporhamphus melanoch!r to show a seasonal dietary shift from macroflora in the summer to amphipods in the winter. Robertson & Klumpp (1983) fo und a diel trophic shift in H. melanochir, from nocturnal camivory to diurnal herbivory, a trophic habit which Klumpp and Nichols (1983) explained in terms of a nutrient balancing behaviour.…”

“…Assuming a gut passage time of approximately six hours for mixed plant and animal ingesta (Robertson & Klumpp, 1983;Klumpp & Nichols, 1983), and since sampling took place at 10:00 hrs, the posterior most material present in the gut would have been ingested a approximately 04:00 hrs (i.e. during darkness).…”